GB2257527A

GB2257527A - Eddy-current tachometer.

Info

Publication number: GB2257527A
Application number: GB9212803A
Authority: GB
Inventors: Wolfgang Wagner; Robert Krott; Manfred Strohrmann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1991-07-12
Filing date: 1992-06-17
Publication date: 1993-01-13
Also published as: FR2679036A1; DE4123128A1; JPH05188069A; GB9212803D0

Description

225)7527 - 1 2 11 Eddv-current tachometer The invention relates to a

tachometer using the eddycurrent effect.

Prior art

Described in DE-A-1,673,439 is a tachometer using the eddy-current effect and having the following features:

a magnetic field generating device with air gap; a rotor which traverses the air gap; a magnetic field measuring device which measures the strength of the magnetic field at the points at which the rotor leaves or enters the magnetic field; and a measuring circuit which forms from the measured values a signal which indicates the rotational speed of the rotor.

The magnetic field generating device in this tachometer consists of a magnet core and two permanent magnets, which enclose between themselves the said air gap. The magnetic field measuring device has at least one magnetic sensor, which measures magnetic fields using an arbitrary method. It is preferable to have two or four sensors present which are interconnected in such a way that the difference sensor forms a half bridge or full bridge. If two sensors are present, one is arranged in each case near the two ends of the air gap, seen in the circumferential direction of the rotor. When the rotor is stationary, the bridge is balanced with the aid of a zero adjustment such that the output signal which indicates the rotational speed is zero.

Tachometers using the eddy-current effect have the advantage that they are capable of reliably determining even very small rotational speeds. Consequently, they are suitable in particular for use in servo drives of any type, whether in adj usting members of machine tools or, for example, in vehicle steering systems.

So far, it has been typical to use specially designed tachos in order to measure very small rotational speeds. However, as a rule said tachos need to have a complex design and to be carefully produced for them to operate reliably. In order to compensate thermal changes in length of the shafts, the tacho must be connected as a rule to the servo motor via a bellows coupling. This applies, in particular, to vehicular technology, since there it is necessary to take account of relatively large temperature differences.

1 The considerable thermal fluctuations to which servo drives are frequently subject, such as, for example, in the vehicular technology already mentioned also entail that an eddy-current tachometer of the type mentioned at the beginning cannot be realised, since the air gap would have to be chosen to be very large in order to ensure in the entire range of the axial tolerance that the rotor does not stick at the boundaries of the air gap. However, if the air gap is made large there is an increase in the influence of malfunctions on the magnetic field which serves the purpose of measurement. Moreover, the field strength decreases, and this decreases the sensitivity.

The wish therefore existed to construct a tachometer using the eddycurrent effect so that it delivers reliable measurements in a relatively large temperature range.

Qv i 1 t1 ---1 r 1 Description of the invention

The tachometer according to the invention has the features of the abovementioned tachometer, although the rotor is constructed in the shape of a pot, that is to say with a circumferential skirt extending in the axial direction, and it is arranged relative to the magnetic field generating device in such a way that, and has its skirt constructed so tall that the latter essentially completely penetrates the field in the air gap continuously within the prescribed axial tolerance.

owing to the fact that measurement is now undertaken with the aid of a skirt on the rotor that extends in the axial direction, the skirt is essentially completely penetrated continuously by the air gap f ield of the magnet in all axial positions of the rotor. It is thus guaranteed that measurement is virtually entirely independent of the axial position of the rotor, which depends decisively on the particular temperature.

Since most types of magnetic field sensors have a temperature response, it is advantageous if the measuring circuit of the tachometer has a temperature compensation in order to compensate such a temperature response.

Owing to the additional skirt on the rotor of the tachometer according to the invention, there is an increase in the moment of inertia of the rotor. In order, nevertheless, to keep the latter at small absolute values, it is advantageous to construct the rotor as a plastic part in which at least one circumferential wall of the skirt is metal-coated.

It is possible to achieve a high measurement sensitivity during scanning of the skirt if the magnetic field generating device has a magnet core whose boundaries extend radially and/or which, seen in the direction of the rotor axis, is constructed in the shape of a U on both sides of the skirt and with limbs pointing towards the skirt.

Drawing Figure 1: Partial longitudinal section through a rotor, a magnetic field generating device and a magnetic field measuring device of a tachometer, along the line I-I of Figure 2, and block diagram of a measuring circuit;

Figure 2: Partial cross-section through the rotor, the magnetic field generating device and the magnetic field measuring device in accordance with Figure 1, along the line II-II in Figure 1;

Figure 3: Block diagram of a measuring circuit for determining the rotational speed of the rotor in the tachometer in accordance with Figure 1; and 1 Figure 4: Detailed representation of a measuring circuit similar to the block diagram of Figure 3.

Description of exemplary embodiments

The tachometer in accordance with Figures 1 and 2 has a rotor 10 with a rotor disc 10.1 and rotor skirt 10.2, a magnetic field generating device 11 with a magnet core 11.1 and four permanent magnets 11.2. a magnetic field measuring device with two magnetores is tors as sensors 12a and 12b, and a measuring device 13.

