GB2265719A - Method for determining a reference position - Google Patents

Description

2265719 Method for determininq a reference position

Prior art

The invention is based on a method for determining a reference position, of the generic type of the main claim.

Devices or methods for determining a reference position, in which a moving part which has a reference marking is scanned by a stationary sensor, for example a Hall sensor, which emits a characteristic output signal when it moves past the reference marking, are already known, for example in DE-A 3,423,664.

As laboratory tests have shown, temperature errors occur when digital Hall sensors are used, which temperature errors lead to inaccuracies in determining reference positions precisely. In this case, both the switching-on point and the switching-off point of the Hall sensors change as a consequence of temperature changes.

Advantages of the invention The method according to the invention, having the characterising features of the main claim, in contrast has the advantage that very accurate determination of a reference position is possible, since the temperature 2 dependencies of the digital Hall sensor, or of the digital Hall sensors, are obviated. For this purpose, according to the invention, the moving part is moved past a Hall sensor in two directions with the magnetic element which defines the reference position, until the switching-on threshold and the switching-off threshold of the sensor are reached in each case. The switching points are stored and the reference position is defined as the centre between identical switching points.

The method can be used in a particularly advantageous manner for determining a reference position in conjunction with learning and moving to a memory position, for example in the case of a mirror adjustment. Another, second sensor, which is sensitive to magnetic fields, is then used, which sensor records the revolutions of the spindle with whose aid the moving part, which comprises the mirror and the magnetic element defining the reference position, is displaced.

Further advantageous refinements and improvements of the method specified in the main claim are possible by means of the measures outlined in the subclaims.

Drawing An exemplary embodiment of the invention is shown in the drawing and is explained in more detail in the following description. In this case, Figure 1 shows a schematic arrangement of the components which are essential to the invention, Figure 2 shows the magnetic flux density when the reference magnet moves past the digital Hall sensor at the side as well as the switching thresholds and the associated signal response of the Hall sensor for a first temperature. and Figure 3 shows the same state of affairs for a second temperature.

Description of the exemplary embodiment

Figure 1 shows the essential components of the arrangement according to the invention schematically. in this case, 10 designates a moving part which can be moved via a spindle 11 in the directions which are indicated by the arrow.

The spindle 11, or the movable part 10, is driven with the aid of a drive motor 12 and possibly a shaft 13, it being possible for there also to be a mechanical transmission 19 in between.

The drive motor 12 is an electric motor whose rotation direction likewise changes in the event of a change in the direction of the current flow.

A mirror 14, whose position is intended to be displaceable, is connected to the moving part 10. In order to define a reference position, a magnetic element 15 is fitted on the moving part 10, the magnetic field emerging from said magnetic element 15 being recorded by a first digital Hall sensor 16. A second digital Hall sensor 17 is arranged in the vicinity of the motor 12 or of the shaft 13 so that it supplies an output signal with whose aid the number of revolutions of the drive motor 12 can be determined.

An evaluation device 18, which can have a pC [sic] or is also a component a [sic] controller, is provided to evaluate the output signals of the Hall sensors 16 and 17.

In the case of the arrangement shown in Figure 1, two digital Hall sensors 16 and 17 are used to determine the position of the mirror 14 or of the moving element 10, and another sensor, by means of which the number of revolutions of the drive motor can be determined, can also be used instead of the digital Hall sensor 17.

The Hall sensor 17 is intended to supply one pulse per revolution of the shaft 13, this being achieved, for example, by means of a magnetic marking on the shaft. The number of revolutions of the shaft or of - 4 the drive motor 12, determined therefrom in the evaluation device, allows the relative movement s which the moving part 10 travels to be determined, when the spindle dimensioning is known. In this case, the resolution of this relative movement measurement is only dependent [sic] on the step-up ratio of the mechanical transmission 19 via which the spindle is finally driven.

For absolute movement measurement, the position of the magnetic element 15,, which marks a reference position, must be determined. This Position is determined with the aid of the digital Hall sensor 16.

Since the switching threshold of the digital Hall sensors as well as the magnetic flux density of the magnetic element change as a function of temperature, the determination of the reference position is dependent on the temperature, and hence so is the overall movement measurement. The method according to the invention, which is now proposed and is explained with the aid of Figure 2, is used for compensation of this temperature dependency.

in Figure 2, M indicates the magnetic flux density which occurs when moving past the magnetic element at the side. This magnetic flux density is recorded by the Hall sensor 16. This Hall sensor 16, which is constructed as a digital Hall sensor, has a first threshold Sl (switchingon threshold) and a second threshold S2 (switching-off threshold).

As long as the magnetic f lux density is less than the threshold Sl, the output signal of the digital Hall sensor indicates a f irst state,, and according to the selected example, the output signal exhibits a "high" state.

If the magnetic flux density exceeds the threshold value Sl, the digital Hall sensor suddenly switches over into its other state, and its output signal then exhibits a "low" state.

