GB2280268A

GB2280268A - Method and apparatus for calibrating a speedometer

Info

Publication number: GB2280268A
Application number: GB9318159A
Authority: GB
Inventors: Philip Charles Brown; Brian Anthony Knight; Thomas Mckeown
Original assignee: Delco Electronics Overseas Corp
Current assignee: Delco Electronics Overseas Corp
Priority date: 1993-07-24
Filing date: 1993-09-01
Publication date: 1995-01-25
Anticipated expiration: 2013-09-01
Also published as: GB2280268B; GB9318159D0; GB9315367D0

Abstract

Calibration apparatus for calibrating a speedometer having a magnet and operable under magnetic induction, includes a calibration bed having a driver for driving the worm of the instrument, a camera and vision system for producing a signal indicative of the position of the instrument pointer (1), a magnetiser for magnetising the magnet and a control unit. Drift of the pointer (1) from a rest position is measured and the magnet is then magnetised to a calibration zone (202 - 206) which is dependent upon the position of the pointer (1) after drifting. Calibration thus takes into account imbalances in the instrument. <IMAGE>

Description

METHOD AND APPARATUS FOR CALIBRATING AN INSTRUMENT The present invention relates to calibration apparatus and to a method of calibrating an instrument.

A common type of instrument, as used for example in vehicle speedometers includes a magnet coupled to the speedometer cable and rotatable therewith. Disposed over the magnet is a speed cup, to which the pointer spindle is fixed. The speed cup is rotatably housed in a metal casing and is restrained by a restraining spring. A pointer is located on the spindle for rotation therewith.

As a result of the flux field generated by the magnet, rotation thereof induces rotation in the speed cup and thus rotation of the spindle and pointer. The amount of rotation of the speed cup is limited by the restraining spring, such that a greater rate of rotation of the magnet induces a greater rotating torque in the speed cup and hence greater rotation of the spindle against the force produced by the restraining spring.

A known method of calibrating such an instrument relative to the scale of a dial of the instrument involves placing the pointer on the spindle when the magnet is in a demagnetised state, saturating the magnet, driving the magnet at a predetermined speed of rotation and demagnetising the magnet until the pointer falls within the correct zone of the dial scale for the predetermined speed of rotation of the magnet. The speed of rotation of the magnet is then changed and the position of the pointer checked to determine whether it falls within the correct zone of the dial scale for that speed of rotation of the magnet.

It has been found in practice that in about 2% of cases, the pointer falls outside the limits of the second zone, around half falling at a scale position below the zone and the other half falling above the zone. Inaccuracies of this type are due to imbalances in the speedometer components and other manufacturing tolerances. When an inaccuracy occurs, the magnets are demagnetised, the pointer is removed from the spindle and then replaced at a higher or lower position, relative to the dial scale, so as to offset the detected inaccuracy.

A problem with the above method is that the recalibration process is time consuming and expensive since it has to be carried out individually for each speedometer.

The present invention seeks to provide improved calibration apparatus and an improved method of calibrating an instrument.

According to an aspect of the present invention, there is provided calibration apparatus for calibrating an instrument which comprises a pointer movable along a scale of a dial and pointer driving means for driving the pointer, the pointer driving means including one or more magnets and being operable by magnetic induction; the apparatus comprising instrument driving means for driving the instrument, pointer detecting means for measuring the position of the pointer, magnetising means for magnetising the magnet or magnets and control means adapted to control the driving means to set the instrument into a predetermined state, to control the pointer detecting means to measure the position of the pointer at the predetermined state and to control the magnetising means to magnetise the magnet or magnets to an amount dependent upon the measured position of the pointer.

Any imbalances in the instrument, caused for example by pointer or speed cup imbalance, the restraining spring or other manufacturing tolerances, can be accounted for directly by looking at the effect on the pointer itself. Subsequent calibration of the instrument is then automatically adjusted. This has been found to reduce considerably calibration failures.

Preferably, the control means is adapted to cause the driving means to set the instrument in a non-sensing state, the measured position of the pointer being the rest state of the pointer. At this position, drift of the pointer from the pointer set position can provide the best indication of the effect of instrument imbalances.

In a preferred embodiment, the control means is adapted to determine a calibration window along the instrument scale from the measured position of the pointer and to control the magnetising means to magnetise the magnet or magnets to an amount at which the pointer moves to within the calibration window.

