Method and apparatus for driving a gauge

Abstract: An air core gauge assembly comprises a sensor (24) for sensing a parameter, a controller (28) receiving the sensed parameter and operative to determine an ideal gauge position responsive to the sensed parameter, to determine a centre position representing an assumed magnetic hysteresis centre, to compare the ideal position to the centre position and to determine an offset value therefrom, and to determine a command signal responsive to the ideal signal and the difference signal. An air core gauge assembly includes an air core gauge, a pointer (58), and graphics (60), the pointer (58) being rotatable in response to commands given to the air core gauge to indicate information in the display, and an air core gauge driver (40) for driving the gauge on the basis of the command signal to rotate the pointer to an angular position representative of the sensed parameter, compensating for effects of magnetic hysteresis in the air core gauge.

Description: This invention relates to a method and apparatus for driving an air core gauge. A typical air core gauge comprises a magnetic rotor, two or more coils wound around the magnetic rotor, a spindle attached to the shaft of the magnetic rotor and a pointer tip on the end of the spindle. The magnetic rotor is pivotally mounted within the coils, to which an electric current is applied. When the electric current is applied to the coils, the coils generate a magnetic flux which acts upon the magnetic rotor causing rotation of the rotor until the rotor is aligned with the composite magnetic vector generated by the coils. The magnetic rotor causes the spindle to rotate, in turn rotating the pointer, which displays information by means of a gauge scale. Typically, a metal shield surrounds the coil and magnet assembly to prevent external fields from disrupting the magnetic vector created by the coils. This metal shield may be referred to as the "can". In operation of the gauge, the can exhibits a slight residual magnetic field that affects the pointer deflection angle causing magnetic hysteresis. Air core gauges also have a mechanical hysteresis which affects the gauge operation but which is reduced by normal driving vibrations in the vehicle. The magnetic hysteresis in the can resists changes to the magnetic field created by the coils, thus affecting the resultant magnetic vector to which the rotor aligns itself and causing positional error in the pointer of the gauge. The present invention seeks to provide an improved method and apparatus for driving a gauge. EP-A-0379816 discloses an arrangement as detailed in the preambles of claims 1 and 5. EP-A-0457134 discloses a method for correcting for internal friction in load cells or transducers of force measuring devices. GB-A-2043268 discloses an analogue indicator which uses a hysteresis circuit to discharge a capacitor into the windings of a motor. Gauge driving apparatus and a method of driving an air core gauge in accordance with the present invention are characterised over EP-A-0379816 by the features specified in the characterising portions of claims 1 and 5 respectively. In some embodiments, the adverse effects of the above-described mechanical limitations of air core gauge mechanisms can be reduced and increased accuracy, for example in vehicle information systems employing air core gauges can be provided. Advantageously, there may be provided a vehicle speedometer with increased accuracy and/or a vehicle tachometer with increased accuracy. In a preferred embodiment, there is provided a method including the steps of: (a) determining an ideal gauge position, (b) determining a centre angle representative of an assumed centre position of a magnetic hysteresis, (c) comparing the ideal gauge position to the centre angle, (d) developing a gauge position command responsive to 1) the ideal gauge position and 2) the comparison between the ideal gauge position and the centre angle and (e) driving the air core gauge according to the determined command. In another embodiment, there is provided apparatus comprising an air core gauge, means for measuring a vehicle parameter, means for determining an ideal gauge position responsive to the parameter measurement, means for determining a centre position representative of an assumed centre of magnetic hysteresis, means for comparing the centre position with the ideal gauge position, means for determining a gauge command responsive to the ideal gauge position and the comparison, means for outputting the gauge command to the air core gauge and means for driving the air core gauge responsive to the output gauge command. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a magnetic hysteresis curve of an air core gauge; Figures 2 and 3 illustrate pointer error caused by magnetic hysteresis in an air core gauge; Figure 4 is a graph illustrating an embodiment of compensation to be applied to an air core gauge; Figure 5 illustrates schematically the compensation of Figure 3 applied to an air core gauge; Figure 6 is a block diagram of an embodiment of compensation apparatus; Figure 7 is a flow chart of an embodiment of routine for the apparatus of Figure 6; and Figure 8 is a graph of experimental results showing improved accuracy in a speedometer system based on the apparatus of Figure 6. Referring to Figure 1, the graph shown illustrates the problem of gauge hysteresis. Line 11 represents an ideal pointer position against drive angle for an air core gauge. If an angular position command is applied to the gauge, the actual angular position of the gauge follows the hysteresis curves represented by traces 10 and 12. If the angular position of the gauge is increasing, the actual position of the gauge lags the ideal position, being slightly less than the ideal position, as shown by trace 10. Similarly, if the angular position of the gauge is decreasing, the actual position of the gauge is slightly higher than the ideal position, as indicated by trace 12 on the graph. Figures 2 and 3 depict the effect of hysteresis in relation to an air core gauge. For the gauge display 14 shown in Figure 2, if the angular position of the gauge is increasing, as represented by arrow 20, the actual position of pointer 18 is slightly less than the drive angle, represented by line 16. Similarly, in Figure 3, when the angular position of pointer 18 is decreasing, as represented by arrow 21, the actual angular position of pointer 18 is slightly greater than the drive angle 17. These variances of pointer position in relation to the drive angle applied to the air core gauge are due, in large part, to magnetic hysteresis in the gauge mechanism. Referring to Figure 4, trace 22 in the graph shown illustrates an embodiment of control structure for compensating for magnetic hysteresis in an air core gauge. The horizontal axis represents the difference between the desired pointer position and a centre angle representing a position of assumed zero magnetic hysteresis. In general, the centre angle is determined through the assumption that the angular position of the rotor (and pointer) at which there is zero magnetic hysteresis lags the commanded pointer position by 5°. For example, at start-up both the initial pointer position and centre angle are assumed to be zero. If the pointer is then commanded to move to the 15° position, the centre angle is assumed to lie at the 10° angular position. In a similar manner, if the pointer is again returned to the 0° position, the centre angle is assumed to move to the 5° angular position. The command structure shown on Figure 4 illustrates the offset provided to the air core gauge to compensate for magnetic hysteresis. If the ideal gauge position is within 1° of the centre position, as determined in the manner described above, no offset is provided and the air core gauge is driven with a command representing the ideal gauge position. If the ideal gauge position is between 1-4° away from the centre angle, the gauge command is offset by a first amount, preferably 0.35° from the ideal command, in the direction away from the centre angle. For example, if the ideal position is between 1-4° greater than the centre angle, the air core gauge is driven with a command representing the ideal gauge position with a 0.35° offset added. Similarly, if the ideal gauge position is between 1-4° less than the centre angle, the air core gauge is driven with a command representing the ideal gauge position with a 0.35° offset subtracted from the ideal position. If the ideal gauge position is further than 4° away from the centre angle, then a second, greater, offset is added to the ideal position in the direction away from the centre angle to determine the actual command applied to the gauge. A suitable number for the second offset is 0.7°. Figure 5 illustrates the operation of the above embodiment with relation to pointer 18 and angular positions on the gauge display. Assuming position 24 represents the determined centre angle, if the ideal pointer position is within the region 26, which falls +/-1° of centre angle 24, no compensation is made and the gauge command is set equal to the ideal pointer position. If the ideal pointer position falls in the region 30, between 1 and 4° greater than centre angle 24, the first offset is added to the ideal pointer position to determine the gauge command. If the ideal pointer position falls within the region 34, which represents any position greater than the region 30, the second offset is added to the ideal pointer position to determine the gauge command. Similarly, if the ideal pointer position falls within the region 28, between 1° and 4° less than centre angle 24, the first offset is subtracted from ideal gauge position to determine the gauge command and if the ideal pointer position falls within the region 32, which represents any position less than region 28, the second offset is subtracted from the ideal gauge command to determine the applied gauge command. Regions 36 and 38 are regions in which the ideal angular positions are greater than or less than the centre angle by at least 5°. If the ideal pointer position