Field

This invention is in the field of mechanical arms, and particularly but not exclusively, mechanical balance arms for robotic applications.

Background

The motors in robotic arms often have to deal with high torgues at extended reach. Larger torgues reguire more powerful and therefore heavier actuators to move the loads. Much of the force or torgue generated by the actuator is used to balance the weight of the arm itself, which means that a large proportion of the energy used by the arm does not go into useful work. Additionally, the torgue that the motors must supply changes significantly as the arm moves through different orientations as the moment of inertia changes, which greatly complicates controlling the robot.

A high reduction gearbox, harmonic drive, ball screw or other transmission device is reguired to generate sufficient torgue to counteract the weights of the links, these transmissions introduce mechanical impedance which makes the robot difficult to back drive for manual position teaching, active control loops with force sensors are typically used to address this problem. This also introduces safety concerns, the strength of traditional industrial robots means they are dangerous for humans to interact with, more modern collaborative robots use sensors to detect the presence of humans so maintain safety, however they still reguire motors powerful enough to hold the weight of the arm which could easily injure human colleagues if their sensors or control loops fail.

High reduction transmissions can also introduce mechanical play or backlash which adversely affects the position accuracy of the robot arm, the transmission must therefore be of high guality which introduces significant cost.

The present invention seeks to provide a solution that can balance the weight of the arm and a variable payload, for all position combinations of 3 or more rotational axes. The solution is scalable and can be theoretically applied to a kinematic chain of any number of links and revolute joints although the mechanical complexity increases significantly beyond 5. The balancing force provided by the following disclosure is theoretically optimal and only reguires active control of the balancing mechanism when the payload changes, at other times it is sufficient to keep the piston pressure constant.

In the design described herein, forces for 3 axes (the 4 ^th rotational axis plane is perpendicular to the gravity direction and does not require balancing) are combined so that a single pneumatic cylinder can be used to provide the opposing force to the gravity load. The balance cylinder operates in parallel to the motors and can provide near 100% of the force required to maintain a load in a static position.

A high reduction transmission is not required, the arm is easily back driven, it is inherently compliant and does not require force or torque sensors in an active control loop to follow external forces. Furthermore, the power consumption of the arm is significantly reduced as the motors are only used to accelerate or decelerate the arm and are not required to hold static loads, thus can be less powerful and inexpensive.

US 4,500,251 discloses a two-joint load system balanced by a pivotable spring or hydraulic cylinder.

US 2,665,102 discloses a lamp stand where a fixed spring balances the weight of two links with a static payload.

The prior art to date has not combined forces for 3 axes into a single force balancing system and the balance force is not optimal for all position poses. Moreover, in the prior art, active control is required to balance varying position poses and the solutions are mechanically complex. The present invention aims to address these shortcomings.

Brief summary of the invention

The present invention provides a balance mechanism and a method of balancing a load as claimed. Further preferred embodiments are defined in the dependent claims.

Brief description of the figures

So that the present disclosure may be understood more readily, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIGURE 1 is a schematic of a balance mechanism for balancing two rotational joints with two load arms, with an unbalanced hand carrying a payload, in accordance with a first example of the present disclosure;

FIGURE 2 is a schematic of a balance mechanism for balancing three rotational joints with two load arms and a hand carrying a payload, in accordance with a second example of the present disclosure;

FIGURE 3 is a schematic of a similar balance mechanism to FIGURE 2 that uses additional force balance members in order to balance variable loads, in accordance with a third example of the present disclosure;

FIGURE 4 is a schematic of a compact balance mechanism for balancing two rotational joints with two load arms, with an unbalanced hand carrying a payload, in accordance with a fourth example of the present disclosure;

FIGURE 5 is a schematic of the balance mechanism shown in FIGURE 4 with an additional shoulder roll axis in accordance with a fifth example of the present disclosure; and

FIGURE 6 is a schematic of a compact balance mechanism for balancing three rotational joints with two load arms and a payload, in accordance with a sixth example of the present disclosure.

Detailed description

Figure 1 illustrates a first configuration of a balance mechanism used to balance the weight of a load mechanism comprising two arm sections and an optional gripper/hand with a payload.

The load mechanism of figure 1 comprises first and second load arms: upper arm A1 and forearm A2, having respective masses M1 and M2 and lengths LA1 , LA2, the arms A1 , A2 being pivotally connected in series at a load arm elbow pivot 2. The upper arm A1 is also connected at its other (distal) end to a shoulder pivot joint 7, which itself is fixed with respect to the ground 11. A hand or gripper with payload 9 is also shown connected in series to the distal end of the forearm A2 at load arm wrist pivot joint 8. The load mechanism therefore provides a series load mechanism in the order: shoulder pivot joint 7 - upper arm A1 - elbow pivot joint 2 - forearm A2 - wrist pivot joint 8 - hand with payload 9. The balance mechanism of figure 1 comprises two balance arms or levers B1 , B2 having lengths LB1, LB2. The balance arms B1 , B2 are pivotally connected in series from the shoulder pivot 7 and pivotally connected to each other at a first balance pivot 1. In this example, the first balance arm B1 is also connected at its other (distal) end to the first load arm A1 via the shoulder pivot 7 and forms a rigid extension of the first load arm A1, such that the angle between the first load arm A1 and the first balance arm B1 is fixed, preferably at 180°. The second balance arm B2 is also pivotally connected at its other (distal) end to a balance force member 5, configured to exert a balancing force on the second balance arm B2. In the figure 1 configuration, the balance force member 5 is slidable with respect to the ground along a single axis (the x-axis) via a slider joint 6 and the balance force member 5 comprises a pressure regulator 10 to adjust the balancing force applied to the second balance arm B2. In other embodiments, the balance force member 5 may be connected to different or multiple balance arm(s) B1 , B2... BN.

The balancing force required is dependent on the maximum torques at the load arm shoulder and elbow joints 7, 2, these torques are a product of the masses M1, M2 of the upper arm A1 and forearm A2 sections, the centres of those masses and the lengths of arms A1 and A2. The maximum gravitational torques apply when the load arms are horizontal. In this configuration, the optional payload 9 is not balanced.

Preferably, at least one of the load or preferably the balance arms BI, B2... BN has an adjustable length LB1 , LB2... LBN to balance varying loads. In this configuration, the second balance arm B2 has an adjustable length (LB2). In other configurations, at least one of the load or preferably the balance arms (B1, B2... BN) has an adjustable mass (M1, M2... MN) to balance varying loads.

The balance mechanism can generally be considered as a scaled reflection of the load mechanism. The first balance lever B1 for the first load arm A1 is preferably a rigid extension of the first load arm A1 itself, on the opposite side of the shoulder pivot 7 and is preferably an (inverted) supplement of its respective arm section. In other words, the first balance lever B1 and first load arm A1 are rigidly connected (e.g. directly or via a connecting member), where the angle therebetween is fixed, preferably at 180°, and cannot change.

