GB2373574A

GB2373574A - Method and apparatus for gauge calibration

Info

Publication number: GB2373574A
Application number: GB0107148A
Authority: GB
Inventors: Christopher Hitchen; Ralph Butcher; Damon Grigg
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2001-03-22
Filing date: 2001-03-22
Publication date: 2002-09-25
Anticipated expiration: 2021-03-22
Also published as: GB2373574B; GB0107148D0

Abstract

A calibration apparatus 2 for a stepper motor gauge comprises a laser L which directs a beam B at a pointer P a lateral effect silicon photodiode PD which determines the location of the point of incidence of the deflected portion of the beam and hence the angle of the pointer. A stator field S is rotated incrementally from an initial orientation to an orientation at which the pointer hits a stop S to determine the number of increments of rotation needed to reach the stop position and is then rotated a further 180{ until the rotor flips to enable this value to be confirmed. The number of increments of rotation needed to reach the zero position from the stop position is then determined. The total is stored in NVRAM as a calibration parameter to enable the gauge to be initialised at zero.

Description

Method and Apparatus for gauge calibration

The present invention relates to a method and apparatus for calibrating gauges, particularly but not exclusively gauges driven by stepper motors. Stepper motors are widely used in gauges eg to indicate speed or fuel level in vehicles.

Typically the rotor of a stepper motor used in such a gauge will drive the gauge pointer via reduction gearing, a typical reduction ratio being in the range 30: 1 to 180: 1. The rotor, which may have single or multiple pole pairs, is driven by a stator field which in turn is generated by a set of stator windings which are energised by approximately sinusoidal currents generated in stepwise fashion under microprocessor control. Provided that the correspondence between the orientation of the stator field and the pointer is known, the pointer can be stepped forwardly or backwardly by the stator field to indicate correctly a variable (such as speed or fuel level for example) which is represented by a digitised output signal from an appropriate transducer.

In practice the pointer may be moved by eg a mechanical shock and as a result the synchronisation between the number of increments of rotation of the stator field and the value on the gauge scale indicated by the pointer will be lost. Consequently, each time the system is powered up, synchronisation must be re-established. This is typically achieved by stepping the stator field anticlockwise to force the pointer against a stop (normally located slightly below zero on the gauge scale) and continuing to step the field anticlockwise until, when the stator field is displaced by just over 1800 from the rotor field, the rotor flips forwardly by about 1800. A stepwise anticlockwise rotation of the stator field by 1800 will

then return the pointer to the stop position in a defined relationship with the stator field.

US 5,665, 897 discloses a calibration procedure for a stepper motor gauge in which the rotor is stepped backwards by stepwise reverse rotation of the stator field until flip back is detected and the number of steps corresponding to the stop position is inferred in the manner outlined above. Accordingly the stop position can be found relative to the stator field orientation and is permanently stored in non-volatile memory. Assuming that the stop position is accurately known in relation to the gauge scale the pointer can then be accurately stepped forward to zero or any other value commanded by its control microprocessor and associated drive circuitry.

US 5,665, 897 also discloses the detection of flip back (which manifests itself as an abrupt forward movement of the pointer from the stop position) with a machine vision system. However, owing to the reduction gearing, this angular movement is very small, of the order of one degree, and the precise detection of flip back and hence of the stop position is not easy with such an arrangement. Furthermore in some cases the rotor may flip back by an extra turn, so that the stop position is not 1800 away from the flip back position but 540 away.

DE-OS 3,921, 462 and US 5,287, 050 disclose stepper motor synchronisation arrangements involving detection of the stop by detecting the absence of back emf in a field winding when the pointer is arrested by the stop. However such electrical arrangements involve detection of rather weak signals and are susceptible to electrical interference.

An object of the present invention is to provide a calibration arrangement in which at least some of the above disadvantages are alleviated.

In one aspect the present invention provides a calibration procedure for a gauge having a pointer mechanism coupled to a rotatable external field for moving the pointer mechanism, the travel of the pointer mechanism being limited by a stop, the procedure comprising the steps of: deflecting a light beam with the pointer mechanism; rotating the external field so as to bring the pointer mechanism to the limit of its travel imposed by the stop; detecting a resulting discontinuity in the movement of the deflected portion of the light beam, and thereby determining the orientation of the field at which the stop is reached.

Since the light beam adds no inertia to the arrangement and magnifies the movement of the pointer, the discontinuity can be detected with great sensitivity and accuracy.