falls within region 36, the centre angle is adjusted to a new centre angle that is 5° less than the commanded gauge position. Similarly, if the ideal gauge position falls within region 38, after the gauge command is determined, the centre angle is adjusted to be 5° greater than the commanded gauge angle. Figure 6 illustrates an embodiment of apparatus suitable for use in a vehicle speedometer (or tachometer) system. The apparatus shown includes a sensor 24, a microprocessor controller 28 including a microprocessor 32, ROM and RAM memories 34 and 36, and input unit 30. Serial data link 38 connects microprocessor controller 28 to a sine/cosine gauge driver circuit 40, including a sine and cosine function generator 42, and digital-to-analogue converters 44 and 46. Gauge driver circuit 40 is connected to and drives gauge 52. In operation, sensor 24 picks up a rotational speed signal from the vehicle transmission to sense vehicle speed in a manner well known to those skilled in the art. The signal is fed on line 26 to microprocessor controller 28. The microprocessor controller 28 converts the signal on line 26 to a numerical signal representing vehicle speed. The vehicle speed signal is then converted to an angular position signal representing the ideal angular position of pointer 58 in relation to graphic scale 60 of gauge 52. Developing the ideal angular position signal may be accomplished in a manner such as looking up, in a look-up table, an angular position signal in response to the developed speed signal. Other processing may be involved such as adding a term to compensate for the pointer tip weight. This may also be done by obtaining the compensation term from a look-up table in response to the angular position signal and is in the form of a reference angle which is then added to the angular position signal to obtain the ideal angular position signal. Hysteresis compensation is then applied to the ideal angular position signal to determine the gauge command in the manner described above and as set forth in more detail below with respect to Figure 7. The gauge command, typically in 10-bit format in this embodiment, is output on serial data link 38 which transfers the gauge command to sine/cosine driver 40. Sine/cosine drivers, such as sine/cosine driver 40, are well known to those skilled in the art and come in a variety of forms responsive to digital and/or analogue signals. In the digital implementation shown, sine/cosine driver 40 receives the 10-bit word in an input latch (not shown), which latches in the gauge command and feeds it to function generator 42. Function generator 42 generates the sine and cosine components of the gauge command represented by the 10-bit word received through line 38. The sine and cosine components of the command are converted to analogue signals through digital-to-analogue converters 44 and 46 and output to gauge 52 through lines 48 and 50. Line 48 provides the sine component of the command to coil 56 and line 50 provides the cosine component of the gauge command to coil 54, which is mounted perpendicularly to coil 56. Coils 56 and 54 generate magnetic vectors comprising the sine and cosine components of the desired magnetic vector, respectively. The magnetic vectors generated by the two coils together form a composite magnetic vector operating on magnetic rotor 57, which rotates to align itself with the composite magnetic vector at the commanded angle (positive/negative hysteresis affect), in turn causing rotation of pointer 58 in relation to graphic scale 60 to provide speed information to the vehicle driver. Referring to Figure 7, an embodiment of routine for controlling the controller 28 is shown. At start-up, step 62 sets the variables COMMAND, IDEAL and CENTRE to zero. At step 64 an input signal is received representing vehicle speed. The input signal can be in a variety of forms such as a frequency signal, a pulse count or any other known signal representative of vehicle speed. The apparatus in Figure 6 shows the input signal coming from speed sensor 24. However, this is an example implementation and the signal may come from other sources such as the vehicle microprocessor control unit, which itself receives a speed signal from a speed sensor, such as sensor 24. Step 66 carries out general signal processing well known to those skilled in the art, where the input signal is converted to a unit of measure such as miles-per-hour or kilometres-per-hour, or in the case of tachometers, rotations-per-minute. Once this signal is developed, a reference angle may be determined from a look-up table. The reference angle represents the desired angular position for the pointer on the gauge corresponding to the determined unit of measure. Other processing at step 66 may also be carried out. For example, if the pointer is tip-heavy, a compensation term may be obtained from a look-up table in computer memory in response to the reference angle. The signal IDEAL, representing the ideal drive angle for the gauge, is then developed equal to the reference angle plus the pointer