DESCRIPTION

Electric motors and generators of the synchronous class require continuous knowledge of the rotor position relative to the stator so that power can be supplied at the optimum timing. Typically rotor position is monitored using a position sensor and in motors to be used on vehicle, and in particular mounted within wheels of vehicles ("in-wheel" motors), the ruggedness and simplicity of the sensor are of paramount concern.

For optimum efficiency the position sensor must provide precise positional information and must do this in situations where electrical noise, temperature variation and mechanical vibration may be substantial. In a wheel mounted system, the sensor must also be very light so as to minimize the unsprung mass of the wheel.

Known systems in the art are based on optical or magnetic or electromagnetic principles and are either heavy, costly, environmentally delicate or a combination of these.

The present applicant has identified the need for an improved position sensor that overcomes or at least alleviates problems associated with the prior art whilst providing an accurate and simple solution.

In accordance with the present invention, there is provided, apparatus for determining the relative position of first and second relatively movable parts of a device, the apparatus comprising: a capacitor assembly comprising a first conductor region (e.g. first conductor plate) provided on the first part and a second conductor region (e.g. second conductor plate), the first and second conductor regions being spaced apart to define opposed sides of a passageway (e.g. notional passageway) extending between the first and second conductor regions, wherein the capacitor assembly is configured to generate a capacitance that varies as the second part moves relative to at least one of the first and second conductor parts; and a sensor for monitoring changes resulting from variation in capacitance of the capacitor assembly as the second part moves relative to at least one of the first and second conductor regions.

In this way, apparatus for determining positioning of the second part relative to the first part (e.g. rotor position or the electrical angle corresponding to the rotor position in a rotary device) is determined by monitoring changes resulting from variation in capacitance of the capacitor assembly during operation (e.g. cyclic operation) of the device. The sensor may be configured to generate an output indicative of a position of the first part relative to the second part.

In one embodiment the device is an electric motor (e.g. electric motor for powering a wheel-driven vehicle). Advantageously, positional feedback from the sensor may be supplied to a control system for determining when to apply voltage, and at what level to apply the voltage, during each phase of the cycle of operation of the electric motor. In addition, positional feedback from the sensor may be used to provide information about the speed of the second part relative to the first part (e.g. for use in determining a speed of an electric vehicle).

In one embodiment, the second conductor region is provided on the second part and is configured to move between first and second positions relative to the first conductor region (e.g. with the first and second conductor regions substantially overlapping (at least along the passageway) as the second conductor regions moves between the first and second positions).

In one embodiment, the first conductor region comprises (at least) first, second, third and fourth discrete (e.g. electrically isolated) conductor sub-regions arranged along the passageway in this order; and the second conductor region has a length along the passageway configured so that in any position along the passageway between the first and second positions a first part of the second conductor region is substantially overlapping (e.g. substantially wholly overlaps) along the passageway with at least one of the first to fourth conductor sub-regions whilst simultaneously a second part of the second conductor region is substantially non-overlapping (e.g. substantially wholly non-overlapping) along the passageway with at least one other of the first to fourth conductor sub-regions (e.g. with different combinations of the first to fourth sub-regions providing the substantially overlapping and/or substantially non-overlapping relationship with the second conductor region as the second conductor region moves between the first and second positions).

In this way, apparatus for determining positioning of the second part relative to the first part is provided in which one conductor sub-region is always fully overlapping with the second conductor region along the passageway (e.g. in a maximum charge state) and one conductor sub-region is always fully non-overlapping with the second conductor region along the passageway (e.g. in a minimum charge state), thereby allowing the apparatus to calibrate the range of state values between these two extremes.

In one embodiment, the sensor is configured to monitor changes resulting from variation in capacitance of the capacitor assembly relative to (e.g. maximum and minimum) measurements taken at the substantially overlapping and substantially non-overlapping conductor sub-regions.

In one embodiment, the second conductor region has a length along the passageway substantially equal to or greater than a minimum spacing along the passageway of opposed outer edges of adjacent pairs of the first to fourth conductor sub-regions.

In one embodiment, the second conductor region has a length along the passageway that is less than a minimum spacing along the passageway between inner edges of the first and fourth conductor sub-regions.

In one embodiment, the second part further comprises a further region adjacent the second conductor region along the passageway.

In one embodiment, the further region is an electrically insulating region.

In one embodiment, the further region is a conductive region adjacent the second conductor region held at a different electrical potential to the second conductor region.

In one embodiment, the second conductor region and/or conductive further region is held at a constant electrical potential.

In one embodiment, the second part comprises a further conductor region spaced from the second conductor region along the passageway by the further region.

In one embodiment, the second conductor region and further region have a combined length along the passageway substantially equal to spacing along the passageway of opposed outer edges of adjacent pairs of the first to fourth conductor sub-regions.

