Field

The present invention relates to a method and a system related to a mechanical drive apparatus. More particularly, the invention relates to a Geneva drive system configured to intermittently rotate a payload for facilitating a maytagging operation.

Background

Maytagging refers to a mitigation process used to remove offset of an inertial sensor by flipping the measurement axis of the inertial sensor by ±180 degrees. Difference of the measurements in the two different (opposite) directions gives twice the actual sensed signal and enables nulling the offset.

Maytagging may be used in the application of gyrocompassing, which refers to finding of true north by means of gyroscopes measuring in-plane components of the angular velocity of the Earth. Maytagging in gyrocompassing applications is also referred to as two-point gyrocompassing. North finding based on micromachined gyroscopes, such as microelectromechanical (MEMS) gyroscopes is an attractive possibility with numerous applications.

Description of the related art

Flipping of measurement axis of an inertial sensor by 180 degrees may be implemented by a stepper motor or a servo motor. However, stepper and servo motors and their supporting control electronics are costly and require high electrical power to operate that could lead to elevated temperatures inside a closure containing one or more gyroscopes. Elevated temperatures may degrade the performance of inertial sensors, for example by increasing required stabilization time after power-on. Summary

An object is to provide an apparatus so as to solve the problem of intermittently flipping an inertial sensor, such as a gyrocompass, preferably by ±180 degrees. The objects of the present invention are achieved with a mechanical drive apparatus according to the claim 1 and with an inertial sensor apparatus according to claim 6.

The preferred embodiments of the invention are disclosed in the dependent claims.

The present invention is based on the idea of utilizing a Geneva drive mechanism for flipping a payload comprising the inertial sensor preferably by ±180 degrees. A Geneva drive is also referred to in the literature as a Maltese cross. The invented mechanical drive apparatus applies the operation principle of a Geneva drive to achieve the wanted payload rotation. Rotating motion of the payload is intermittent meaning that the payload is stationary between two consecutive rotations which occur alternately in opposite directions. The operation principle is easily adjustable for other rotation angles, for example any selected angle in the range between 160 to 200 degrees.

According to a first aspect, a drive mechanism is provided. The mechanism comprises two or four circular drive wheels rotationally coupled to each other. Two of the rotating drive wheels have a drive pin. The mechanical drive mechanism comprises a drive motor coupled to one of the drive wheels. The drive motor is configured to continuously rotate the respective drive wheel, and consequently all other drive wheels. The drive mechanism comprises a driven wheel having two outwardly opening radial slots arranged symmetrically on opposite sides of the driven wheel. The driven wheel is configured to be driven into an intermittent rotation motion about its central axis by drive pins of the two rotating drive wheels alternately engaging with one of the radial slots. Direction of the intermittent rotation motion alternates between two consecutive rotation phases. The mechanism comprises a first gearing wheel uniaxially coupled to the driven wheel and a second gearing wheel rotationally coupled to the first gearing wheel. A payload is uniaxially coupled to the second gearing wheel. The second gearing wheel and the payload are configured to be rotated in the range of 160 to 200 degrees, preferably in the range of 170 to 190 degrees, most preferably by 180 degrees in a first direction during engagement of a first drive pin of a first drive wheel with one of the two outwardly opening radial slots. The second gearing wheel and the payload are configured to be rotated in the range of 160 to 200 degrees, preferably in the range of 170 to 190 degrees, most preferably by 180 degrees in a second direction opposite to the first direction during engagement of a second drive pin of a second drive wheel with the other one of the two outwardly opening radial slots.

According to a second aspect, the driven wheel, the first gearing wheel, the second gearing wheel and the payload are stationary between intermittent rotations.

According to a third aspect, the first drive wheel and the second drive wheel each comprise a blocking disc configured to disable rotation motion of the driven wheel between intermittent rotations.

According to a fourth aspect, the blocking disc comprises a convex portion configured to engage with a respective concave portion of the driven wheel between said intermittent rotations for temporarily disabling rotation motion of the driven wheel.

According to a fifth aspect, gearing ratio of the first and second gearing wheels is 2: 1 and wherein duty cycle of the drive mechanism is 50%.

According to another aspect, an inertial sensor apparatus is provided that comprises the drive mechanism according to any one of the above aspects. The drive mechanism is configured to rotate the payload by 180 degrees during each rotation period for maytagging at least one inertial sensor comprised in the payload. According to a further aspect, the payload comprises a MEMS gyroscope and/or a MEMS accelerometer.

According to some aspects, the inertial sensor apparatus is a MEMS gyrocompass.