Moreover, where A’° is the zenith angle of the first balance arm B1 from vertical, A° is the zenith angle of the first load arm A1 from the same vertical and therefore A° = - (180-A’°). Angles A° and A’° are supplementary as they add up to 180° and inverted since they are taken from vertical in different directions (A’ anticlockwise from vertical; A clockwise from vertical). Note that the language ‘with respect to’ is intended to be understood as independent of direction (i.e. could be either direction). Figure 1 also shows that A’° = (A’°).

The second balance arm B2 can also be considered a (inverted) supplement to the second load arm A2, since the angles of the second load arm A2 and the second balance arm B2 from the shoulder pivot 7 with respect to vertical also add up to 180°.

Furthermore, in the figure 1 configuration, the proximal end at balance pivot 1 of the second balance lever B2 is coupled or linked to the proximal end at the elbow pivot 2 of the second load arm A2, via a transmission 20. The transmission 20 is generally configured to transfer the forces and/or torques of one or more of the load arms A1 , A2...AN to one or more of the balance arms B1 , B2,... BN. In this embodiment, the transmission 20 comprises a pair of pulleys, sprockets, gears or equivalents 21 , 22 with a first pulley or sprocket 21 rigidly attached to the second balance arm B2 at the balance pivot 1 and a second pulley or sprocket 22 rigidly attached to the second load arm A2 at the elbow pivot 2, the pulleys or sprockets 21 , 22 being connected via a first timing (synchronous) belt or chain 31. Angle B’° of the second balance lever B2 from the first balance lever B1 equals the angle B° of the second load arm A2 from the first load arm A1.

The system is preferably configured to adjust the relative angles of the balance arms B1, B2... BN and/or the lengths LB1 , LB2...LBN of one or more of the balance arms B1, B2...BN to balance the torques of the load arms A1 , A2...AN, thus accounting for variable loads.

One core aspect of the figure 1 and subsequent arrangements is that the first balance lever B1 provides a first balance pivot 1 offset from the shoulder pivot 7 at a distance LB1 , and the number of balance arms B1, B2 (2) matches the number of degrees of freedom (2) to be balanced. In the prior art, balance elements are generally on the same rotation axis as the first load arm shoulder joint 7.

To balance the system, the force produced by the balance force member(s) induces a maximum torque on each balance arm pivot joint 7, 1 , 3 that substantially matches the maximum torque induced by gravity at the corresponding load arm pivot joint 7, 2, 8. (In other words, the shoulder pivot joint 7 has both gravitational load and balance torques that are balanced; the gravitational torque at elbow pivot 2 is balanced by the torque at the first balance pivot joint 1 and in embodiments with a balanced third load arm A3, the gravitational torque at wrist pivot 8 is balanced by the torque at the second balance pivot joint 3.) It can be shown that if the balance lever lengths LB1 and LB2 as well as the balance piston force F are set according to the equations below, the torques T induced by gravity at the corresponding load arm joints: shoulder joint T7 and elbow joint T2 respectively, then the force F from the balance force member 5 can be set as equal to and opposite to the forces induced by gravity and the system will be perfectly balanced.

Equations governing the balance arm lengths LB1, LB2 and the balance piston force, F are:

T2 LB2

T7 ~ LB1 + LB2

T2 T7 F ~ LB2 ~ LB1 + LB2

Where T7 and T2 are the maximum gravitational torques experienced at the shoulder joint 7 and elbow joint 2 respectively and F is the force enacted by the balance force member 5. From these equations, it can be seen that the lengths LB1 and LB2 are not fixed provided the above ratio at least substantially holds and thus can be adjusted to change the needed balance force F.

The balance force member 5 produces a balancing torque T on the balance pivot 1 and on the shoulder joint 7 via the balance levers B1, B2, to balance the load torques on the elbow joint 2 and the shoulder joint 7 from the first and second load arms A1, A2, transmitted via the pulleys/sprockets 21 , 22 and belt/chain 31. It can be seen that the sinusoidal change in torque at the shoulder 7 and elbow 2 joints which occurs as they change angle with respect to the gravity plane is perfectly cancelled by the sinusoidal change in force transmission from the balance piston 5 to the levers B1, B2 as they rotate in the opposing orientation to the upper arm A1 and forearm A2 sections. Thus, the piston pressure in the balance force member 5 can remain constant for all possible combinations or rotations of the load arms A1 , A2 and perfect balance will be maintained. Under constant load conditions, no active control of the balance force member 5 is required, providing easy control.

The balance force member 5 is preferably a single low friction pneumatic cylinder 5 and is preferably mounted on a horizontal slider joint 6 free to move sideways, thus its force vector is always parallel with the gravity vector. As the robotic arm grasps or releases an optional payload 9, the balance system will require adjustment to accommodate the change in load (torque) presented at the shoulder and elbow joints 7, 2. This adjustment can be achieved by changing the pressure in the balance cylinder 5 and (preferably simultaneously) altering the appropriate lengths LB1 , LB2 of the balance levers B1, B2 e.g. with a servo motor and ball screw or a telescoping balance lever arrangement, or using an additional balance lever(s) B3..BN such as shown in Figure 2 and described below, or by using multiple balance cylinders as shown in Figure 3 and described below.

The balance system is intended to work in parallel with electric servo motor actuators which position the joints, where the torque output requirement for these motors is minimised by the balance mechanism.

In further configurations, additional load arms A3... AN and balance arms B3... BN may be used, such as described below.

Figure 2 Illustrates the configuration of figure 1 extended to balance three rotational joints in the vertical plane: shoulder 7, elbow 2 and wrist 8, with optional payload 9, but with fixed length balance arms B1, B2, B3.

In this configuration, the lengths LB1-LB3 and balance force member 5 force F are set according to the formulae given below:

Where T7, T2 and T8 are the maximum gravitational load torques experienced at the shoulder joint 7, elbow joint 2 and wrist joint 8 respectively and F is the force enacted by the balance force member 5.

The balance arms therefore do not necessarily need to be length adjustable, although this provides for balancing varying loads. In this configuration, the balance force member 5 enacts a balancing torque on the balance pivot joints 3, 1, 7 (TB3, TB1, TB7) such that the balancing torques TB3, TB1, TB7 are equal and opposite to the gravitational torques T8, T2 and T7, respectively.

As for figure 1 , the first balance lever B1 is a rigid extension of the first load arm A1 itself, on the opposite side of the shoulder pivot 7 and therefore is also an (inverted) supplement of its respective arm section, i.e. where A’° is the zenith angle of B1 from vertical, A° is the zenith angle of A1 from the same vertical and therefore A° = - (180-A’°). Again, in this example the first balance lever B1 and first load arm A1 are rigidly connected where the angle therebetween is fixed at 180°. Further, similar to the figure 1 embodiment, the pulleys or sprockets 21 , 22 at the balance pivot 1 and elbow pivot 2 respectively are connected via a first timing (synchronous) belt or chain 31 so that angle B’° of the second balance lever B2 from the first balance lever B1 equals the angle B° of the second load arm A2 from the first load arm A1.