Advantageously, the control means is adapted to determine from the measured pointer position whether the pointer is in a central measurement zone representative of an instrument reading when in the predetermined state, an upper measurement zone representative of an instrument reading greater than the reading at the predetermined state or a lower measurement zone representative of an instrument reading less than the reading at the predetermined state. This can provide simple and rapid correction for pointer imbalances.

Preferably, the control means is adapted to position the pointer in a zone within the calibration window on the basis of the measurement zone in which the pointer lies when the instrument is in the predetermined state.

Advantageously, the control means is adapted to position the pointer in a zone substantially in the centre of a calibration window when the pointer is in the central measurement zone in the predetermined state, to position the pointer in a zone representative of a lower instrument reading than at the centre of the calibration window when the pointer is in the upper measurement zone in the predetermined state, and to position the pointer in a zone representative of a greater instrument reading than at the centre of the calibration window when pointer is in the lower measurement zone in the predetermined state.

In an embodiment, the control means is adapted to set one or more test windows, to cause the driving means to set the instrument into one or more test states, and to determine from the measured pointer position whether the pointer falls within the or a respective test window. Use of test windows can provide a simple and effective way of testing the accuracy of the calibration.

The control means may be adapted to set the or each test window at a location representative of a lower instrument reading than the location of the or a calibration window. Locating the calibration window at a greater measurement than the test windows, and preferably as close as reasonably possible to the greatest instrument reading ensures means that any inaccuracies in calibration at the chosen location are diminished further down the instrument scale.

In a preferred embodiment, the control means is adapted to control the magnetising means to demagnetise the magnet or magnets substantially completely, to control the sensing means to measure the position of the pointer, to control the magnetising means to magnetise the magnet or magnets substantially to saturation and to demagnetise the magnet or magnets in steps until the pointer moves to the or a calibration zone. Demagnetisation of the magnet or magnets ensures that the pointer stays in its rest position even if the worm is rotating, since the magnet or magnets would have no effect on the speed cup. It has also been found that gradual demagnetisation of the magnet or magnets from a state of saturation provides a convenient way of finding the correct magnetisation level.

According to another aspect of the present invention, there is provided a method of calibrating an instrument comprising a pointer movable along a scale of a dial and driving means for driving the pointer, the driving means including one or more magnets and being operable by magnetic induction; the method comprising the steps of setting the instrument into a predetermined state, measuring the position of the pointer at the predetermined state and magnetising the magnet or magnets to an amount dependent upon the measured position of the pointer.

The instrument is preferably a speedometer.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a side elevational view in partial cross-section of the principal components of a speedometer; Figure 2 is a front elevational view of a speedometer dial showing an embodiment of calibration zones; Figure 3 is a block diagram of the principal components of an embodiment of calibration apparatus; and Figure 4 is a flow chart of the principal steps of an embodiment of calibrating routine.

Referring to Figure 1, the speedometer shown includes a coupling worm 10 rotatably located within a shaft 12 of a casing 14 of the speedometer. The worm 10 has a bore 16 therein for receiving an end of a speedometer cable (not shown) of conventional form.

The bore 16 includes a square cross-section portion 18 which in use makes mating contact with a square crosssection end of the speedometer cable and a guide portion 20 tapering into the bore 16 for assisting the insertion of the speedometer cable into the bore 16.

Fixed to the other end of the worm 10 is a disc 22 of plastics material within which a magnet (not shown) is embedded, at diametrically opposite sides relative to the worm 10.

A speed cup 24 , in this example made from aluminium, is disposed over the disc 22 and includes a generally flat upper surface 26 and a depending wall 28 extending along substantially the whole perimeter of the upper surface 26. The upper surface 26 is raised around its centre point to avoid contact with the worm 10.

Projecting from the raised portion of the upper surface 26 and fixed thereto is a spindle 30. A restraining spring (not shown) is coupled to the spindle 30 and applies a restraining force against rotation of the spindle 30. A casing 32, in this example made from a magnetically insulating material, is disposed over the speed cup 24 and has a crosssectional shape similar to that of the speed cup 24.

The magnet in the disc 10 produces a flux field which passes through the speed cup 24.

When the speedometer cable is made to rotate, the worm 10 rotates as well, causing rotation of the disc 22 and of the magnet. Rotation of the magnet produces rotation of the flux field, which induces rotating torque in the speed cup 24. The greater the rotation of the magnet, the greater is the amount of torque induced in the speed cup 24. As a result, the speed cup 24 and spindle 30 are able to rotate until the restraining force produced by the restraining spring is equal to the rotating torque induced in the speed cup 24.