The present invention has the advantage that slip rings for power and communication are not needed. The switching arrangement can be made compact, such that it needs only about 2 to 3 times the lateral area needed for the payload. The design of the invented mechanism can be easily adjusted to implement rotation angles differing from the preferred angle of 180 degrees typically used for maytagging purposes.

Brief description of the drawings

In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached drawings, in which

Figure 1 illustrates drive wheels according to a first embodiment.

Figure 2 illustrates drive wheels according to a second embodiment.

Figure 3 illustrates drive wheels and a driven wheel.

Figure 4 illustrates a side view cross section of drive wheels and a driven wheels according to the second embodiment.

Figure 5 illustrates the driven wheel and gearing wheels.

Figure 6 illustrated the driven wheel, gearing wheels and a payload.

Figures 7a to 7f illustrate operation of the Geneva drive mechanism.

Detailed description

Unless otherwise stated, in the following figures, the Geneva drive mechanism is shown as a projection in which a plane determined by circular disc-formed drive wheels (10) is parallel with the drawing sheet (xy-plane) and direction towards the reader (z-axis) is referred to with terms "up", "upwards", "above" and like and direction away from the reader is "down" and "downwards", "below" and like. This projection used for illustration purposes should not considered limiting position of a physical device.

The Figure 1 illustrates drive wheels according to a first embodiment as seen from above, i.e. in direction of rotation axes of the drive wheels. According to the first embodiment, there are four circular rotating drive wheels (10) rotationally coupled to each other and thus rotating in synchronized manner. Drive wheels are preferably formed as essentially flat discs with non-zero thickness and all drive wheels are preferably disposed mutually on the same horizontal plane. Each drive wheel is essentially circular and rotates about its own central axis. A first drive wheel (10-A) and a second drive wheel (10-B) both have a drive pin (11- A; 11-B) fixedly attached to the drive wheel at a predetermined distance from the central axis (15) of the respective drive wheel (10-A; 10-B). The first and second drive wheels (10-A, 10-B) have equal radiuses. Two smaller drive wheels (10-C, 10-D) in this embodiment convey rotation between the first and second drive wheels (10-A, 10-B) while enabling maintaining a distance between outer circumferences of the first and second drive wheels. As a result, the first and second drive wheels (10-A, 10-B) rotate in mutually opposite directions as illustrated by the curved arrows. Radiuses of the two smaller drive wheels (10-C, 10-D) are preferably mutually equal and smaller than radius of the first and second drive wheels (10-A, 10-B). Drive pins (11-A; 11-B) are disposed on one side of the respective drive wheel and extending preferably upwards, orthogonally away from the plane formed by upper surfaces of the drive wheels (10).

Both the first and second drive wheel (10-A, 10-B) further preferably comprises a blocking disc (12-A; 12-B) disposed on one face of the respective drive wheel. The blocking disc (12-A; 12-B) extends upwards from the plane formed by upper surfaces of the drive wheels, in other words it extends in the same direction as the respective drive pin (10).

The Figure 2 illustrates drive wheels according to a second embodiment. In this embodiment, there are only two drive wheels (10-A, 10-B). The first drive wheel (10-A) and second drive wheel (10-B) are directly and rotationally coupled to each other, thus configured to rotate in mutually opposite directions. The first and second drive wheels (10-A, 10-B) have equal radiuses. This configuration enables a simpler drive wheel construction but leaves no space on the direct line between the central axes (15) of the two drive wheels for additional functional parts. As in the first embodiment, both the first and second drive wheel (10-A, 10-B) preferably comprise blocking discs (12-A, 12-B).

For causing all drive wheels (10) to rotate continuously, it is sufficient that one of the drive wheels (10) is rotated. Rotation is preferably implemented by a continuously rotating simple electric motor that is coupled to one of the drive wheels to provide motion.

The Figure 3 illustrates drive wheels (10) and a driven wheel (20). According to the first embodiment, the central axis (25) of the driven wheel (20) is placed on the direct line between central axes (15) of the first and second drive wheels (10- A, 10-B). The driven wheel (20) is disposed in a plane that is parallel to the plane determined by the drive wheels and preferably coplanar with the blocking discs (12-A, 12-B). In this example, the driven wheel (20) is disposed above the upper surfaces of the drive wheels (10) (towards the viewer) such that the drive pins (11) can engage with the driven wheel (20) for intermittently rotating it.