Preferably the field (which is suitably a magnetic field generated by stator windings magnetically coupled to a rotor which drives the pointer mechanism) is rotated in stepwise fashion and the number of steps between a predetermined gauge reading (eg zero or a stable rest position of the pointer mechanism) and said limit is counted by a counter. In a preferred embodiment this count value is stored in a memory associated with the gauge during the calibration procedure to enable the gauge circuitry, during subsequent use, to reach the predetermined reading (eg zero) from the stop.

In certain embodiments the rotation of the external field is continued beyond that needed to bring the pointer mechanism to its limit of travel imposed by the stop until

the pointer mechanism flips away from the stop as a result of reversal of the orientation of the external field and the orientation of the field at which the stop is reached is inferred from the discontinuity in the movement of the deflected portion of the light beam arising from the flip of the pointer mechanism. For example if the pointer mechanism is driven by a two-pole rotor which is coupled to a stator field then the orientation of the field at which the stop is reached will be 1800 back from the orientation of the field at which the pointer mechanism flips.

In one embodiment the light beam is deflected to a photodetector which is arranged to detect the location of the point of incidence of the beam.

The preferred photodetector is a lateral effect silicon photodetector which generates an output indicating the offset of the centroid of the incident light relative to the centre of the photodetector in continuous analogue form. Such photodetectors are capable of resolving optical changes at several MHz. As a result, the position of the incident light beam can be monitored virtually continuously, enabling the discontinuity in the movement of the pointer to be detected with great accuracy. Conversely, a machine vision system as proposed in the prior art noted above is inherently discontinuous because of the relatively low frame rate inherent in video systems.

Preferably the light beam is reflected by the gauge pointer. In one embodiment the pointer spindle carries a reflective element which is disposed in the path of the light beam.

In another aspect the invention provides a calibration apparatus for a gauge, the gauge having a pointer

mechanism coupled to a rotatable external field for moving the pointer mechanism, the travel of the pointer mechanism being limited by a stop, the apparatus comprising: means for directing a light beam at beam-deflecting means carried by the pointer mechanism; means for rotating the external field so as to bring the pointer mechanism to the limit of its travel imposed by the stop, and means for detecting a resulting discontinuity in the movement of the deflected portion of the light beam, to determine the orientation of the field at which the stop is reached.

A preferred embodiment of the invention is described below by way of example only with reference to Figures 1 to 3 of the accompanying drawings, wherein: Figure 1 is a diagrammatic representation of a stepper motor gauge for calibration in accordance with the invention; Figure 2A is a diagrammatic representation of a calibration arrangement comprising the stepper motor gauge of Figure 1 coupled to calibration apparatus in accordance with the invention; Figure 2B is a diagrammatic representation of the calibration arrangement of Figure 2A at a further stage in the calibration procedure; Figure 2C is a diagrammatic representation of the calibration arrangement of Figure 2A at a still further stage in the calibration procedure; Figure 2D is a diagrammatic representation of a variant of the calibration apparatus of Figure 2A, and

Figure 3 is a graph of beam displacement : stator field angle for the arrangement of Figures 2A to 2C.

Referring to Figure 1, the stepper motor gauge comprises a scale SC having a stop S located below (displaced anticlockwise from) the zero gradation or chaplet. The gauge includes a pointer mechanism comprising a pointer P mounted on a driven gear G2 which is engaged with a drive gear Gl. Gear Gl is mounted on a two-pole magnetic rotor (not shown) whose magnetic field is represented by the vector R. Gears Gl and G2 constitute reduction gearing with a gear ratio of (say) 180: 1 although this ratio is not critical and may for example be in the range 30: 1 to 180 : 1.

The magnetic rotor is magnetically coupled to a rotatable stator field (represented by vector S, and shown laterally displaced from vector R purely for ease of illustration) which is generated by field windings Wl and W2 in a conventional manner. Windings Wl and W2 are energised by appropriate drive circuits of a gauge circuit 1, which also includes a microprocessor mP provided with nonvolatile RAM (NVRAM). The microprocessor mP is arranged to rotate the stator field S about the axis of the rotor in stepwise fashion in dependence upon a digital input signal from a transducer (not shown) such as a speedometer or fuel gauge for example.

As described thus far, the arrangement of Figure 1 is conventional.

Since there is inevitably some variation amongst different gauges in the angular separation d between the anticlockwise limit of travel of the pointer P imposed by the stop S and the zero position of the pointer, it is necessary to establish this angular separation, eg in

terms of the number of steps of rotation of the stator field needed to bring the pointer from its rest position against the stop to the zero position. This can then be permanently stored as a count value in the NVRAM of circuitry 1.