If the magnetic flux density falls again, the digital Hall sensor switches over suddenly from the "low" state into the "high" state again on falling below the second threshold S2.

This property of the digital Hall sensor is utilised to define precisely the reference position of the magnetic element 15. For this purpose, the moving part 10 is initially moved into a position in which the magnetic element 15 is so far away from the Hall sensor 16 that the magnetic f lux density is less than both thresholds of the Hall sensor. The moving part is then located, for example, at position A.

The moving part 10 is displaced with the aid of the drive motor 12 in such a manner that the magnetic element 15 finally reaches the position F. During this displacement, the magnetic flux density M, which is recorded by the Hall sensor 16, changes in the manner shown in the curve a) in Fig. 2.

As long as the magnetic element 15 is located between the position A and C, the output signal of the digital Hall sensor 16 is "high", once the switching threshold S1, (switching-on threshold), [sic] has been exceeded the output signal jumps to "low" and remains at "low" until, at position E, the magnetic flux density M falls below the threshold S2, (switchingoff threshold). At this point, the output signal of the digital Hall sensor 16 jumps to "high" again and remains in this state as long as the magnetic flux density remains below the threshold S1.

The signal response of the output of the digital Hall sensor 16 is shown in curve b) in Figure 2, the movement of the magnetic element being intended to take place in the direction of the arrow.

Curve c) in Figure 2 shows the same state of affairs for the case when the magnetic element 15 moves from the position F to the position A.

For this purposer the drive motor is reversed so that the moving part, including the magnetic element 15 and the mirror 14 which is to be adjusted, moves f rom right to left.

The output signal of the digital Hall sensor 16 is in this case initially "high", it jumps to "low" on reaching the position D, when the magnetic flux density M falls below the threshold S1, and remains in this state until, at the point B, the magnetic flux density falls below the threshold S2 again. This state is maintained until position A.

The position of the switching points C and D is stored, for example, in the evaluation device 18, and the reference position can be defined very accurately by halving the distance between C and D.

It would also be possible to store the points B and E and to define the position of the reference position exactly by halving the distance between B and E.

Figure 3 shows the same curves as in Figure 2, but for a different temperature. It has been shown that the thresholds S1 and S2 of a digital Hall sensor are displaced considerably as a function of the temperaturer so that in the case of an arrangement which is otherwise identical and in the case of an identical magnetic flux density M. the intersections between the magnetic flux density M and the thresholds S1 and S2 displace [sic] - If the determination of the reference position is carried out in the manner described in the explanation of Figure 2, so that the magnetic element 15 is initially moved past the Hall sensor 16 in the one direction and subsequently in the other direction, and if the determination of the reference position is then once again carried out in such a manner that half the distance between the positions H and J, and G and K, respectivelyr defines this reference position, then the temperature dependency of the thresholds and of the digital Hall sensor, respectively, is compensated for, independently of the prevailing temperature.

An evaluation can take place for so long [sic], f or as long as the magnetic f lux density M exceeds the threshold S1 and is less than the threshold S2.

The explanations given up to this point apply only when the moving part 10 can be moved between the positions A and F without the spindle, which finally ensures the movement, striking one of its stops. If the spindle, starting from the initially random original point A, reaches a stop before reaching a switching point of the digital Hall sensor, the motor direction must be reversed. The position of the switching point thus determined on the second Hall sensor is then stored for the subsequent centre determination.

once the motor has moved away from the recognised stop, the response shown in Figure 2 is produced again. The reference position can be determined in the previously described manner.

Claims (9)

Claims

1. Method for determining a reference position using a moving part which has a magnetic element which defines the reference position, and using a stationary sensor. preferably a digital Hall sensor, which is sensitive to magnetic fields and whose switching-on threshold and switching-off threshold are different, characterised in that the moving part (10) is displaced in one direction until the sensor (16) has switched on and off again, in that the moving part (10) is then displaced in the opposite direction until the sensor (12) has switched on and of f again, the position of at least two switching points (C. D or H, J, respectively) or (B. E or G, Kt respectively) being stored in an evaluation device (18), and the reference position being defined as the centre between the two identical switching points (C. D or H, J, respectively) or (B, E or G. K. respectively).

2. Method according to Claim 1, characterised in that the moving part 10 is displaced with the aid of a drive motor 12, which drives a spindle 11.

3. Method according to Claim 2. characterised in that the change in the direction of displacement is carried out by changing the direction in which current flows through the drive motor.

4. Method according to Claim 2 or 3, characterised in that the direction of movement is reversed when the spindle runs onto one of its stops, the stop position being stored and taken into account in the definition of the reference position.

5. Method according to one of the preceding claims, characterised in that the evaluation device (18) is a microcomputer.

6. Method according to one of the preceding claims, characterised in that a holder for an adjustable mirror is provided as the moving part.

7. A method for determining a reference position substantially as herein described with reference to the accompanying drawing.

8. An apparatus for determining a reference position substantially as herein described with reference to the accompanying drawing.

9. An apparatus for carrying out the method as claimed in any of claims 1 to 6.