GB2079473A - Improvements in a tachometer device controlled by an electric motor - Google Patents

Abstract

A tachometer device is constituted by an electric motor (2), a magneto- electrical transducer (8) sensitive to the HALL effect a speed indicator (7) provided with means (17) sensitive to magnetic field variations, a a permanent magnet (5) coupled angularly to a shaft (6) of said motor (2) and arranged to activate both the indicator (7) and the transducer (8). The magnet (5) has four or more poles and the transducer (8) is mounted on a ferromagnetic plate (25) extending over an arc great enough to span the angle between two adjacent poles of of opposite sign of the magnet (5). <IMAGE>

Description

SPECIFICATION Improvements in a tachometer device controlled by an electric motor This invention relates to a tachometer device con trolled by an electric motor.

In particular, this invention relates to a tachometer device which enables both rotary motion to be transmitted from a first to a second shaft by an electrical system, and the angular speed of said motion to be measured. More particularly, this invention relates to a tachometer device of the type comprising substantially: - a first generator which provides electrical pulses at a frequency proportional to the angular speed of rotation of the first shaft; - an electric motor, the shaft of which can be angularly coupled to said second shaft; - a speed indicator comprising means sensitive to magnetic field vibrations; - a second pulse generator provided with a magneto-electrical transducer; and - a permanent magnet coupled angularly to the shaft of said motor and arranged to activate both those means of the speed indicator which are sensitive to magnetic field variations, and the said magneto-electrical transducer. A device of the afore said type is illustrated in detail in U.K. patent application No. 2,020,028A entitled "Tachometer device controlled by an electric motor" and pub lished on 7th November 1979, the contents of which are incorporated herein for simple reference.In the said application, the magneto-electrical transducer used is an electrical contact element of reed type, the use of magneto-electrical transducers of other types being considered only from a theoretical viewpoint both because of the considerable and unfavourable cost difference between these and the reed contact, and because of the difficulties encountered in cor rectly positioning such transducers relative to said permanent magnet, in that extremely accurate assembly is required in order to obtain correct signals from such magneto-electrical transducers.

The use of the reed contact element however gives rise to certain drawbacks both of technical nature (reliability) and of application (for example as a tachometer in the automobile field). From the tech nical viewpoint, it has been found that the behaviour of the reed contact element is considerably influ enced by the speed of rotation of the permanent magnet carried by said motor shaft. In this respect, it is possible that the facing terminals constituting the electrical contact element vibrate at their own reso nance frequency, or that they remain in permanent contact with each other, so falsifying the reading provided by said indicator.Moreover, as the reed contact element is substantially a mechanical compoanent its terminals are subject to wear in that they are continuously subjected to bending stresses, so that progressive changes in their response to the magnetic field generated by the permanent magnet occur with time. Finally, the continuous alternating opening and closure of the electrical contact be tween said terminals gives rise in the long term to a progressive increase in the resistance at the contact zones. This increase can be prevented by using elements with silver-plated terminals, but this requires considerable additional expense.

It has also been noted that when used as a tachometer in the automobile field, the ticking noise generated by the reed contact element is often annoying to the driver, and cannot be easily reduced in that the tachometer must of necessity be mounted on the instrument dashboard in a zone immediately in front of the driver's seat.

The object of the present invention is to provide a tachometer device which is free from the aforesaid drawbacks of known devices.

The present invention provides a tachometer device of the type constituted by an electric motor, a speed indicator provided with means sensitive to magnetic field variations, a magneto-electrical transducer, and a permanent magnet coupled angularly to a shaft of said motor and arranged to activate both said means of said instrument and said magnetoelectrical transducer, characterised in that said magneto-electrical transducer comprises an element sensitive to the HALL effect which is rigidly coupled to and supported by a profiled member disposed between the casing of said motor and said permanent magnet.

The present invention will be more apparent from the description of a preferred embodiment given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 is a partly sectional perspective view of a tachometer device according to the present invention, with some parts removed for clarity; and Figure2 is a section through the device of Figure 1 on a vertical plane.

In Figure 1,the reference numeral 1 indicates overall a portion of a tachometer generator constructed according to the present invention and comprising substantially an electric motor 2 which receives a supply signal through cables 3 and 4; a permanent magnet 5 of cylindrical shape angularly coupled to a shaft 6 of the electric motor 2; a speed indicator 7 of known type, and a magneto-electrical transducer 8 both arranged to be controlled by the magnetic field generated by the permanent magnet 5. The device 1 also comprises a counter unit 10 of known type substantially constituted by a plurality of totalising drums 11 mounted on a shaft 12 which is angularly coupled to the shaft 6 of the motor 2 by means of a kinematic linkage 13.This latter comprises two shafts 14, 15, each of which has a first end coupled to the first end of the other shaft, and a second end coupled respectively to the shaft 6 of the motor 2 and the shaft 12 of the counter unit 10 by a worm-helical gear coupling. The indicator 7 is also of known type and comprises substantially a cup member 17 angularly coupled to one end of a shaft 18, on the other end of which is mounted a pointer 19, and a spiral return spring 20 having one end connected to the shaft 18 and its opposite end fixed to a fixed point of a support frame, not shown in Figure 1.