It may be seen f rom the top view of Figure 2 that seen in the direction of the axi..b 114 of the rotor 1n. the magnet core 11. 1 has radial boundaries, and that it is constructed in the shape of a U on both sides of the skirt 10.2 and with limbs pointing towards the skirt. One each of the four permanent magnets 11. 2 is accommodated in 61 i 3 11, each of the four limbs. In this case. said magnets are arranged with respect to their polarity such that in the two outer limbs the north poles point towards the skirt 10.2, while in the two inner limbs the south poles are directed towards the skirt. The polarity of the magnets can also be chosen inversely. Embedded in each case in the ends of the two outer limbs is one of the two sensors 12a and 12b, respect JILVely. The latter could, however, also be mounted on the ends of the limbs.

1 The rotor 10 consists of plastic, with a metal coating 10.3 on the outside of the skirt 10.2. Such a coating can, however, also be applied to the inside or both sides. Eddy currents are induced in this metal coating when the skirt 10.2 moves through the air gap 11.3 of the magnet core 11, that is to say when the rotor 10 rotates. Where the metal coating 10.3 leaves the air gap 11.3, the magnetic f ield generated by the eddy currents is directed such that the field in the air gap is amplified, while it is directed at the entry side such that the field in the air gap is attenuated. In other words, a polarity arises on the exit side which tends to restrain the metal coating moving out of the air gap, while on the entry side the polarity is directed such that it tends to obstruct the penetration of the coating into the air gap. The f aster the rotor 10 rotates, that is to say the faster the metal coating 10.3 moves through the air gap 11. 3, the more dif f erent the magnetic f ields which the two sensors 12a and 12b measure become.

As may be seen from Figure 1, the skirt 10. 2 with the metal coating 10.3 projects quite a way through the air gap 11.3. This projection is dimensioned such that even when the rot-or 10 movec upwards,,7-;thin the scope of axial play, for example due to a thermal expansion (in accordance with the representation of Figure 1), the magnetic conditions in the air gap 11. 3 remain essentially unchanged, because the skirt with its metal coating still essentially completely penetrates the magnet gap even then.

The measuring circuit 13 represented only as a single block in Figure 1 is represented in Figure 3 as a block diagram and in Figure 4 as a detailed circuit diagram with a few changes by comparison with a the simplified block diagram of Figure 3. In accordance with these circuit diagrams, the sensors 12a and 12b are supplied with constant current from a constant-current source 15. Each of the sensors 12a and 12b is connected to a d1_fferential amplifier 16a and 16b respectively, via a respective compensation circuit 17a and 17b for offset correction and thermalcompensation. The difference of the signals 16a. and 16b is formed in an output amplifier 18 and output as a signal which indicates the rotational speed w of the rotor 10. In order to ensure an output signal of value zero when the rotor is stationary, a zero adjustment 19 is further present. Reference is made to Figure 4 for the construction of the circuit sections 15 to 19. Here, it may be noted merely that the of f set correction is performed with the aid of potentiometers Pla and Plb in the respective compensation circuit 17a and 17b, and the zero adjustment is performed with the aid of a potentiometer P2 in the zero-adjustment circuit 19. A temperature-dependent resistor NTCa and NTCb, respectively, is present in each case for the purpose of thermal compensation in the compensation circuits 17a and 17b. These resistors are selected such that they have a temperature response which corresponds essentially to that of the sensors 12a and 12b. At least these temperature-dependent resistors are fitted such that essentially they continuously have the same temperature as the sensors 12a and 12b. However, the entire measuring circuit 13 can be arranged in effective thermal contact with the magnet core 11 and thus with the sensors.

J1 When a rotor 10 having a skirt diameter of 5 cm and a field strength of 270 mT was used, a sensitivity of 33 MV/(U/min) was measured using magnetoresistors as sensors, while with the use of Hall sensors the sensitivity was 17 mVI(Ulmin).

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Claims

1 1. Tachometer using the eddy-current effect, having: - a magnetic field generating device (11) with air gap (11.3); - a rotor (10) which traverses the air gap; - a magnetic field measuring device (12a, 12b) which measures the strength of the magnetic field at the points at which the rotor leaves or enters the air gap; - and a measuring circuit (13) which forms from the measured values a signal which indicates the rotational speed of the rotor; characterised in that the rotor (10) is constructed in the shape of a pot, that is to say with a circumferential skirt (10.2) extending in the axial direction, and is arranged relative to the magnetic field generating device (11) in such a way that, and has its skirt constructed so tall that the latter essentially completely penetrates the field in the air gap (11.3) continuously within the prescribed axial tolerance.

2. Tachometer according to Claim 1, characterised in that the magnetic f ield generating device (11) has a magnet core (11. 1) with, seen in the direction of the rotor axis (14), radially extending boundaries.

3. Tachometer according to one of Claims 1 or 2, characterised in that the magnet core (!'L. 1), seen in the direction of the rotor axis (14), is constructed in the shape of a U on both sides of the skirt (10.2) and with limbs pointing towards the skirt.

4 4. Tachometer according to one of Claims 1 to 3, characterised in that the rotor (10) is a plastic part in which at least one circumferential wall of the skirt is metal-coated (10.2).

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5. Tachometer according to one of Claims 1 to 4.. characterised in that the measuring circuit (13) has thermal compensation (17a, 17b, NTCa, NTCb) for compensating the temperature response of the magnetic field measuring device (12a, 12b).

6. A tachometer substantially as herein described with reference to the accompanying drawings.

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