tip compensation term. This is one example of the functions which may be performed at step 66 and any functions which develop a signal IDEAL, representing the ideal drive angle for the air core gauge, may be alternatively substituted. After step 66, the routine moves to step 68 where the variable IDEAL is compared to the variable CENTRE, representing the assumed centre position of magnetic hysteresis where there is zero hysteresis effect. If the difference between IDEAL and CENTRE is less than 1°, the variable COMMAND, representing the output command used to drive the air core gauge, is set equal to IDEAL at step 70 and output at step 94. If the difference between the variable IDEAL and the variable CENTRE is not less than 1°, the routine moves to step 72 where it is determined if the difference, IDEAL - CENTRE, is between 0 and 4 degrees. If the difference IDEAL - CENTRE falls within the range of 0 to 4 degrees, the routine moves to step 74 where the variable COMMAND is set equal to the variable IDEAL + 0.35° offset. The routine then moves to step 94 to output COMMAND. If the difference, IDEAL - CENTRE, was not between 0 and 4 at step 72, the routine moves to step 76 where the difference, IDEAL - CENTRE is compared to zero. If IDEAL - CENTRE is greater than zero, then it is known at this point that IDEAL - CENTRE is at least 4° and at step 78 the variable COMMAND is set to IDEAL + 0.7°, the maximum offset. The routine then moves to step 80 where the difference, IDEAL - CENTRE is compared to 5°. If IDEAL - CENTRE is greater than 5°, then a new centre angle is determined at step 82 as CENTRE = COMMAND - 5°. The routine then moves to step 94 to output the variable COMMAND to drive the air core gauge. If at step 76 the difference, IDEAL - CENTRE, is not greater than zero, then it is known that the angular position desired is less than the centre position. The routine then moves to step 84 where the difference, CENTRE - IDEAL, is determined. If CENTRE - IDEAL is within the range of 0 to 4, the routine moves to step 86 where COMMAND is set equal to IDEAL - 0.35°. The routine then moves to step 94 where the variable COMMAND is output to drive the gauge. If at step 84 the difference, CENTRE - IDEAL, was not between 0 and 4, the routine moves to step 88 where it is known that the difference, CENTRE - IDEAL, is at least 4° so the variable COMMAND is set equal to the variable IDEAL - 0.7°, the maximum offset. The routine then moves to step 90 where the difference, CENTRE - IDEAL is compared to 5°. If the difference, CENTRE - IDEAL, is greater than 5°, the routine moves to step 92 where a new centre position is determined by according to CENTRE = COMMAND + 5°. If the difference, CENTRE - IDEAL, was not greater than 5° at step 90, the routine moves to step 94. At step 94 the variable COMMAND is output to the sine/cosine drive unit 40 and the routine moves to step 64 to await the next input signal. Implementation of the routine shown in Figure 7 and the computer controller 28 shown in Figure 6 provides operation of the gauge apparatus as shown in Figures 4 and 5 and as described above. Implementation into an air core gauge system, such as in a vehicle speedometer can result in improved accuracy of the indicated display, as shown in Figure 8. The horizontal axis of Figure 8 indicates the gauge drive angle, which ranges from zero to 210° and the vertical axis indicates the pointer error with a zero line corresponding to zero pointer error. Trace 106 represents the pointer position during ascent up the pointer scale and trace 100 represents pointer position during descent of the pointer down the pointer scale. The relatively large error differences between ascent and descent of the pointer due to magnetic hysteresis are exemplified by the large distance between traces 100 and 106. Traces 104 and 102 represent positional error during angular ascent and descent when the method and apparatus as described above are implemented. As can be seen, lines 102 and 104 are much closer together than lines 100 and 106, illustrating the reduction in the error due to magnetic hysteresis. The improved results illustrated in Figure 8 represent an increase in accuracy of 1.4° or almost 1.6 km/hr (1 mile per hour) in the experimental speedometer tested in which 1.5° angular movement corresponded to 1.6 km/hr (1 mile per hour) on the speedometer graphics. In the implementation set forth above with reference to Figure 7, the centre angle CENTRE is computed as lagging the variable COMMAND by 5°. Alternatively, CENTRE may be determined as lagging the variable IDEAL by 5°. This alternative can easily be implemented by those skilled in the art by substituting the variable IDEAL for the variable COMMAND when determining the variable CENTRE. The above-described embodiment can also be used in a vehicle tachometer system, in which an engine speed signal is measured (instead of vehicle speed) and indicated by gauge 52. Likewise, any other air core gauge in which magnetic hysteresis plays an effect may be used with the above described embodiment.