In accordance with the invention, the required calibration is achieved with the calibration arrangement 2 shown in Figure 2A.

Referring to Figure 2A, the calibration arrangement comprises a laser L which directs a laser beam B at the pointer P. The beam is specularly reflected to a lateral effect two dimensional linear silicon photodetector PD which generates an output signal indicating the offset (indicated at xl) along the x-axis of the centroid of the incident light relative to the centre of the photodetector in continuous analogue form. This output signal, which represents the angular position of pointer P is input, via analogue-to-digital conversion circuitry A/D, to a calibration circuit 4 which also includes a drive circuit and microprocessor similar to that of circuit 1 of Figure 1 and outputs stepped drive signals to stator field windings W1 and W2. The amplitude ratio of these drive signals defines the angular orientation of stator field vector S and this vector can be rotated in stepwise fashion by varying the drive signals stepwise in accordance with respective phase-displaced generally sinusoidal patterns.

A counter 3 is provided in calibration circuit 4 which counts the number of steps by which the stator field drive signals are incremented from a predetermined initial condition. The counter value is output to the NVRAM of circuit 1 (Figure 1) as will subsequently be described. It should be noted that the circuit 4, being similar in operation to circuit 1, could be substituted by a modified

form of circuit 1. In particular the circuit 1 could be supplemented by a counter similar to counter 3 but having an output coupled internally to the circuit's NVRAM.

Before describing the operation of the calibration arrangement with reference to Figures 2B and 2C, it should be noted that in the orientation of field vector S shown in Figure 2A, the pointer P is assumed to be resting gently against stop S, for ease of description. However in general, at initialisation of the gauge circuitry, the pointer will have an arbitrary rest position displaced from the stop S, and some appreciable clockwise rotation of the stator field will normally be needed to bring the pointer into contact with the stop. Importantly, the resulting discontinuity D1 (Figure 3, see below) in the pointer movement with rotation of the stator field can be detected directly with the present arrangement.

Referring now to Figure 2B, in which certain elements of the calibration arrangement have been omitted purely for the sake of clarity, it will be noted that stator vector S has been rotated clockwise by fractionally less than 1800. The position x2 of the point of incidence of beam B on photodetector PD is unchanged however (ie x2 = xl) because of the limit imposed by stop S.

Turning now to Figure 2C, a fractional further clockwise rotation of stator field vector S causes the rotor field vector R to flip anticlockwise by 1800 as the rotor flips to align itself with the stator field, causing the pointer P to kick away from the stop S as shown. This angular movement will only be about one degree (assuming a gear ratio of 180: 1) but will be detected by photodetector PD as a new position of incidence x3 of the reflected portion of the beam B.

Referring now to Figure 3 in conjunction with Figure 2C, the relationship between the displacement of the point of incidence x of the beam B on photodetector PD and the angular position qS of the stator field S is shown (together with the points at which the pointer reaches the stop, the rotor flips back and the stator re-synchronises with the stator field), and exhibits a first discontinuity D1 at xl (= x2) as noted above and a second discontinuity D2 at x3 separated by 1800 in qS (due to flipback of the

rotor). The first discontinuity D1 can be detected (eg by double differentiation of the plot of Figure 3) to find directly the number of increments of rotation of the stator field from initialisation to the stop position (ie the limit of travel of the pointer).

This count value can be confirmed by subtracting a number of increments of rotation of the stator field between detected positions xi and x3 equivalent to 1800. In some cases an additional number of increments of rotation equivalent to a further complete rotation-ie 540 in total will be necessary because the rotor flips forward by an extra turn. Since such behaviour is undesirable, individual gauges with this characteristic may need to be modified.

Having found the stop position in relation to the initial stator field, the stator field is then advanced by a standard number of increments to advance the pointer from the stop position to nominally the zero position. Owing to manufacturing tolerances, the pointer will in practice reach a position slightly displaced either side from the stop position and this position is detected eg by a machine vision system (not shown) and the number of increments of rotation needed to reach the stop zero position exactly is calculated and added to the number of increments needed to reach the stop position from the rest position on initialisation. The total is then stored

permanently in the NVRAM of circuitry 1 (Figure 1) to enable the gauge to advance the pointer P to the zero position each time the gauge circuitry is switched on (eg when the ignition switch of the vehicle is turned on). A similar calibration procedure is followed in respect of other gauge readings and the correct number of increments of rotation corresponding to each gauge reading is determined and similarly stored permanently in the NVRAM of circuitry 1.