In this embodiment, the wrist rotation angle C° is transmitted to a third balance lever B3 via three timing belts or chains 32, 33, 34 and four timing pulleys or sprockets 23, 24, 28 and 29. In this embodiment, a pulley 23 is rigidly connected to the third balance arm B3 and a pulley 28 is rigidly connected to the payload 9 or a third load arm A3. The pulleys 23, 28 in this example are linked via the three timing belts 32, 33, 34 via additional pulleys 24, 29 at the first balance pivot joint 1 and elbow pivot joint 2, respectively, as shown. In other embodiments, the pulleys or sprockets 23, 28 may be linked directly, i.e. not via additional pulleys 24, 29.

In some embodiments, the timing belts 32, 33 share a common pulley 29 at the elbow pivot 2, which couples the rotation of the belts 32, 33, but is otherwise free to rotate on the elbow pivot 2. In the same way, the timing belts 33, 34 might share a common pulley 24 at the balance pivot 1 , which is free to rotate on the first balance pivot 1. In other embodiments, the pulleys 24, 29 might each comprise a pair of pulleys: 24a, 24b on the same axle at the first balance pivot 1 ; and 29a, 29b on the same axle at the elbow pivot 2, where the pulleys in each pair 24a, 24b; 29a, 29b are rigidly fixed with respect to one another, to link their rotational motion.

Here, further to the link between the second balance arm B2 and second load arm A2 comprising pulleys or sprockets 21 , 22 as described above for figure 1 , the system is additionally configured such that an angle C° of the payload 9 or a third load arm A3, from the second load arm A2 equals an angle C’° of the third balance arm B3 from the second balance arm B2. Thus, the angle C’° of the wrist balance lever B3 with respect to the forearm balance lever B2 is maintained, the torque required to balance the wrist joint 8 is transmitted back through the timing belts 32, 33, 34 from the balance cylinder 5 to the wrist joint 8.

It can be seen that this arrangement can also be rotated to the horizontal plane at the shoulder joint 7 and the balance mechanism remains effective.

By extension, ‘N’ number of series joints on the same plane can be balanced - although the complexity of the transmission would be cumbersome beyond 5.

Figure 3 Illustrates a similar configuration to Figure 2 but with two additional balance force members 51, 52 with corresponding slider joints 53, 54 and corresponding pressure regulators 55, 56 to adjust the balancing forces applied to the balance arms B1, B2, B3, to allow for a variable payload mass M3 and/or load arm masses M1 , M2. The balance forces are preferably transmitted through the same pulley/belt or sprocket/chain transmission 20 as described above with reference to Figure 2.

The balance force members 5, 51, 52 are connected to the respective balance levers B1-B3 so that every balance arm B1-B3 has a balance force member 5, 51 , 52 attached. Each balance force member 5, 51, 52 is connected to a separate corresponding slider joint 6, 53, 54 so that they may translate freely with respect to each other and each slider is connected to a corresponding pressure regulator 10, 55, 56, so that the force applied by each balance force member 5, 51 , 52 can be varied independently.

In contrast to figures 1-2 above (and 4-5 below), in this figure 3 configuration, the lengths LB1- LB3 of the balance levers B1-B3 are set so that they match the ratio of the lengths LA1-LA3 of the load arms A1-A3, i.e. LA1 :LA2:LA3 = LB1:LB2:LB3, since the balance for each load arm and the payload can be balanced by varying the pressures for each balance force member 5, 51, 52. The balance levers B1-B3 do not need to be length-adjustable. The pressure for each balance force member 5, 51 , 52 is set such that the torque applied to each balance member pivot joint 7, 1 , 3 matches the maximum torque that each load arm mass M1 , M2, M3 enacts on each load arm pivot joint 7, 2, 8. As an example, the force in balance force member 52 is preferably set so that the torque that it enacts on the second balance pivot 3 matches the gravitational torque that load mass M3 enacts on the wrist pivot 8 when the load arm A3 is horizontal (i.e. maximum).

This is demonstrated in the equations below that show how the balance force members, 51 , 52 forces F5, F51 , F52 and the balance arm lengths LB1, LB2 and LB3 are calculated.

Where T7, T2 and T8 are the maximum gravitational torques experienced at the shoulder joint 7, elbow joint 2 and wrist joint 8 respectively and F5, F51 and F53 are the forces enacted by the pneumatic pistons 5, 51 , 52 respectively. Lengths LA1, LA2, LA3 are the lengths of the load arms A1, A2 and A3 respectively. Lengths LB1 , LB2, LB3 are the lengths of the balance lever arms respectively. It can be shown that this allows the gravitational load on each load arm pivot joint 7, 2, 8 to be perfectly matched by the balancing torques applied through the force balance members 5, 51, 52 in any rotational configuration of the mechanism.

A variation in any of the load arm masses M1 , M2, M3 can be accommodated by varying the force in the corresponding balance force member 5, 51, 52. This figure 3 example therefore only requires a change in pressure for a change in payload, thus is easy to control. The mechanism can be extended to N number of additional arm joints using N number of balance force members acting on N different balance arms connected in series.

In this example, all of the pivot joints lie on the same pitch axis, however as long as the orientation of the balance lever axis matches the orientation of the corresponding arm axis and the motions are linked, the balance mechanism will work.

As an example, the elbow pivot joint 2 could be rotated by 90 degrees to rotate the second load arm A2 in the roll axis instead of the pitch axis. To accommodate this, the first balance pivot joint 1 would be rotated by the same angle and provided that the motions of the second load arm A2 and the second balance lever B2 are still linked (e.g. through a mechanical linkage or transmission, or hydraulic/pneumatic actuators), it can be shown that the gravitational and balancing loads will still cancel out in all joint orientations.

Figure 4 illustrates a more compact configuration for balancing two load arms A1, A2 with optional unbalanced payload 9, which may be used to balance the arm of a humanoid robot or animatronic figure, where, in contrast to figures 1-3, the balance force member 5 is mounted at the front of the first and second balance arms B1 , B2 with its base fixed to a rotating pivot joint 6.

In figures 1-3, the balance arms B1 , B2... BN are connected in series to the load arms A1 , A2...AN, which are also connected in series. In the compact configuration of figure 4, the first load arm A1 is pivotally connected at a first end thereof to a shoulder pivot 7 that is preferably fixed with respect to the ground; and the first load arm A1 is also pivotally connected to the second load arm A2 at its second end at the elbow pivot 2. A balance arm B2, having a length LB2, is pivotally connected at a first end thereof to the first load arm A1 at a balance pivot 1, where the balance pivot 1 is at a distance LB1 along the first load arm A1 from the shoulder pivot 7, effectively forming a first balance arm B1 having length LB1, as indicated in figure 4. As for the figure 1 arrangement, first balance pivot 1 is offset from the shoulder pivot 7 at a distance LB1, and the two effective balance arms B1 , B2 match the number of degrees of freedom (i.e. 2) to be balanced.