Claims: Gauge driving apparatus comprising a sensor (24) for sensing a predetermined parameter; a gauge assembly including an air core gauge, a pointer (58), and a graphic display (60); and a driver (40) for providing a drive signal to rotate the pointer to an angular position representative of the sensed parameter; characterised by processing means (28) operable in dependence upon the sensed parameter to compute an ideal gauge position and a centre angle representative of an assumed centre position of magnetic hysteresis, to compare the ideal gauge position to the centre position to obtain therefrom an offset value representative of the difference between the ideal gauge position and the centre position and to compute a command signal in response to the ideal gauge position and the offset value; and in that the driver is arranged to provide the drive signal in response to the command signal. Gauge driving apparatus according to claim 1, wherein the processing means is adapted to compute a command signal substantially corresponding to the ideal gauge position when the centre position and the ideal gauge position are within a first range of each other, to compute a command signal substantially corresponding to a sum of the ideal gauge position and a first command offset when the centre position and the ideal gauge position are within a second range of each other, and to compute a command signal substantially corresponding to a sum of the ideal gauge position and a second command offset when the centre position and the ideal gauge position are within a third range of each other. Gauge driving apparatus according to claim 1 or 2, wherein the processing means is adapted to set the centre position substantially to correspond to a previous centre position if the previous centre position and the ideal gauge position are within a predetermined distance of each other and substantially to correspond to a position lagging the ideal gauge position if the centre position and ideal position are outside the predetermined distance of each other. Gauge driving apparatus according to claim 3, wherein the processing means is adapted to set the centre position to lag the ideal gauge position by substantially 5°. A method of driving an air core gauge comprising the step of sensing a predetermined parameter; characterised by the steps of computing an ideal gauge position on the basis of the sensed parameter; computing a centre angle representative of an assumed centre position of magnetic hysteresis of the air core gauge; obtaining a difference signal on the basis of a comparison of the centre position and the ideal gauge position; computing a command signal on the basis of the ideal gauge position and the difference signal; and driving the air core gauge on the basis of the command signal. A method according to claim 5, wherein the step of computing the command signal comprises the steps of determining if the difference signal is within a first, a second or a third range of values; if the difference signal is within the first range of values, setting the command signal substantially to correspond to the ideal gauge position; if the difference signal is within the second range of values, setting the gauge command signal substantially to correspond to a sum of the ideal gauge position and a first command offset; and if the difference signal is within the third range of values, setting the gauge command signal substantially to correspond to a sum of the ideal gauge position and a second command offset. A method according to claim 5 or 6, wherein the step of computing the centre signal comprises the steps of comparing the centre signal to the ideal gauge position; maintaining the centre signal substantially unchanged if the centre signal is within a predetermined distance of the ideal gauge position; and adjusting the centre signal to lag the ideal gauge position if the centre signal is outside the predetermined distance of the ideal gauge position. A method according to claim 7, wherein the centre signal is set to lag the ideal gauge position by substantially 5°.

Inventors: HAWES KEVIN JOSEPH, WEBER REYNOLDS PATRICK, HAWES, KEVIN JOSEPH, WEBER, REYNOLDS PATRICK
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1993
Main classification: G01D3
IPC codes: G01R7