The photodetector PD may be calibrated with the arrangement shown, since the difference between x2 and x3 is known to amount to 1800 of rotation of stator field S.

Hence the angular separation between the stop S and the zero chaplet can be measured in terms of the output of the photodetector and converted to an angle and hence a count value.

In the variant shown in Figure 2D, a plane mirror facet M is formed on the boss of the spindle of pointer P and is inclined at 450 to the spindle axis. The laser beam B is aligned with the spindle axis and is reflected at 900 to the spindle axis by mirror facet M ie parallel to the plane of pointer P. In other variants, the beam may be deflected with a refractive element. The light beam need not be in the visible spectrum, but could be an IR beam for example.

Claims

1. A calibration procedure for a gauge having a pointer mechanism coupled to a rotatable external field for moving the pointer mechanism, the travel of the pointer mechanism being limited by a stop, the procedure comprising the steps of: deflecting a light beam with the pointer mechanism; rotating the external field so as to bring the pointer mechanism to the limit of its travel imposed by the stop; detecting a resulting discontinuity in the movement of the deflected portion of the light beam, and thereby determining the orientation of the field at which the stop is reached.

2. A calibration procedure according to claim 1 wherein the field is rotated in stepwise fashion and the number of steps between a predetermined gauge reading and said limit is counted by a counter.

3. A calibration procedure according to claim 2 wherein said predetermined gauge reading is zero or a stable rest position of the pointer mechanism.

4. A calibration procedure according to claim 2 or claim 3 wherein the number of steps counted by the counter is stored in a memory associated with the gauge.

5. A calibration procedure according to any preceding claim wherein the field is a magnetic field generated by stator windings magnetically coupled to a rotor which drives the pointer mechanism.

6. A calibration procedure according to any preceding claim wherein the movement of the deflected portion of the light beam is monitored continuously and the extent of rotation of the external field at which the discontinuity

due to the arresting of the travel of the pointer mechanism by the stop is measured.

7. A calibration procedure according to any preceding claim wherein the rotation of the external field is continued beyond that needed to bring the pointer mechanism to its limit of travel imposed by the stop until the pointer mechanism flips away from the stop as a result of reversal of the orientation of the external field and the orientation of the field at which the stop is reached is inferred from the discontinuity in the movement of the deflected portion of the light beam arising from the flip of the pointer mechanism.

8. A calibration procedure according to any preceding claim wherein the light beam is deflected to a photodetector which is arranged to detect the location of the point of incidence of the beam.

9. A calibration procedure according to claim 8 wherein the photodetector is a lateral effect photodetector.

10. A calibration procedure according to any preceding claim wherein the light beam is reflected by the gauge pointer.

11. A calibration procedure according to claim 10 wherein the pointer spindle carries a reflective element which is disposed in the path of the light beam.

12. Calibration apparatus for a gauge, the gauge having a pointer mechanism coupled to a rotatable external field for moving the pointer mechanism, the travel of the pointer mechanism being limited by a stop, the calibration apparatus comprising:

means for directing a light beam at beam-deflecting means carried by the pointer mechanism ; means for rotating the external field so as to bring the pointer mechanism to the limit of its travel imposed by the stop, and means for detecting a resulting discontinuity in the movement of the deflected portion of the light beam, to determine the orientation of the field at which the stop is reached.

13. Calibration apparatus according to claim 12 further comprising a counter arranged to count the number of steps between a predetermined gauge reading and said limit as the field is rotated in stepwise fashion.

14. Calibration apparatus according to claim 12 or claim 13, comprising a photodetector in the path of the deflected portion of the light beam and which is arranged to detect the location of the point of incidence of the beam thereon.

15. Calibration apparatus according to claim 14 wherein the photodetector is a lateral effect photodetector.

16. Calibration apparatus according to any of claims 12 to 15 wherein the light beam is arranged to be reflected by the gauge pointer.

17. Calibration apparatus according to claim 16 comprising a reflective element which in use is carried by the gauge pointer and is disposed in the path of the light beam.

18. Calibration apparatus substantially as described hereinabove with reference to Figure 2A optionally as modified in accordance with Figure 2D of the accompanying drawings.

19. A calibration procedure for a gauge having a pointer mechanism coupled to a rotatable external field for moving the pointer mechanism, the calibration procedure being substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.