In some embodiments, a distinct first balance arm B1 may be provided, the first balance arm B1 having a length LB1 and being connected to the shoulder pivot 7 at a first end and to the second balance arm B2 at the balance pivot 1 at a second end (see figure 4).

The balance lever orientations are coupled to load arm sections A1, A2 and in this example the first and second balance lever lengths LB1, LB2 are fixed (but preferably adjustable in other examples) and are calculated using the ratio of maximum torques at the load shoulder and elbow pivot joints 7, 2 as described above for figure 1, however the force direction is not inverted as in Fig. 1. In the figure 4 compact configuration, a zenith angle A° of the first load arm A1 from vertical equals a zenith angle A’° of the first balance arm B1 (when present) from the same vertical; and an angle B° of the second load arm A2 from the first load arm A1 equals an angle B’° of the second balance arm B2 from the first load arm A1 (when no distinct first balance arm B1 is present) or from the first balance arm B1 (when present, as shown in figure 4). In the embodiment of figure 4, the balance pivot joint 1 and the elbow pivot joint 2 are linked via pulleys or sprockets 21 , 22 - the first pulley 21 is rigidly attached to the second balance arm B2 at the balance pivot 1 ; and the second pulley 22 is rigidly attached to the second load arm A2 at the elbow pivot 2. The pulleys or sprockets 21 , 22 are connected via a first timing (synchronous) belt or chain 31 , to link and preferably synchronise the angles B°, B’°.

In this configuration, in contrast to that of figures 1-3, the balance force member 5 connected to the second balance arm B2 is connected to the ground 11 via rotating pivot joint 6 instead of slider joint 6. The rotating pivot joint 6 means that the compensation forces are sub optimal in comparison to the sliding base joint 6 used in the arrangement of Figures 1-3.

To compensate for the fixed base of the balance force member 5, a ratio between the timing pulleys 21, 22 can be introduced, effectively providing a timing ratio between the balance pivot 1 and the elbow pivot 2, to keep balance force member 5 perpendicular or co-linear to the second balance arm B2 at peak and minimum load respectively - this improves the balance optimisation for the forearm section A2. The ratio is calculated from the angular travel of the balance force member 5 with respect to its base pivot joint 6 as it rotates to follow the motion of the second balance lever B2.

Figure 5 illustrates the configuration of Figure 4 with an additional shoulder roll rotation axis 60.

In this embodiment, the shoulder joint 7 comprises a shoulder roll pivot joint 41 to allow the whole configuration of Figure 4 to rotate around this axis. Two additional roll pivots 42, 43 are provided in this embodiment: balance force member joint 6 comprises a balance force member roll pivot 42 at the base of the balance force member 5; and the second balance lever B2 comprises a balance lever roll pivot 43 at the connection between the balance force member 5 and the second balance lever B2, allowing the balance force member 5 to stay attached to the mechanism without restricting movement. This additional shoulder roll axis 60 may be actuated by any suitable means such as a servo motor or a ball-screw.

When shoulder pitch axis 61 is horizontal, the mechanism behaves exactly as described for figure 4. As the shoulder roll rotation axis 60 is perpendicular to gravitational forces, the gravitational torques on the shoulder 7, elbow 2 and wrist 8 joints are scaled by the sine of the angle between the shoulder pitch axis 61 and vertical axis 64, with the gravitational torques dropping to 0 when the shoulder pitch axis 61 becomes vertical and aligned with the vertical axis 64.

In this example, the balance force member 5 is attached to the second balance lever B2 through an additional rotational axis 43, the rotational torque applied to the second balance lever B2 by the balance force member 5 is given by the sine of the angle between second balance lever B2 link axis 63 and balance force member axis 62 and decreases to zero when the axes 62, 63 become aligned, therefore the torque transmitted to the forearm section A2 via the belt 31 also reduces to zero as the arm moves to the horizontal plane, matching the gravitational load at the elbow pivot 2.

This same principle can be applied to any of the configurations shown in figures 1-3, for example to provide an additional degree of freedom whilst maintaining the function of the balance mechanism for robotic arms. As long as any balance force members 5, 51 , 52 are connected before the roll axis 41 in the kinematic chain as shown in figure 5 and the arm and balance levers rotate relative to the horizontal plane, the balance torques and gravitational loads will scale proportionally to each other. In this example, the balance force members 5, 51, 52 should be coupled to their respective balance arms B1 , B2, B3 via universal joints to accommodate the roll axis motion.

Figure 6 illustrates an alternative compact configuration where the balance force member 5 is attached to the upper arm A1 section and is free to slide, preferably on a linear bearing slider joint 6, so the balance force member 5 moves with the upper arm A1 and can be contained within it. In this particular example, both shoulder pivot 7 and first balance pivot 1 are fixed with respect to the ground 11. The first balance arm B1 is also fixed with respect to the ground 11 and remains vertical. Furthermore, the lengths LB1 , LB2, LBN of all balance arms B1 , B2, B2 are fixed. In other examples, any combinations of fixed and non-fixed pivots, arms and balance arm lengths may be used.

The load arms A1 , A2 each form parts of a parallelogram comprising link elements E1 ... E4 (first parallelogram: B1, A1 , E1, E2; second parallelogram: E1, A2, E3, E4) and it can be seen that the vertical first (elbow) and third (wrist) link elements E1, E3 (where the first link element E1 is connected between the elbow joint 2 and co-joint 2’ and the third link element E3 is connected between the wrist joint 8 and co-joint 8’) remain vertical for all configurations of the shoulder, elbow and wrist axes.

The vertical parallelogram link elements E1, E3 are spanned by 2 pairs of gears 16, 17; 18, 19. A first elbow gear 16 at the elbow joint 2 is rigidly fixed to the forearm A2, a second elbow gear 17 in the elbow pair 16, 17 operates as a planetary gear and is coupled to an elbow timing pulley or sprocket 22 and a first belt or chain 31 transmits the motion to the forearm balance lever B2.

It can be seen that the relative motion of the elbow planetary gears 16, 17 cancels the relative motion of the balance force member 5 with respect to the ground plane 11. Thus, the rotation of the forearm balance lever B2 with respect to the balance force member 5 is kept in phase with the angle of the forearm A2 with respect to the ground plane 11.

In the three-axis system as shown, balance for the (optional) hand or gripper payload 9 is accomplished via a wrist gear 18 which is rigidly fixed to the hand and coupled to its planetary gear pair 19, its motion being coupled to the hand balance lever B3 in a similar fashion to the coupling of the forearm A2 to its balance lever B2, via corresponding wrist pulley 28, elbow pulley 22 and additional belts or chains 32, 33. In figure 6, three balance gears 15 mounted on second balance lever B2 transmit motion to the hand balance lever B3.

This configuration is also extensible to ‘N’ rotational joints on the vertical plane.