The driven wheel (20) comprises two outwardly opening radial slots (21) arranged symmetrically on two opposite sides of the driven wheel (20). The driven wheel (20) is configured to be driven about its central axis (25) alternately and intermittently by drive pins (11-A, 11-B) of the first and second drive wheels (10-A, 10-B). Rotation of the driven wheel (20) occurs while one of the driving pins is engaged with one of the radial slots (21), which causes the driven wheel (20) to rotate about its central axis (25) in the opposite direction in comparison to the respective drive wheel (10) that is currently engaged with the driven wheel (20) via its driving pin (11). Purpose of the blocking disc (12-A; 12-B) is to enable locking of the driven wheel in a fixed position between intermittent rotating phases, when neither of the two drive pins (11-A, 11-B) is engaged with one of the radial slots (21).

The figure 4 shows a simplified side view cross section of drive wheels (10-A, 10- B) and driven wheel (20) according to the second embodiment. The drawing is not in scale. For enabling disposing central axis (25) of the driven wheel on the direct line between central axes (15) of the drive wheels (10-A, 10-B) in similar manner to the first embodiment, the axis (25) of the driven wheel (20) extends away from the plane of the drive wheels (10-A, 10-B). Driving pins (11-A ,110- B) extend above the drive wheel (10-A, 10-B) in the z-axis direction on the same face as the blocking discs (12-A, 12-B).

The Figure 5 illustrates the driven wheel (20) and gearing wheels (30-A, 30-B). Drive wheels (10) have been omitted from the Figure 5 for clarity. A drive pin (11-A) is however illustrated. Gearing wheels (30) are preferably circular and disc-shaped and disposed in a plane that is parallel with the plane of the drive wheels. In this example, gearing wheels (30) are disposed below the driven wheel (20) and preferably also below the plane of the drive wheels.

Purpose of the gearing wheels is to determine angle of the intermittent rotation motion of the payload. In the preferred embodiment, the gearing wheels cause the preferred 180-degree rotation motion driven by a 90-degree rotation motion of the driven wheel (20).

According to embodiments of the invention, gearing wheels (30) comprise a first gearing wheel (30-A) which is uniaxial with the driven wheel (20). A second gearing wheel (30-B) is rotationally coupled to the first gearing wheel (30-A). Radius of the second gearing wheel (30-B) is smaller than the radius of the first gearing wheel (30-A). In the shown example, gearing ratio of the gearing wheels is 2: 1. Alternative gearing ratios are applicable for causing rotation motions of the second gearing wheel (30-B) that differ from the exemplary 180-degrees. If the driven wheel rotates an angle other than 90 degrees, the gearing ratio can be used to tune the rotating of the payload to the preferred 180 degrees, or to another rotation angle.

The Figure 6 illustrated the driven wheel (20), gearing wheels (30-A, 30-B) and a payload (40). The payload (40) is fixed to the second gearing wheel (30-B) and/or the rotation axis (35) thereof such that the payload (40) rotates uniaxially with the second gearing wheel (35). The payload (40) is preferably disposed in a plane that is parallel with the gearing wheels as well as the drive wheels and the driven wheel but has a non-zero distance from any of these such that the payload (40) can rotate without colliding with any of the wheels or axis thereof.

Figures 7a to 7f illustrate operation of the Geneva drive mechanism over a full operation cycle. The first drive wheel (10-A) and the second drive wheel (10-B) rotate 360 degrees during the operation cycle.

This exemplary operation cycle takes time of 8*dt, in which dt refers to any selected time unit. The payload (40) is stationary for 2*2*dt = 4*dt, while its rotation operation to each direction takes 2*dt. Thus, duty cycle of this exemplary embodiment is 50%. Duty cycle can be increased by increasing the gearing ratio and/or moving the drive pin (11-A, 11-B) location closer to the respective rotation axis (15) of the rotating drive wheels (10-A, 10-B). In this example, rotation operation can also be referred to as a flipping operation, since the payload (40) is flipped about the rotation axis (35) by 180 degrees.

The Figure 7a illustrates a first phase at moment t = tO+dt, in which the payload is stationary. At this moment, the drive pin (11-A) of the first drive wheel (10-A) is just about to engage with one of the radial slots (21) of the driven wheel (20), which initiates a rotation phase of the driven wheel (20).

The Figure 7b illustrates a second phase at moment t = t0+2dt. The drive pin (11-A) of the first drive wheel (10-A) rotates the driven wheel (20), driving the first gearing wheel (30-A) to a clockwise rotating motion, which is conveyed to a counterclockwise rotation motion of the second gearing wheel (30-B) that rotates the payload (40). At the shown point of time, the counterclockwise rotation motion of the payload (40) about its rotation axis (35), caused by the drive pin (11-A) of the first drive wheel (10-A) engaging to the radial slot (21) of the drive wheel (20), is half-way through, and this rotation continues as long as the drive pin (11-A) remains engaged with the radial slot (21). A concave portion at the outer circumference of the blocking disc (12-A) of the first drive wheel (10-A) allows undisturbed rotation of the driven wheel (20).