In one embodiment, the sensor is configured to monitor changes resulting from variation in capacitance of each of the conductor sub-regions as the second part moves relative to the first part.

In one embodiment, the capacitor assembly further comprises a third conductor region (e.g. third conductor plate) provided on an opposed side of the second conductor region to the first conductor region and spaced from the second conductor region to define opposed sides of a further passageway extending between the second and third conductor regions.

In one embodiment, the sensor monitors variation of a state (e.g. electrical state) of the capacitor assembly over a period of time.

In one embodiment, the sensor monitors the time it takes for the capacitor assembly to reach a predetermined state. For example, the sensor may monitor the time it takes the capacitor assembly to reach a predetermined level of charge (e.g. by monitoring the time it takes for the capacitor assembly to reach a predetermined voltage when supplied by a constant current source). In one embodiment, the sensor comprises a timer (e.g. digital counter) for measuring the time taken to reach the predetermined state.

In another embodiment, the sensor monitors the change in a state of the capacitor assembly over a predetermined time period (e.g. by monitoring the voltage change achieved across the capacitor assembly during a predetermined period of constant current charging).

In one embodiment, the sensor comprises an analogue sensor for measuring the change in state of the capacitor assembly.

In one embodiment, the second part is rotatable relative to the first part and the apparatus determines a relative rotational position of the first and second relatively movable parts.

In one embodiment, the second part is linearly movable relative to the first part and the apparatus determines a relative linear position of the first and second relatively movable parts.

In one embodiment, the second part is the moveable part of the apparatus (e.g. rotor part of a motor).

In one embodiment, the first part is a stationary part of the apparatus (e.g. a stator part of a motor). Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic view of apparatus in accordance with a first embodiment of the present invention;

Figure 2 is a schematic illustration of measurement circuit for use in the sensor of the apparatus of Figure 1; and

Figure 3 is a schematic view of apparatus in accordance with a second embodiment of the present invention.

Figure 1 shows a position encoder 10 (for the sake of simplicity a linear encoder is shown but the position encoder may equally be a rotary encoder) for determining the relative position of first and second relatively movable parts 20, 30 of a device (e.g. fixed and moving parts respectively of an electric motor or generator), the position encoder 10 comprising: a capacitor assembly 40 comprising a first conductor region 50 provided on the first part 20 and a second conductor region 60 provided on the second part 30; and a sensor 70.

The first and second conductor regions 50, 60 are spaced apart (e.g. by approximately 1mm or potentially an even a larger spacing) to define opposed sides of a passageway 80 extending between the first and second conductor regions.

The first conductor region 50 comprises first, second, third and fourth discrete (e.g. electrically isolated) identical conductor sub-region plates 50A, 50B, 50C and 50D respectively arranged along the passageway 80 in this order over a required measurement cycle distance. Each of the first to fourth conductor sub-region plates 50A-D are independently electrically connected to sensor 70.

The second conductor region 60 comprises a conductor plate 62 held at a constant electrical potential and having a length Li along the passageway 80 (e.g. in the direction of relative motion) that is: i) equal to the spacing along the passageway of opposed outer edges of adjacent pairs of the first to fourth conductor sub-region plates 50A-D; and ii) less than the spacing along the passageway between inner edges of the first and fourth conductor sub- region plates 50A, 50D. Adjacent conductor plate 62 along the passageway 80 is a further region 64 (also of length Li) that is either electrically insulating or a conductive region held at a different constant electrical potential to conductor plate 62. Conductor plate 62 and further region 64 have a combined length L ₂ along the passageway that is equal to spacing along the passageway of opposed outer edges of the first and fourth conductor sub-region plates 50A, 50D. The pairings of a conductor plate 62 and a further region 64 of combined length L ₂ are repeated along second part 30 as shown.

As second part 30 moves relative to first part 20, capacitor assembly 40 is configured to generate a capacitance that varies as conductor plate 62 moves relative to first to fourth conductor sub-region plates 50A-D. The geometry of conductor plate 62 is configured so that in any position along the passageway between first and second positions a first part of conductor plate 62 is substantially wholly overlapping along the passageway 80 with at least one of the first to fourth conductor sub-region plates 50A-D whilst simultaneously a second part of conductor plate 62 is substantially wholly non-overlapping along the passageway 80 with at least one other of the first to fourth conductor sub-region plates 50A-D, with different combinations of the first to fourth sub-region plates 50A-D providing the substantially wholly overlapping and/or substantially wholly non-overlapping relationship with conductor plate 62 as conductor plate 62 moves between the first and second positions. In this way, a position encoder is provided in which one conductor sub-region plate is always fully overlapping with conductor plate 62 along the passageway (e.g. in a maximum charge state) and one conductor sub-region plate is always fully non-overlapping with conductor plate 62 along the passageway (e.g. in a minimum charge state).