In a simpler two-axis system, this configuration does not require the wrist gears 18, 19, the transmission belts 32, 33 or the links E3, E4. The wrist balance lever B3 would also not be required, with the force balance member 5 instead being coupled to the forearm balance lever B2. Preferred embodiments

At least two arm sections (A1 , A2) constrained in series such that one can rotate relative to the other, the weight of these arm sections being supported by the following mechanism:

• a lever (B2) attached to the upper link, free to rotate with respect to the upper arm link but constrained to it with a revolute joint (1). A force producing device (5) actuates this lever;

• said lever actuating force producing device can move freely such that its force vector is always directly below the distal end of the lever, however the force producing device is constrained by two links (6) such that its height relative to the floor is constant and the force producing device has somewhere to push against;

• said lever is rigidly attached to a pulley (21) which actuates a timing belt or chain (31), said timing belt transmits torque from the lever, through the pulley (21) to a second pulley (22) connected to the second link in the kinematic chain;

• thus the pulleys and belt transmit torque from the lever to the fore arm link (A2), said torque, from the force producing device onto the lever applies a torque onto the elbow joint (2) that balances some fraction of the torque that arises from the weights of the various appendages that are on the distal end of the elbow joint such that that some or all of the load on the elbow joint due to masses attached to the elbow joint is mitigated;

• the distance (LB1) between the base of the lever where it attaches to the upper arm, and the shoulder joint is non zero, so that the force producing device pushes against the upper arm link at some non zero radius from the shoulder joint thus applying a torque to the shoulder joint;

• furthermore, due to the angular coupling between the fore arm (A2) and the lever (B2) the sinusoidal changes in torque on the elbow joint (2) due to the distance between the centre of mass of the appendages attached on the distal side of the elbow joint, is directly proportional to the sinusoidal changes in torque that the force producing device (5) imparts on the lever, as the lever is constrained by the timing belt (31) to be at the same angle with respect to the force producing device as the angle the fore arm is at with respect to the gravitational field vector;

• additionally the change in torque on the shoulder joint (7) due to differing orientations of the fore arm joint angle (B°) producing different torques as the radius between the centre of mass of the appendages attached on the distal side of the elbow joint change with the orientation of the elbow joint (2), is proportional to the torque from the force producing device (5) into the shoulder joint as the distance between the end of the lever and the shoulder joint changes in concurrence with the distance between the fore arm centre of mass and the shoulder joint;

• the difference in torque on the shoulder joint (7) due to changes in the angle of the shoulder link with respect to the gravitational field vector are proportional to the changes in torque of the force producing device (5) into the shoulder joint, as the force producing devices force vector is always aligned with the gravitational field vector as the force producing device is constrained to point in this direction;

Preferably wherein: the distance between the shoulder joint (7) and the distal lever joint (1) can be varied; and/or the base of the force producing device is fixed and the angle of its force vector changes to match the position of the lever instead of keeping the vector direction constant and changing its position.

Preferably wherein: a hand link is added via a revolute joint (8) to the distal end of the fore arm link, a pulley (28) is rigidly connected to the distal end of the hand link on which a fore arm timing belt (32) is placed, said timing belt travels along the fore arm onto an idler pulley (29), the axis of which is coincident with the elbow joint (2) such that the length of fore arm timing belt stays constant as the elbow joint moves, rigidly connected to the elbow joint idler pulley is a second pulley with an axis coincident to the first, this second pulley transmits the motion of the wrist joint along another timing belt (33) that travels along the upper arm link to a lever pulley (24), whose axis is coincident with the proximal axle of the original lever (B2) described above;

• the lever pulley (24) is free to rotate about the proximal lever axis (1), the lever pulley is rigidly coupled to a pulley which uses a belt (34) to transmit the torque from the wrist joint (8) along the original pulley to another pulley (23) at the distal end of the original lever (B2). Rigidly connected to the pulley at the distal end of the original lever is a second lever (B3), the arrangement of pulleys and belts previously described transmit torque from the wrist joint to this second lever and torque from the second lever to the wrist joint;

• the transmission just described also act to hold the second lever (B3) parallel or at a fixed angular offset to the wrist link;

• the force producing device (5) now attaches to the distal end of this second lever (B3). Preferably where the upper (A1) and fore arm (A2) links form two 4 bar linkages;

• the force producing device (5) in this version is now parallel to the upper arm, the force producing device is free to slide on a linear rail (6) that is fixed perpendicular to the longitudinal axis of the upper arm such that the force producing device can move in a direction perpendicular to the upper arms longitudinal axis but cannot move in the same direction as the upper arm’s longitudinal axis so that the force producing device can produce forces that push against the upper arm and the force producing device’s force vector stays parallel to the upper arm regardless of the angle of the shoulder joint (7);

• the upper arm link (A1) has additional links (B1 , E1) pivotally jointed to the upper arm link at the shoulder joint and elbow joint (2), a third link (E2) is pivotally jointed to these additional links to form a 4 bar linkage where the upper arm link described above is now the bottom link;

• the fore arm (A2) link has an additional link (E3) pivotally jointed to the fore arm link at the wrist joint (8), a third link (E4) is pivotally jointed to this additional wrist joint link going between the distal end of the additional wrist joint link (E3) and the distal end of the additional link (E1) attached to the elbow joint to form a second 4 bar linkage where the fore arm link described above is now the bottom link;

• the hand link is pivotally attached to the additional wrist joint link (E3) and is rigidly connected to a planetary gear (18), a second planetary gear (19) whose axis is coincident with and rigidly connected to the pulley (28) at the distal end of the fore arm link (E4) such that these planetary gears transmits the torque of the wrist joint to the secondary timing belt system (33), and through the secondary timing belt system the torque from the wrist joint is transmitted to the secondary lever (B2);

• the pulley (22) that is rigidly connected to the fore arm link that is described above is no longer rigidly connected to that axle but is free to rotate, the aforementioned pulley is rigidly connected to a planetary gear (17) which meshes with another planetary gear (16) rigidly connected to the top link of the distal four bar linkage, so that torques transmitted along the timing belt described above now go through two planetary gears and transmit the torque from the force producing device to the second four bar linkage and balance the weight of this second linkage and connected appendages.

Where there is a single balance force member, the balance lever lengths are in the same ratio as the respective joint torques (figs. 1 , 2, 4-6). Where there are multiple balance members, the balance lever lengths are in the same ratio as their respective arm lengths (fig. 3). Other variants

In some embodiments, the balance force member 5 comprises a low friction pneumatic cylinder, but in other embodiments other force members such as hydraulic cylinders, a pneumatic or hydraulic piston, or a gas strut, servo systems or other electric or electromechanical members, springs or other resilient members may be used singly, separately or in any combination.

In some embodiments, adjustment of the balance force to accommodate a change in load (torque), e.g. arising from an optional payload 9, is preferably by a fluid (gas or liquid) pressure regulator 10, but may be by any other suitable means. This adjustment can be achieved be by changing the pressure in the balance cylinder 5.