Referring to Figure 2, the shown example of speedometer dial 40 includes a circular aperture (not shown) for receiving the spindle 30 of the speedometer. Pointer 1 (shown in Figure 2) is then placed on the spindle 30. Rotation of the spindle 30 causes the pointer 1 to rotate and thus to move along the speed scale 44 to give an indication of the speed of rotation of the speedometer cable.

The speedometer dial 40 also includes two substantially rectangular apertures, as is conventional, for odometers.

The pointer 1 is fitted onto the spindle 30 when the worm 10 is not rotating and the magnet is in a demagnetised state. In this condition, the pointer 1 would be in a position in which it indicates no speed, that is at the centre of the pointer set position, or measurement zone, 100 shown in Figure 2.

Due to imbalances in the components of the speedometer and to other manufacturing tolerances, the pointer 1 may come to rest in any one of three windows in the pointer set position 100. If the pointer 1 is generally balanced, it will remain in the central window 102. However, if imbalances exist, the pointer 1 may move in either direction of the central window 102.

Calibration of the speedometer is carried out by driving the worm 10 at a predetermined speed, in this example 3000 rpm, saturating the magnet so that the pointer 1 moves to the opposite end of the scale 44 and then gradually demagnetising the magnet until the pointer 1 moves into the calibration window 200. The accuracy of calibration is then checked by driving the worm 10 at two different speeds, in this example 2000 rpm and 1000 rpm, and determining whether the pointer 1 moves into the limits of the respective test windows 110 and 112.

If the pointer 1 is substantially balanced when set on the spindle 30, demagnetisation of the magnet to a state in which the pointer 1 moves into the central zone 202 of the calibration window 200, will generally mean that the pointer 1 will fall within the test windows 110 and 112 when the worm is driven at 2000 rpm and 1000 rpm, respectively.

However, if, due to instrument imbalances, the pointer 1 moves from the central pointer set position 102 in an increasing speed direction to the upper pointer set position 104, calibration of the pointer 1 into the central zone 202 of the calibration window 200 can cause the pointer 1 to fall outside the test windows 110 and 112, to a side of these test windows representative of greater speed.

On the other hand, if the pointer 1 moves from the central pointer set position 102 in a decreasing speed direction to the lower pointer set position 106, calibration of the pointer 1 into the central zone 202 of the calibration window 200, can cause the pointer 1 to fall to a lower speed position outside the test windows 110 and 112.

The preferred embodiment compensates for such potential inaccuracies by determining whether and how the pointer 1 has moved from-the central pointer set position 102 and adjusting the calibration of the speedometer on the basis of this movement. More specifically, if it is determined that the pointer 1 has moved to the upper pointer set position 104, the magnet is demagnetised until the pointer 1 moves into the lower zone 204 in the calibration window 200. On the other hand, if it is determined that the pointer 1 has moved to the lower pointer set position 106, the magnet is demagnetised until the pointer 1 moves into the upper zone 206 in the calibration window 200. All three calibration zones 202, 204, 206 fall within the calibration window 200, representing an acceptable accuracy of the speedometer.

In this manner, the initial movement of the pointer 1 is accounted for by adjusting the level of magnetisation of the magnet. When, as in the preferred embodiment, the calibration and adjustment is made at the high speed end of the scale 44, the position of the pointer 1 will be more accurate at the lower speed positions, thereby ensuring that the speedometer is satisfactorily calibrated for all speeds along the scale 44. Of course, calibration could be carried out in a lower speed window, such as one of the windows 110 and 112.

Referring to Figure 3, the embodiment of calibration apparatus shown includes a CCD camera 300 coupled to a vision system 302 of any suitable known type. A central control unit 304 is coupled to the vision system 302, to a magnetiser 306 and to a calibration bed 312. The calibration bed 310 includes first and second magnetising coils 308,310 and a worm driver 314. Also included in the calibration bed 312 is means (not shown) for holding a speedometer in place during calibration thereof, in a position in which the worm 10 of the speedometer engages the worm driver 314 and the magnet in the disc 22 is within the area of the coils 308,310 for magnetisation thereby.

The camera 300 is disposed in front of the calibration bed 312 and in use focused onto the pointer 1 and dial 40 of a speedometer to be calibrated. The vision system 302 is programmed to receive the images from the camera 300, to determine therefrom the position of the pointer 1 and to translate this position into a value representative of the angle of orientation of the pointer 1. This value is sent to the control unit 304 which determines therefrom the steps to be taken in calibrating the speedometer, as explained in further detail below.

