GB2186979A

GB2186979A - Device for measurement of angular velocity of a rotating body

Info

Publication number: GB2186979A
Application number: GB08703147A
Authority: GB
Inventors: Helmut Steffes
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1986-02-25
Filing date: 1987-02-11
Publication date: 1987-08-26
Anticipated expiration: 2007-02-11
Also published as: FR2594956B1; FR2594956A1; DE3605995C2; GB8703147D0; GB2186979B; DE3605995A1

Abstract

A device for measurement of angular velocity of a rotating body comprises a transducer (1) composed of a core (2), an exciting winding (4) and a measuring winding (3). Said transducer (1) is arranged at the periphery of a toothed disc (10) co-rotating with the rotating body. The current flowing through the exciting winding (4) and thus the intensity of the magnetic field in the air gap between the transducer (1) and the toothed disc (10) is controlled in dependence upon the magnitude of the voltage induced in the winding (3) in such a fashion that, in the event of a standstill or a low angular velocity, maximum current flows, which current reduces when the angular velocity increases. In this way the induced signal strength is rendered less variable. An evaluation circuit is also connected to winding (3), the frequency of the signal voltage being a measure of the speed of disc (10). The arrangement may be used for measuring wheel speed in an anti-lock braking system. <IMAGE>

Description

SPECIFICATION Devicefor measurement of angular velocity of a rotating body This invention relates to a device for measurementof angular velocity of a rotating body, i.e. a wheel, a shaft or the like, and for the generation ofan alternating signal whose frequency is proportional to the angularvelocity, including a measuring data emitter in the form of a toothed disc, and a stationary, inductive transducer arranged atthe periphery ofthe toothed disc and separated from the toothed disc by an air gap, which transducer creates a magnetic field penetrating the air gap.

Devices with transducers, i.e. sensors of this kind are, for instance, used for the measurement of the rotational behaviour of wheels in electronically controlled anti-wheel-lock brake systems. Within a controller of such brake systems, the sensor signals are logically combined and processed by means of wired or programme-controlled electronic switching circuits. In this manner, control signals are generated which are, for instance, transmitted to electromagnetically actuated hydraulic multi-directional control valves, and thus lead to the desired brake pressure modulation and finally to wheel slip control.

Inductive transducers ofthis kind are known in a great many forms. In West German printed and published patent specification 201901 a sensor is described which comprises a permanent mangnet, and which is also arranged at the periphery of a toothed disc serving as a measuring data emitter and co-rotating with the rotating body. Between the pole shoes of the magnet and the toothed disc, there is a small air gap wherein a magneticfield generated by the permanent magnet is formed. Due to the passing from tooth to tooth gap, the magnetic field is changed during the rotation of the toothed disc and induces an alternating signal in a measuring coil wound around the core ofthe permanent magnet, the frequency of said alternating signal being proportional to the rotational movement ofthe toothed disc.The amplitude of the measured signal also increases almost linearly with the rotational movement. This is disadvantageous in that at slow revolutions, a measured signal having only a very low amplitude is available, which measured signal can only be separated from the inevitable unwanted signals using a great deal of electronic equipment. In addition, it cannot be avoided that during standstill ofthe wheel, the toothed disc is magnetized by means of the permanent magnet field, which also results in unwanted signals during operation.

Finally, the functioning of this known sensor and/or measuring device is, in addition, impaired by means of ferrous particles which may accumulate at the poles underthe influence of the permanent magnet field and may lead to short-circuit bridges.

Devices for the measurement of velocities and/or transducers of a similartype are, e.g., also illustrated and described in West German printed and published patent specification 2113307 and West German printed and published patent application 34 00 870.

The present invention has an object to overcome the above-mentioned disadvantages of known sensors and to develop a device forthe measurement of the angular velocity of a rotating body which device supplies, overthe whole working range, in particular even at low angular velocities, a measured signal or waned signal which can be separated from unwanted signals and which can be evaluated by means of comparatively little electronic equipment. A high operational reliability even over longer periods of time is, in addition, required.

According to the present invention there is provided a device for measurement of ang u lar velocity of a rotating body, and for the generation of an alternating signal whose frequency is proportional to the angularvelocity, including a measuring data emitter in the form of a toothed disc, and a stationary, inductive transducer arranged at the periphery of the toothed disc and separated from the toothed disc by an air gap, which transducer creates a magneticfield penetrating the air gap, characterised in that the magneticfield is created by means of an exciting winding wound around a coil core, through which exciting winding a controlled direct currentflows when the measuring device is in operation, and in that a second winding, namely a measuring winding, is provided wherein an output signal of the transducer representing a measured signal can be induced in the event of a change ofthe magneticfield as a resultofa rotational movement of the toothed disc.