In some embodiments, at least one of the load arms A1, A2...AN has an adjustable length LA1, LA2... LAN.

In some embodiments, at least one of the balance arms B1, B2... BN has an adjustable length LB1 , LB2... LBN.

In some embodiments, at least one of the arms A1 , A2...AN; B1 , B2... BN comprises multiple parts connected together.

In some embodiments, one or more of the pivots, preferably the shoulder pivot 7 and/or the first balance pivot 1, is/are translationally-fixed.

In some embodiments, a transmission 20 is configured to transfer the forces and/or torques of one or more of the load arms (A1 , A2...AN) to one or more of the balance arms (B1, B2,... BN), preferably by linking the relative angles of the load and balance arms. In other embodiments, the timing belts or chains with their associated pulleys or sprockets can be replaced by a hydraulic or pneumatic rotary vane actuator pairs, which will couple the rotations of the load arms to their respective balance arms, synchronising the rotations and transmitting the required balance force.

Preferably, the transmission (20) synchronously links: the relative angles (B°, B’°) of the second load arm (A2) with respect to the first load arm (A1) and the second balance arm (B2) with respect to the first load arm (A1) or the first balance arm (B1), preferably via one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s); and/or the relative angles (C°, C’°) of the payload (9) or third load arm (A3) with respect to the second load arm (A1) and the third balance arm (B3) with respect to the second balance arm (B2), preferably via one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s).

In some embodiments, the synchronous connections and optional transmission 20 may generally comprise any one or more of the following: one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s)and/or electronic equivalents. The transmission preferably provides a synchronous connection to link the relative angles of the load and/or balance arms.

In some embodiments, one or more of the pivot(s) connecting the balance and/or load arms is/are or comprise(s) a pulley, gear or sprocket, and/or a revolute, pin, hinge or universal joint.

The various ‘arms’ and ‘levers’ (terms used interchangeably) may be described as having one or more ends, to clarify how these elements are connected. The numeric labelling (first, second) of these ends is arbitrary, to aid understanding, particularly with reference to the figures, which best illustrate particular embodiments. Other terminology such as ‘proximal’ and ‘distal’ may be used, with respect to the other elements described and/or shown in the figures.

The present disclosure further contemplates corresponding methods of balancing load arms in accordance with the above, as well as kits of parts comprising the various elements used (which may or may not include the load mechanism elements to be balanced by the balance mechanism) and computer-readable instructions that perform the method.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

Protection may also be sought for any features disclosed in any one or more published documents incorporated by reference in combination with the present disclosure. Representative features

1. A balance mechanism for balancing at least first and second load arms (A1, A2...AN) having respective lengths LA1 , LA2... LAN, the first and second load arms (A1, A2) being connected in series and pivotable with respect to one another at an elbow pivot joint (2), comprising: at least first and second balance arms (B1 , B2... BN) that are pivotable with respect to one another at a balance pivot joint (1), wherein at least the first balance arm (B1) is connected to the first load arm (A1) at a shoulder pivot joint (7) in use; a transmission (20) configured to transfer the forces and/or torques of one or more of the load arms (A1, A2...AN) to one or more of the balance arms (B1, B2,... BN); and a balance force member (5, 51, 52) pivotally connected to at least one of the balance arms (B1, B2, BN) in use and configured to exert a balancing force thereon in use, wherein, in use, the force produced by the balance force member(s) (5, 51, 52) induces a maximum torque on each balance arm pivot joint (7, 1, 3) that substantially matches the maximum torque induced by gravity at the corresponding load arm pivot joint (7, 2, 8).

2. A balance mechanism for balancing at least first and second load arms (A1, A2...AN) having respective lengths LA1 , LA2... LAN, the first and second load arms (A1, A2) being connected in series and pivotable with respect to one another at an elbow pivot (2), the first load arm (A1) being pivotally connected to a shoulder pivot joint (7) in use, comprising: at least one balance arm (B2), the balance arm (B2) having a length LB2 and being pivotally connected at a first end thereof to the first load arm (A1) at a balance pivot joint (1) in use, the balance pivot joint (1) being at a distance LB1 along the first load arm (A1) from the shoulder pivot joint (7); a transmission (20) configured to transfer the forces and/or torques of one or more of the load arms (A1, A2...AN) to one or more of the balance arms (B1 , B2,... BN); and a balance force member (5) pivotally connected at a first end thereof to a second end of the balance arm (B2) and pivotally connected at a second end thereof to a balance force member joint (6) in use and configured to exert a balancing force on the balance arm (B2) in use, wherein, in use, the force produced by the balance force member (5) induces a maximum torque on each balance arm pivot joint (7, 1) that substantially matches the maximum torque induced by gravity at the corresponding load arm pivot joint (7, 2) 3. The balance mechanism of clause 2, comprising a first balance arm (B1) having length LB1 connected to the shoulder pivot (7) at a first end of the first balance arm (B1) and connected at a second end to the second balance arm (B2) via the balance pivot (1).

4. The balance mechanism of any preceding clause, wherein: a ratio of the lengths (LB1, LB2... LBN) of the balance arms (B1, B2,... BN) is configured to be substantially the same as either a ratio of the lengths (LA1, LA2... LAN) of the load arms (A1, A2...AN) or a ratio of the maximum torques induced by gravity at the respective corresponding load arm joints (7, 2, 8).

5. The balance mechanism of any preceding clause, wherein: at least one of the load arms (A1, A2...AN) has an adjustable length LA1 , LA2... LAN; and/or at least one of the load arms (A1 , A2...AN) has an adjustable mass; and/or at least one of the balance arms (B1 , B2... BN) has an adjustable length LB1 , LB2... LBN; and/or at least one of the balance arms (B1, B2... BN) has an adjustable mass.

6. The balance mechanism of any preceding clause, comprising a first balance force member (5, 51, 52) pivotally connected to the first balance arm (B1) in use and configured to exert a balancing force thereon in use; and further comprising: a second balance force member (5, 51 , 52) pivotally connected to the second balance arm (B2) in use and configured to exert a balancing force thereon in use; and optionally a third balance force member (5, 51 , 52) pivotally connected to the third balance arm (B3) in use and configured to exert a balancing force thereon in use; and optionally a further balance force member pivotally connected to the Nth balance arm (BN) in use and configured to exert a balancing force thereon in use.

7. The balance mechanism of any preceding clause, further comprising the first and second load arms (A1, A2) that are connected in series and pivotable with respect to one another at the elbow pivot (2); and optionally a payload (9) or a third load arm (A3) pivotally connected to the second load arm (A2) via a wrist pivot (8), preferably further comprising a third balance arm (B3) pivotally connected to the first and/or second balance arms (B1, B2), preferably via a second and/or third balance pivot (3, 4), for balancing the payload (9) or third load arm (A3).