A first output signal from the control unit 304 is sent to the calibration bed and operates the worm driver 312 at selected speeds for rotating the worm 10 of a speedometer placed on the calibration bed 312.

A second signal from the control unit 304 is fed to the magnetiser 306 and is representative of the level of magnetising or demagnetising voltage which should be fed to the coils 308,310.

An embodiment of calibrating routine is shown in Figure 4. The routine is commenced when a speedometer to be calibrated is placed on the calibration bed 312. In the first step in the routine, step 400, the control unit 304 commands the magnetiser 306 to demagnetise completely the magnet of the speedometer so that any rotation of the worm 10 has no effect on the movement of the pointer 1.

Then, under control of the control unit 304, an image from the camera 300 is processed by the vision system 302, in known manner. The vision system determines from the processed image the angle of orientation of the pointer 1. A-value representative of this angle then sent to control unit 304.

At step 402, the control unit 304 determines in which pointer set zone 102-106 the pointer 1 lies.

If the pointer 1 is in the central pointer set position 102, the routine passes to step 404; while if it is in the upper pointer set position 104, the routine passes to step 406; and if it is in the lower pointer set position 106, the routine passes to step 408.

Saturation and subsequent demagnetisation of the speedometer magnet is carried out in steps 404,406 and 408. In the first stage in each of these steps, the control unit 304 instructs the magnetiser to saturate the speedometer magnet, which it does by energising the coils 308,310 at a very high voltage, for example 400 volts. The second stage involves a gradual demagnetisation of the magnet, again by instructing the magnetiser 306 to apply a demagnetising voltage to the coils 308,310.

During demagnetisation, the control unit reads the position of the pointer 300 from the signal from the vision system 302 and determines the amount of demagnetisation which should be produced in the next step of demagnetisation of the magnet. In this manner, when the pointer 1 is at a significant distance from the particular calibration zone in which it is to be set, demagnetisation can be carried out in large voltage steps, for example in steps of 40 or 50 volts. On the other hand, as the pointer 1 approaches the appropriate calibration zone, demagnetisation is carried out in progressively smaller steps until the pointer 1 falls within the zone.

At step 404, the control unit 304 controls the demagnetisation of the magnet until the pointer 1 falls within the central zone 202 of the calibration window 200. In step 406, the control unit 304 controls the demagnetisation of the magnet until the pointer 1 falls within the lower zone 204 of the calibration window 200, while in step 408, the control unit 304 controls the demagnetisation of the magnet until the pointer 1 falls within the upper zone 206 of the calibration window 200.

Once the pointer 1 has moved into the appropriate calibration zone 202,204,206, the routine passes to step 410 in which the control unit causes the worm driver 314 to rotate first at 2000 rpm and then at 1000 rpm. At both of these speeds, the control unit 304 determines from the signal from the vision system 302 whether the pointer 1 is within the appropriate lower speed window 110,112. If this is the case, calibration is determined to have been successful and the routine is left.

If the test carried out at step 410 shows that the pointer 1 is not within one of the windows 110,112, it is determined that calibration has not been successful. At this point, the system may determine that the speedometer is faulty and can not be calibrated. In an alternative embodiment, the system may repeat the routine of Figure 4 but using a calibration zone within the calibration window 200 which on the basis of the failure of the previous calibration may produce the required accuracy.

The above-described embodiment measures the position of the pointer 1 when in its rest state. In another embodiment, the magnet could be magnetised to a predetermined amount and the worm 10 driven at a predetermined speed, the position of the pointer then being measured. Imbalances in the speedometer would result in differences in the position of the pointer 1. Thus, the pointer set position 100 would be located along the scale 44 on the basis of the magnetisation of the magnet and the speed of the worm 10.

Claims

Claims:

1. Calibration apparatus for calibrating an instrument which comprises a pointer movable along a scale of a dial and pointer driving means for driving the pointer, the pointer driving means including one or more magnets and being operable by magnetic induction; the apparatus comprising instrument driving means for driving the instrument, pointer detecting means for measuring the position of the pointer, magnetising means for magnetising the magnet or magnets and control means adapted to control the driving means to set the instrument into a predetermined state, to control the pointer detecting means to measure the position of the pointer at the predetermined state and to control the magnetising means to magnetise the magnet or magnets to an amount dependent upon the measured position of the pointer.