Sensor 70 is configured to monitor changes resulting from variation in capacitance of capacitor assembly 40 as second part 30 moves relative to first part 20 by measuring voltage at each of first to fourth sub-region plates 50A-D. The conductor sub-regions plates in the maximum and minimum charge states are used to calibrate the measured output of the two remaining conductor sub-region plates which will be varying with position, one rising and the other falling.

As illustrated, first conductor sub-region plate 50A will progressively record an increasing capacitance as the first illustrated conductor plate 62 travels in the direction of the arrow. Simultaneously, third conductor sub-region plate 50C will show a corresponding decrease in capacitance as second illustrated conductor plate 62 moves away from being fully overlapping with the conductor sub-region plate 50C. Accordingly, there will be a linearly increasing and a linearly decreasing value being measured by sensor 70. Whilst this process is occurring, second conductor sub-region plate 50B remains fully non-overlapping with either of the first and second illustrated conductor plates 62 and at its lowest capacitance level, whilst fourth conductor sub-region plate 50D remains at its highest fully overlapping capacitance level. By using the voltage values of second and fourth conductor sub-region plates 50B, 50D to define the voltage scale range, the values for first and third conductor sub-regions can be accurately interpreted in terms of the relative position of the first part 20 relative to the second part 30. Once first illustrated conductor plate 62 reaches the halfway stage, the process then proceeds with first conductor sub-region plate 50A becoming the maximum level reference and third conductor sub-region plate 50C the minimum level reference, with second and fourth conductor sub-region plates 50B, 50D being the rising and falling value plates respectively. Thus any position over the length L ₂ can be determined.

For large distances, for example in large diameter motors where angular position is required for commutation control purposes, the illustrated configuration can be sized to suit the needs of the application. For example, the distance L ₂ can be 360 electrical degrees to represent a commutation cycle or it can be 360 mechanical degrees to represent the full machine cycle. Where L ₂ does not cover all of a motion path, the illustrated configured of conductor sub-region plates may be repeated as many times a required to cover the whole distance.

The position encoder 10 of the present invention advantageously provides a robust and low-cost system for determining position of two relatively movable parts. First to fourth conductor sub-region plates 50A-D and conductor plates 62 may be provided on a PCB/FR4-based substrate. Undesirable effects of mechanical vibrations in the system to be measured are negated or at least significantly reduced by virtue of the maximum and minimum conductor sub-region plates acting as references.

Figure 2 shows a measurement circuit 100 for use in sensor 70. Each of the first to fourth conductor sub-region plates 50A-D is connected to a constant current source 102, a discharge switch 104, a buffer amplifier 106 and a comparator 108. When the measurement cycle begins, discharge switch 104 is opened to allow capacitor assembly 40 to charge. The buffered output is fed to comparator 108 which is set with a predefined threshold voltage level.

At the same time as the cycle is started (by release of the discharge switch 104), a counter 110 is started which continues to count until comparator 108 switches to indicate the end of the charge period. As soon as comparator 108 indicates the end of the count cycle, discharge switch 104 is re-engaged to rest the capacitor charge to zero ready to re- start the measurement/count cycle.

For low capacitance the time will be short and for large capacitance the time will be long. With a high speed counter a high resolution can be achieved. Also, by allowing the capacitor assembly 40 to charge up to a high predetermined voltage a good resolution can be obtained whilst the higher voltage also minimises the effects of any noise interference. This simple counting technique removes the need for any analogue to digital conversion, thereby simplifying the system design and eliminating the need to measure very low voltages - especially beneficial in electrically noisy environments such a motor or generator.

Figure 3 shows a position encoder 10' based on position encoder 10 (features in common are labelled correspondingly) in which second part 30' is centrally located between first part 20' and a third part 120 comprising a third conductor region 130 spaced from second conductor region 60' to define opposed sides of a further passageway 140 extending between the second and third conductor regions 60', 130.

Third conductor region 130 comprises first, second, third and fourth discrete (e.g. electrically isolated) identical conductor sub-region plates 150A, 150B, 150C and 150D respectively arranged along the passageway 140 in this order to mirror first to fourth conductor sub-region plates 50A'-D' on first part 20' . Each of the first to fourth conductor sub-region plates 150A-D are independently electrically connected to sensor 70'.

Any movement of second part 30' towards either of first and second parts 20', 120 will result in capacitance being increased on the side with a reduced gap and decreased on the side with the widening gap. If the first and second fixed parts 20', 120 are mirror images of each other and connected in series or deal with as a pair of algebraically summed values, then the combined capacitance should not change due to the movement, thereby cancelling out any undesired capacitance variation.