8. The balance mechanism of any preceding clause, configured to adjust: the relative angles of one or more of the balance arms (B1 , B2... BN); and/or the lengths LB1 , LB2, LBN of one or more of the balance arms (B1, B2... BN); and/or the balance pressure in one or more of the balance force members (5, 51, 52), to balance the forces and/or torques of the load arms (A1 , A2... AN) and/or payload (9) or third load arm (A3).

9. The balance mechanism of any preceding clause, wherein: the balance force member(s) (5, 51 , 52) is/are translatable and/or rotatable with respect to the ground, preferably by the balance force member joint(s) (6, 53, 54) being either linearly translatable with respect to the ground only along a single axis or translationally fixed but rotatable; and/or the balance force member(s) (5, 51 , 52) is/are or comprise(s) a spring, a piston, preferably a pneumatic or hydraulic piston, or a gas strut.

10. The balance mechanism of any preceding clause, wherein one or more of the pivots, preferably the shoulder pivot (7) and/or the first balance pivot (1), is/are translationally-fixed.

11. The balance mechanism of any preceding clause, wherein the first balance arm (B1) is rigidly connected to the first load arm (A1) via the shoulder pivot (7) in use, preferably such that an angle between the first balance arm (B1) and the first load arm (A1) is 180°.

12. The balance mechanism of any preceding clause, wherein the transmission (20) synchronously links: the relative angles (B°, B’°) of the second load arm (A2) with respect to the first load arm (A1) and the second balance arm (B2) with respect to the first load arm (A1) or the first balance arm (B1), preferably via one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s); and/or the relative angles (C°, C’°) of the payload (9) or third load arm (A3) with respect to the second load arm (A1) and the third balance arm (B3) with respect to the second balance arm (B2), preferably via one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s).

13. The balance mechanism of any preceding clause, wherein the system is configured such that, preferably to adjust such that: an angle (A°) of the first load arm (A1) with respect to vertical equals or supplements an angle (A’°) of the first balance arm (B1) with respect to vertical; and/or an angle (B°) of the second load arm (A2) with respect to the first load arm (A1) equals or supplements an angle (B’°) of the second balance arm (B2) with respect to the first load arm (A1) or the first balance arm (B1).

14. The balance mechanism of any preceding clause, wherein the system is configured such that, preferably to adjust such that: a zenith angle (A°) of the first load arm (A1) from vertical supplements and inverts a zenith angle (A’°) of the first balance arm (B1) from vertical; and/or an angle (B°) of the second load arm (A2) from the first load arm (A1) equals an angle (B’°) of the second balance arm (B2) from the first balance arm (B1).

15. The balance mechanism of any of clauses 1 to 13, wherein the system is configured such that, preferably to adjust such that: a zenith angle (A°) of the first load arm (A1) from vertical equals a zenith angle (A’°) of the first balance arm (B1) from vertical; and/or an angle (B°) of the second load arm (A2) from the first load arm (A1) equals an angle (B’°) of the second balance arm (B2) from the first load arm (A1) or the first balance arm (B1).

16. The balance mechanism of clause 13, 14 or 15, wherein the system is further configured such that, preferably to adjust such that: an angle (C°) of the payload (9) or third load arm (A3) with respect to the second load arm (A2) equals or supplements an angle (C’°) of the third balance arm (B3) with respect to the second balance arm (B2); and/or an angle (N°) of the Nth load arm (AN) with respect to the N-1 load arm (AN-1) equals or supplements an angle (N’°) of the Nth balance arm (BN) with respect to the N-1 balance arm (AN-1).

17. The balance mechanism of any preceding clause, wherein the shoulder pivot (7) and/or the balance pivot (1) and/or the second balance arm (B2) and/or the balance force member joint (6) is or comprise(s) a roll pivot joint (41 , 42, 43), preferably a universal joint, providing a roll rotation axis at the shoulder pivot (7) and/or the balance pivot (1) and/or the second balance arm (B2) and/or the balance force member joint (6), respectively.

18. The balance mechanism of any preceding clause, wherein: the shoulder joint (7) comprises a shoulder roll pivot joint (41); the balance force member (5) is connected to the balance arm (B2) with a roll pivot joint (43); and the balance force member joint (6) comprises a roll pivot joint (42).

19. The balance mechanism of any preceding clause, wherein: in use, the first load arm (A1) is rigidly connected to the first balance arm (B1) via the shoulder pivot (7), which is translationally-fixed; the second balance arm (B2) is pivotally connected at a first end thereof to the first balance arm (B1) in series, via a first balance pivot (1); the second or a subsequent balance arm (B2... BN) is pivotally connected at a second end thereof to a first end of the balance force member (5) via a balance pivot (3, 4); and the balance force member (5) is connected at a second end thereof to a balance force member joint (6) that is linearly translatable along one axis, wherein the transmission (20) is configured such that or to adjust such that: an angle (B°) of the second load arm (A2) with respect to the first load arm (A1) equals an angle (B’°) of the second balance arm (B2) with respect to the first balance arm (B1).

20. The balance mechanism of clause 19, further comprising: a payload (9) or third load arm (A3) pivotally connected to the second load arm (A2) at a wrist pivot (8); and a third balance arm (B3) pivotally connected between the second balance arm (B2) and the balance force member (5, 51 , 52), via second and third balance pivots (3, 4), wherein the transmission (20) is further configured such that or to adjust such that: an angle (C°) of the payload (9) or third load arm (A3) with respect to the second load arm (A2) equals an angle (C’°) of the third balance arm (B3) with respect to the second balance arm (B2).

21. The balance mechanism of clause 2 or any clause dependent thereon, wherein: the shoulder pivot (7) is rotatable but translationally-fixed with respect to the ground; the balance force member joint (6) is rotatable but translationally fixed with respect to the ground; and the transmission (20) is configured such that or to adjust such that: an angle (B°) of the second load arm (A2) with respect to the first load arm (A1) equals an angle (B’°) of the second balance arm (B2) with respect to the first load arm (A1) or the first balance arm (B1) .

22. The balance mechanism of any preceding clause, wherein the transmission (20) is configured to adjust the relative angles of one or more of the balance arms (B1 , B2... BN) and/or the lengths (LB1 , LB2... LBN) of one or more of the balance arms (B1 , B2... BN) to balance the forces and/or torques of one or more of the load arms (A1 , A2...AN) and/or the payload (9), preferably wherein the transmission (20) comprises one or more of the following: timing belt(s) with pulley(s), linkage(s), motor(s), sprocket(s), gear(s) and/or chain(s).

23. The balance mechanism of any preceding clause, wherein: the elbow pivot (2) and the first balance pivot (1) are synchronously connected, preferably by one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s); and/or the wrist pivot (8) and the second balance pivot (3) are synchronously connected, preferably by one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s); and/or the first balance pivot (1) and a second balance pivot (3) are synchronously connected, preferably by one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s); and/or the elbow pivot (2) and the wrist pivot (8) are synchronously connected, preferably by one or more timing belt(s), pulley(s), linkage(s), motor(s), sprocket(s), gear(s), chain(s), hydraulic or pneumatic actuator(s).