The Figure 7c illustrates a third phase at moment t = t0+3dt. The drive pin (11- A) of the first drive wheel (10-A) is just about to exit the radial slot (21) of the driven wheel (20), which causes the driven wheel (20) to stop. Also gearing wheels (30-A, 30-B) stop, which causes the payload (40) to come to a stop in its current position, which is flipped, in other words rotated 180 degrees from the initial position of the payload (40) shown in the figure 7a. At this phase, a convex portion of the outer circumference of the blocking disc (12-A) of the first drive wheel (10-A) has already engaged with a respective concave portion of the driven wheel (20), and convex portion of the blocking disc (12-B) of the second drive wheel (10-B) is just about to engage with an opposite concave portion of the driven wheel (20). The blocking discs (12-A, 12-B) lock the driven wheel (20) temporarily in a fixed position.

The Figure 7d illustrates a fourth phase at moment t = tO+4dt, in which neither of the drive pins (11-A, 11-B) is engaged with the radial slots (21) of the driven wheel (40). Drive wheels (10) continue their rotating motion, but payload (40) is not rotated. Convex portions of both blocking discs (12-A, 12-B) are engaged with two opposite concave portions of the driven wheel (20) to disable its movement.

The Figure 7e illustrates a fifth phase at moment t = t0+5dt. The stationary period of the payload (40) is about to end, when the drive pin (11-B) of the second drive wheel (10-B) engages with the other radial slot (21) of the drive wheel (20). Also convex portion of the blocking disc (12-A) of the first drive wheel (10-A) disengages from contact with the driven wheel (20), thus enabling rotation thereof.

The figure 7f illustrates a sixth phase at moment t = tO + 6d. This phase is mirrored in comparison to the second phase illustrated in the figure 7b: the effective drive wheel is different and therefore rotation directions of the driven wheel (20), gear wheels (30-A, 30-B) and the payload (40) are reversed from the second phase. The drive pin (11-B) of the second drive wheel (10-B) rotates the driven wheel (20), driving the first gearing wheel (30-A) in a counterclockwise rotating motion, which is conveyed to a clockwise rotation motion the second gearing wheel (30-B) that rotates the payload (40). At the shown point of time, the clockwise rotation motion of the payload (40) about its rotation axis (35), caused by the drive pin (11-B) of the second drive wheel (10-B) engaging to the radial slot (21) of the drive wheel (20), is half-way through, and this rotation continues as long as the drive pin (11-B) remains engaged with the radial slot (21). A concave portion at the outer circumference of the blocking disc (12-B) of the second drive wheel (10-B) allows undisturbed rotation of the driven wheel

(20).

The figure 7e illustrates a seventh phase at moment t = tO + 7d. This phase is mirrored in comparison to the third phase illustrated in the figure 7c. The drive pin (11-B) of the second drive wheel (10-B) is just about to exit the radial slot

(21) of the driven wheel (20), which causes the driven wheel (20) to stop. Also gearing wheels (30-A, 30-B) stop, which causes the payload (40) to come to a stop in its current position, which is flipped, i.e. 180 degrees rotated, back to the initial position of the payload (40) shown in the figure 7a. At this phase, a convex portion of the outer circumference of the blocking disc (12-B) of the second drive wheel (10-B) has already engaged with a respective concave portion of the driven wheel (20), and convex portion of the blocking disc (12-A) of the first drive wheel (10-A) is just about to engage with an opposite concave portion of the driven wheel (20), which locks the driven wheel (20) temporarily in a fixed position, until the operation cycle returns in the first phase (Figure 7a).

Although the first embodiment has been used as an example to explain operation of the Geneve drive mechanisms, a skilled person understands that the same operation principle discussed in connection with figures 3, 5, 6 and 7a to 7g applies equally to the second embodiment.

The payload (40) preferably comprises at least one inertial sensor, such as a MEMS sensor. The MEMS sensor may be for example a gyroscope or an accelerometer. The 180-degree rotation angle according to the disclosed embodiments is particularly suitable for maytagging purposes in a MEMS gyrocompass, in which case the MEMS sensor comprised in the payload (40) is at least one MEMS gyroscope. In maytagging, one or more gyroscopes of the gyrocompass should measure angular velocity about at least two orthogonal axes which are at least approximately parallel with lines extending along the surface of the Earth. The more gyroscopes are provided in the gyrocompass, the better signal to noise ratio can be achieved. Having N gyroscopes in a gyrocompass reduces white noise effect in the true north reading approximately by ratio 1/VN. A 180-degree rotation angle is also useful in a process of compensating offset error of an acceleration sensor.

It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.