Acontrolled magneticfield is thus generated in the air gap between the toothed disc and the transducer instead of a permanent magnet field. When the electrical power supply is switched off, which condition prevails in an automotive vehicle when the ignition has been switched off, the magnetic field is reduced to zero so that iron particles or rubbed-off parts accumulated within the area ofthe air gap at the pole or at the pole shoes can fall off and can thus no longer lead to malfunctions. By means of the control of the exciting current it is, in addition, achieved that a high magnetic field is generated even at low velocities, said magneticfieid creating during this phase of operation a measured signal having a comparatively high amplitude.When the velocity is increased, however, the controlled exciting current decreases, the load on the battery thus being reduced and a constant or only slightly increasing measured signal amplitude being achieved. The evaluabilityofthe measured signal isthusimproved considerably, and the electronic equipment required for this purpose is reduced.

According to a favourable embodiment of the present invention, the transducer is equipped with an electronic closed-loop control ci rcuit for producing the controlled direct currentforthe exciting winding.

The amount of the direct current flowing through the exciting winding is, expediently, controllable in depenence upon the magnitude of the measured signal of the transducer following rectification thereof. In many cases it is favourable if the magnitude of the controlled direct current is variable at least approximately inversely proportional to the magnitude of the rectified measuring signal.

Both windings, namely the exciting winding and the measuring winding, can be wound around a common coil core which includes an end face and/or pole face directed towards the toothed disc, said face in part defining the air gap between the toothed disc and the transducer, which air gap is penetrated by the magnetic field.

The closed-loop control circuit may comprise a variable resistance, e.g. a transistor, through the intermediary of which the exciting winding is connected to a direct voltage or direct current source, which resistance can be varied at least approximately proportional to the magnitude of the rectified measured signal. Between the control connection of the variable resistance- this is, in the aforementioned special case, the base ofthe transistor-and the output of the measuring winding, an amplifier stage with a rectifier arranged downstream thereof, e.g. a half-wave rectifier, is expediently provided.

An embodiment ofthe invention will now be described with reference to the accompanying drawings, in which; Figure 1 illustrates a sectional view of a transducer according to the invention and its arrangement relative to a toothed disc; and Figure2 shows a particularly simple embodiment of a closed-loop control circuit intended to influence the exciting current ofthetransducer.

As shown in Figure 1,thetransducer 1 ofthe measuring device according to the present invention comprises substantially a core 2 andtwo coils or windings 3, 4, namely an exciting winding and a measuring winding. Both coils are wound around a coil carrier 5. The winding terminals are referred to as 6,7 and 8. The core 2 normally consists of ferromagnetic material. The whole arrangement is located in a housing 9 protecting said arrangement against mechanical influences and humidity.

The transducer 1 is, for instance, fixed to vehicle axle (not shown),whereas a toothed disc 10 the form of a measuring data emitter co-rotates with a wheel, shaft or another rotating body (not shown) the angular velocity of which isto be measured.

The core 2 of the transducer 1 extends from its end or pole face 11 directed towards the toothed disc 10 into the housing, and is shaped such that a magnetic field having a relatively high density is created in the air gap between the toothed disc 10, the measuring data emitter, and the pole or end face 11 ofthe transducer 1,so that during operation, when the disc 10 is rotating,the magnetic flux changes so strongly during the transition from one tooth to a tooth gap that even in the event of a low angular velocity, as a consequence ofthis change in flux, a sufficient measuring voltage is induced in the measuring winding. In this case, the frequency ofthe measured voltage is directly proportional to the angular velocity and to the number of teeth of the toothed disc 10.At a constant exciting current, the magnitude of the measured voltage wou Id increase and decrease proportionally to the angularvelocity.

A closed-loop control circuit of the device according to the invention is illustrated in Figure 2. In this case, an exciting current, namely a controlled direct current derived from a vehicle battery U5 here serving as a voltage source, flows via the terminals 7 and 8. The magnitude of this exciting current is controlled in dependence upon the measured signal picked up by the terminals 6 and 7 of the measuring winding by means of a controllable variable resistance, here a transistor T2.For this purpose, an amplifierstageV1 comprising in this case, substantially, a transistor Ti, a voltage divider R1, R2 as well as collector and emitter resistances R3, R4, with a half-wave rectifier D1, C4arranged downstream of said amplifier stage, is connected to the terminals 6 and 7 ofthe measuring winding through the intermediary of a coupling capacitor C1.

The transistor T2 which serves as a variable resistance and which determines the amountofthe exciting current is actuated via a voltage divider R5, R6 at the outlet of the half-wave rectifier Di, C4. "B" designates the base, "E" the emitter and "C" the collector of the transistorT2.