2. Calibration apparatus according to claim 1, wherein the control means is adapted to cause the driving means to set the instrument in a non-sensing state, the measured position of the pointer being the rest state of the pointer.

3. Calibration apparatus according to claim 1 or 2, wherein the control means is adapted to determine a calibration window relative to the instrument scale from the measured position of the pointer and to control the magnetising means to magnetise the magnet or magnets to an amount at which the pointer moves to within the calibration window.

4. Calibration apparatus according to claim 3, wherein the control means is adapted to determine from the measured pointer position whether the pointer is in a central measurement zone representative of an instrument reading when in the predetermined state, an upper measurement zone representative of an instrument reading greater than the reading-at the predetermined state or a lower measurement zone representative of an instrument reading less than the reading at the predetermined state.

5. Calibration apparatus according to claim 4, wherein the control means is adapted to position the pointer in a zone within the calibration window on the basis of the measurement zone in which the pointer lies when the instrument is in the predetermined state.

6. Calibration apparatus according to claim 5, wherein the control means is adapted to position the pointer in a zone substantially in the centre of a calibration window when the pointer is in the central measurement zone in the predetermined state, to position the pointer in a zone representative of a lower instrument reading than at the centre of the calibration window when the pointer is in the upper measurement zone in the predetermined state, and to position the pointer in a zone representative of a greater instrument reading than at the centre of the calibration window when pointer is in the lower measurement zone in the predetermined state.

7. Calibration apparatus according to any preceding claim, wherein the control means is adapted to set one or more test windows, to cause the driving means to set the instrument into one or more test states, and to determine from the measured pointer position whether the pointer falls within the or a respective test window.

8. Calibration apparatus according to claim 7, wherein the control means is adapted to set the or each test window at a location representative of a lower instrument reading than the location of the or a calibration window.

9. Calibration apparatus according to any preceding claim, wherein the control means is adapted to control the magnetising means to demagnetise the magnet or magnets substantially completely, to control the sensing means to measure the position of the pointer, to control the magnetising means to magnetise the magnet or magnets substantially to saturation and to demagnetise the magnet or magnets in steps until the pointer moves to the or a calibration zone.

10. A method of calibrating an instrument comprising a pointer movable along a scale of a dial and driving means for driving the pointer, the driving means including one or more magnets and being operable by magnetic induction; the method comprising the steps of setting the instrument into a predetermined state, measuring the position of the pointer at the predetermined state and magnetising the magnet or magnets to an amount dependent upon the measured position of the pointer.

11. A method according to claim 10, wherein the predetermined state of the instrument is a nonsensing state, the measured position of the pointer being the rest state of the pointer.

12. A method according to claim 10 or 11, comprising the step of determining a calibration window relative to the instrument scale from the measured position of the pointer, driving the instrument to a preset state, and magnetising the magnet or magnets to an amount at which the pointer moves to within the calibration window.

13. A method according to claim 12, wherein the step of determining the position of the pointer includes the step of determining whether the pointer is in a central measurement zone representative of an instrument reading when in the predetermined state, an upper measurement zone representative of an instrument reading greater than the reading at the predetermined state or a lower measurement zone representative of an instrument reading less than the reading at the predetermined state.

14. A method according to claim 13, comprising the step of positioning the pointer in a zone within the calibration window on the basis of the measurement zone in which the pointer lies when the instrument is in the predetermined state.

15. A method according to claim 14, comprising the step of positioning the pointer in a zone substantially in the centre of a calibration window when the pointer is in the central measurement zone in the predetermined state, positioning the pointer in a zone representative of a lower instrument reading than at the centre of the calibration window when the pointer is in the upper measurement zone in the predetermined state, and positioning the pointer in a zone representative of a greater instrument reading than at the centre of the calibration window when pointer is in the lower measurement zone in the predetermined state.

16. A method according to any one of claims 10 to 15, comprising the step of determining one or more test windows, setting the instrument into one or more test states, and determining whether the pointer falls within the or a respective test window.

17. A method according to claim 16, wherein the or each test window is at a location representative of a lower instrument reading than the location of the or a calibration zone.

18. A method according to any one of claims 10 to 17, comprising the step of substantially completely demagnetising the magnet or magnets, measuring the position of the pointer, magnetising the magnet or magnets substantially to saturation, and demagnetising the magnet or magnets in steps until the pointer moves to the or a calibration zone.

19. Calibration apparatus for calibrating an instrument substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

20. A method of calibrating an instrument substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.