The magneto-electrical transducer 8 is constituted by an integrated circuit containing an element sensitive to the HALL effect and provided with a positive supply terminal 22 and negative supply terminal 23, and an output terminal 24 at which a signal of high or low level is present according to whether the magnetic field through the transducer 8 is greater or less than a predetermined value.

According to the present invention, the transducer 8 rests on a plate 25 substantially in the shape of a circular ring sector and constructed of ferromagnetic material to convey the lines of flux of the magnetic field generated by the magnet 5 towards the transducer 8. In the example illustrated, the magnet 5 is of the four pole type, and the plate 25 extends through a circular arc substantially equal to 900 in order for the lines of flux of the magnetic field generated by two adjacent poles of opposite sign of the magnet 5 to be conveyed about the transducer 8. The plate 25 rests in its turn on the upper surface of a printed circuit 26, to which the terminals 22, 23 and 24 of the transducer 8 lead.The circuit 26 is provided with an appendix 27 extending radially outwards which, at the side opposite that facing the plate 25, comprises three conducting tracks 28,29 and 30 (indicated by dashed lines) which enable each terminal 22, 23,24 of the transducer 8 to be electrically connected to respective connecting cables 32,33,34.

Figure 2 shows how the various parts of the tachometer generator 1 are assembled relative to each other in practice. In particular, a lower cup member 36 is provided for containing the motor 2, a framework 37 is provided for substantially supporting the indicator 7 and counter unit 10, and a profiled member 38 is mounted between the cup member 36 and framework 37 for substantially supporting the printed circuit 26, the plate 25 and transducer 8.

The profiled member 38 comprises an axial through bore 40 which allows passage of the shaft 6 of the motor 2, and is also provided with a pin 41 which engages in a seat 42 provided in the casing of the motor 2. The printed circuit 26, the plate 25 and transducer 8 are fixed to the profiled member 38 by hot-riveting of a pair of rivets 43a, 43b (see Figure 1).

In particular, the profiled member 38 has a surface 44 provided with a plurality of steps, on each of which there respectively rest the upper surface of the transducer 8 and part of the plate 25.

The aforesaid mounting arrangement obviates the drawbacks of known devicesJln this respect, the positioning of the transducer 8 relative to the magnet 5 is ensured to be correct both in an axial and radial direction, because said transducer 8 is precisely constrained to the profiled member 38 which rests in its turn on the casing of the motor 2, while the magnet 5 is mechanically constrained to the casing of the motor 2 by the motor shaft 6.

The use of an element sensitive to the HALL effect as the magneto-electrical transducer obviates all the drawbacks deriving from the use of the reed contact element, and consequently contributes to considerably increasing the reliability of the tachometer 1. In particular, by suitably choosing the transducer 8 it is possible to obtain at the output terminal 24 of this latter a signal with a square wave form and a duty cycle of 50%, so considerably facilitating signal processing.

Finally, it is apparent that modifications can be made to the aforesaid embodiment of the tachometer 1 without leaving the scope of the present invention.

For example, in order to obtain a signal having a frequency greater than that obtainable in the present case at the output terminal 24 of the transducer 8 for equal angular rotational speeds of the shaft 6 of the motor 2, it is necessary only to replace the magnet 5 with a magnet having a greater number of poles. In this case, the plate 25 must be sized to extend through an angle which enables the flux of two adjacent poles to be conveyed, i.e. to advantageously extend through an angle obtained by dividing a round angle (360 ) by the number of poles of the magnet used.

Claims (9)

1. A tachometer device of the type constituted by an electric motor (2), a speed indicator (7) provided with means (17) sensitive to magneticfieldvaria- tions, a magneto-electrical transducer (8), and a permanent magnet (5) coupled angularly to a shaft (6) of said motor (2) and arranged to activate both said means (17) of said indicator (7), and said magneto-electrical transducer (8), characterised in that said magneto-electrical transducer (8) comprises an element sensitive to the HALL effect which is rigidly coupled to and supported by a profiled member (38) disposed between the casing of said motor (2) and said permanent magnet (5).

2. A device as claimed in claim 1, characterised by comprising a plate (25) of ferromagnetic material on which said transducer (8) rests.

3. A device as claimed in claim 2, characterised in that said plate (5) is in the shape of a circular ring sector and extends over an angle arranged to convey towards said transducer (8) the flux of two adjacent poles of opposite sign of said permanent magnet (5).

4. A device as claimed in any one of the preceding claims, characterised by comprising a printed circuit (26) provided with conducting tracks (28,29, 30) having a first end connected to a corresponding terminal (22, 23, 24) of said transducer (8), and an opposite end arranged to be connected to a respective cable (32,33, 34).

5. A device as claimed in claim 4 dependent on claim 2 or 3, characterised in that said plate (25) rests on said printed circuit (26), and said printed circuit (26), said plate (25) and said transducer (8) are connected to said profiled member (38).

6. A device as claimed in claim 5, characterised by comprising a pair of rivets (43a, 43b) which connect said printed circuit (26) to said profiled member (38).

7. A device as claimed in any one of the preced- ing claims, characterised in that saidxtransducerA8) rests on a surface (44) of said profiled member (38).

8. A device as claimed in claim 7, characterised in that said surface (44) is shaped in the form of steps, and part of said plate (25) also rests on said surface (44).

9. A tachometer device substantially as described with reference to the accompanying drawing.