An evaluation circuitwherein thefrequency ofthe measured signal and thus the information aboutthe rotational behaviour ofthe wheel is evaluated is, in addition, connected to the terminals 6,7 of the measuring winding 3via signalling lines (not shown).

The closed-loop control circuit depicted in Figure 2 operates as follows: As long as the wheel whose angularvelocity isto be measured, and thus the toothed disc 10, stands still or only rotates at a low velocity, the voltage at the output of the half-wave rectifier and/or across the voltage divider R5, R6 is minimal. After switching the ignition and thus the supplyvoltage U5 on, the transistor T2 is, therefore, switched so as to be open through the intermediary ofthe base resistance R6.

During this phase, the maximum possible exciting currentwhich is, substantially, only limited by means of the emitter resistance R7 and the internal resistance of the exciting winding 4flows via the transistorT2 and via the winding 4.

At first,the transistorT1 ofthe amplifier stage is also switched so asto beopenthroughthe intermediary ofthe base resistance Ri. However, as soon as the amplitude ofthe measured signal, namely the alternating voltage between the terminals 6 and 7, increases due to higher angular velocities of the toothed disc 10, the capacitor C4 at the output of the am plifier stag e V1 is charged during the blocking phase ofthetransistorT1 via the collector resistance R3, capacitor C3 and the diode Di. Avoltage drop which leads to an increase in the potential atthe base B of the transistorT2 and thus to a reduction of the exciting currentflowing via the transistorT2 is generated acrossthecapacitorC4 and thus across the voltage divider R5, R6. This results, on the other hand, in a reduction ofthe magneticfield between the core 2 and the toothed disc 10 and thus of the voltage induced in the measuring winding 3 when the angularvelocity of the disc 10 remains constant. By means of this "counter coupling", the amplitude of the measured signal atthe output of the measuring winding 3 becomes (almost) independent of the angular velocity; in any case, the amplitude variation is less significant as compared with transducers with a constant magnetic field. In this manner, the evaluabilityofthe measured signal, in particularthe separation between unwanted and wanted signal, is improved considerably.

The depicted closed-loop control circuit represents one of many possibilities. When designing the control circuit, attention must be paid, above all, to the fact that the exciting current reaches, in the manner described above, a maximum, after the ignition has been switched on, in the event of a standstill of the wheel or a low angular velocity and is reduced to a minimumvalueatan increasing measuredsignalorwantedsignalin amanner depending on the respective case of application.

Claims

1. A device for measurement of angular velocity of a rotating body, and for the generation of an alternating signal whose frequency is proportional to the angular velocity, including a measuring data emitter in the form of a toothed disc, and a stationary, inductive transducer arranged atthe periphery of the toothed disc and separated from the toothed disc by an air gap, which transducer creates a magnetic field penetrating the air gap, characterised in that the magneticfield is created by means of an exciting winding (4) wound around a coil core (2), through which exciting winding a controlled direct current flows when the measuring device is in operation, and in that a second winding (3), namely a measuring winding, is provided wherein an output signal ofthetransducer (1 ) representing a measured signal can be induced in the event of a change of the magnetic field as a result of a rotational movement of the toothed disc (10).

2. A device according to claim 1, characterised in that the transducer (1) is equipped with an electronic closed-loop control circuit for producing the controlled direct currentforthe exciting winding (4).

3. Adeviceaccordingtoclaim 1 orclaim2, characterised in that the magnitude of the direct current flowing through the exciting winding (4) is controllable in dependence upon the magnitude of the measured signal of the transducer (1) following rectification thereof.

4. A device according to claim 3, characterised in that the magnitude of the direct currentflowing through the exciting winding (4) is variable at least approximately inversely proportional to the magnitude ofthe rectified measured signal.

5. A device according to any one of the preceding claims, characterised in that the exciting and the measuring winding (4, 3) are both wound around the coil core (2) which includes an end face and/or pole face (11) directed towards the toothed disc (10), said face in part defining the air gap between the toothed disc (10) and the transducer (1 ),which air gap is penetrated by the magneticfield.

6. A device according to claim 3 as appendentto claim 2, characterised in that the closed-loop control circuit comprises a variable resistance (T2) through the intermediary ofwhich the exciting winding (4) is connected to a direct voltage or direct current source (UB), which resistance can be varied at least approximately proportional to the magnitude ofthe rectified measured signal.

7. A device according to claim 6, characterised in that a transistor actuated by the rectified measured signal is provided as variable resistance (T2).

8. A device according to claim 6 or claim 7, characterised in that an amplifier stage (Vi) with a rectifier (D1, C4) arranged downstream thereof is provided between a control connection (B) ofthe variable resistance (T2) and the output of the transducer (1 ) and/or the measuring winding (3).

9. A device for measurement of angularvelocity of a rotating body substantially as herein described with reference to and as illustrated in the accompanying drawings.