24. The balance mechanism of any preceding clause, wherein any one or more of the pivot(s) (1 , 2, 3, 4, 7, 8) comprise(s) a pulley(s), sprocket(s) or gear(s), and/or a revolute, pin, slider, hinge and/or universal joint.

25. The balance mechanism of any preceding clause, wherein the first balance lever (B1) extends away from the shoulder pivot 7 such that an angle (A°) of the first load arm (A1) with respect to vertical equals or supplements an angle (A’°) of the first balance arm (B1) with respect to vertical; and/or the transmission (20) comprising a pulley, sprocket, or gear (21) at the first balance pivot (1) and a pulley, sprocket, or gear (22) at the elbow pivot (2), the pulleys, sprockets or gears (21 , 22) being connected via a belt or chain (31) or additional gears, such that an angle (B°) of the second load arm (A2) with respect to the first load arm (A1) equals or supplements an angle (B’°) of the second balance arm (B2) with respect to the first load arm (A1) or the first balance arm (B1); and/or further comprising additional pulleys, sprockets, or gears (23, 24, 28, 29) at one or more of the balance pivots (1, 3) and load pivots (2, 8), the additional pulleys, sprockets, or gears (23, 24, 28, 29) being connected via additional belts, chains or gears (32, 33, 34) such that an angle (C°) of the payload (9) or third load arm (A3) with respect to the second load arm (A2) equals or supplements an angle (C’°) of the third balance arm (B3) with respect to the second balance arm (B2). 26. The balance mechanism of any preceding clause, wherein: in use, the first load arm (A1) is pivotally connected to the first balance arm (B1) at a first end of the first balance arm (B1) via the shoulder pivot (7); the first balance arm (B1) is pivotally connected at a second end thereof to a first end of the second balance arm (B2) via a first balance pivot (1); the balance force member (5) is pivotally connected at a first end thereof to at least one of the balance arms (B1 , B2..BN) via a further balance pivot (3, 4) and in use slidably connected at its second end to the first load arm (A1) via a slider joint (6); and in use, the elbow pivot (2) is synchronously connected to the first balance pivot (1).

27. The balance mechanism of clause 26, further comprising: a payload (9) or third load arm (A3) pivotally connected to the second load arm (A2) at a wrist pivot (8); and a third balance arm (B3) pivotally connected between the second balance arm (B2) and the balance force member (5), via second and third balance pivots (3, 4), wherein: the elbow pivot (2) and the first, second or third balance pivots (1 , 3, 4) are synchronously connected; and the wrist pivot (8) and the first, second and/or third balance pivots (1, 3, 4) are synchronously connected.

28. The balance mechanism of clause 27, wherein: the elbow pivot (2) and wrist pivot (8) are synchronously connected via an elbow gear pair (16, 17) and a wrist gear pair (18, 19) respectively; and/or the second balance arm (B2) is pivotally and/or synchronously connected to the third balance arm (B3) via one or more gear(s) (13).

29. The balance mechanism of any of clauses 26-28, wherein the balance force member (5) is parallel to the first load arm (A1) and free to slide on a linear rail that is fixed perpendicular to the longitudinal axis of the first load arm (A1).

30. The balance mechanism of any of clauses 26-29, comprising two four-bar linkages, each comprising two vertical link elements.

31. The balance mechanism of clause 30, wherein: the first four-bar linkage comprises the first balance arm (B1), the first load arm (A1), a first elbow link element (E1) and a second link element (E2); and the second four-bar linkage comprises the first elbow link element (E1), the second load arm (A2), a third wrist link element (E3), and a fourth link element (E4).

32. The balance mechanism of any of clauses 26-31 , wherein rotation of the second balance lever (B2) with respect to the balance force member (5) is configured to be in phase with an angle of the first load arm (A2) with respect to the ground.

33. A kit of parts comprising the at least first and second balance arms (B1, B2... BN) and the balance force member(s) (5, 51 , 52) of any preceding clause.

34. A method of balancing at least first and second load arms (A1 , A2...AN) having respective lengths LA1 , LA2... LAN, the first and second load arms (A1, A2) being connected in series and pivotable with respect to one another at an elbow pivot joint (2), comprising: pivotally connecting at least first and second balance arms (B1 , B2... BN) at a balance pivot joint (1); connecting at least the first balance arm (B1) to the first load arm (A1) at a shoulder pivot joint (7); configuring a transmission (20) to transfer the forces and/or torques of one or more of the load arms (A1 , A2...AN) to one or more of the balance arms (B1, B2,... BN); and pivotally connecting a balance force member (5, 51 , 52) to at least one of the balance arms (B1, B2, BN) to exert a balancing force thereon wherein, in use, the force produced by the balance force member(s) (5, 51, 52) induces a maximum torque on each balance arm pivot joint (7, 1 , 3) that substantially matches the maximum torque induced by gravity at the corresponding load arm pivot joint (7, 2, 8).

35. A method of balancing at least first and second load arms (A1 , A2...AN) having respective lengths LA1 , LA2... LAN, the first and second load arms (A1, A2) being connected in series and pivotable with respect to one another at an elbow pivot joint (2), the first load arm (A1) being pivotally connected to a shoulder pivot joint (7) in use, comprising: pivotally connecting at least one balance arm (B2) at a first end thereof to the first load arm (A1) at a balance pivot joint (1), the balance pivot joint (1) being at a distance LB1 along the first load arm (A1) from the shoulder pivot joint (7) and the balance arm (B2) having a length LB2; pivotally connecting a balance force member (5) at a first end thereof to a second end of the balance arm (B2) and pivotally connecting the balance force member (5) at a second end thereof to a balance force member joint (6), the balance force member (5) being configured to exert a balancing force on the balance arm (B2) in use; and configuring a transmission (20) to transfer the forces and/or torques of one or more of the load arms (A1 , A2...AN) to one or more of the balance arms (B1, B2,... BN), wherein, in use, the force produced by the balance force member (5) induces a maximum torque on each balance arm pivot joint (7, 1) that substantially matches the maximum torque induced by gravity at the corresponding load arm pivot joint (7, 2).

36. The method of clause 34 or 35, further comprising adjusting: the relative angles of one or more of the balance arms (B1 , B2... BN); and/or the lengths LA1 , LA2, LAN of one or more of the load arms (A1, A2...AN); and/or the lengths LB1 , LB2, LBN of one or more of the balance arms (B1, B2... BN); and/or at least one of the masses of the balance arms (B1, B2... BN) and/or load arms (A1 , A2...AN); and/or the balance pressure in one or more of the balance force members (5, 51, 52), to balance the forces and/or torques of the load arms (A1 , A2...AN).

37. A computer-readable medium comprising instructions that, when executed, perform the method of clauses 34-36.

38. A balance mechanism substantially as shown in any of figures 1-6.

Index to reference numerals