GB2123962A - Electronic tachometer for vehicles having different tyre diameters - Google Patents

Abstract

A tachometer device (1) for vehicles includes a first pulse generator (2) generating an electrical signal the frequency of which is proportional to the speed of the vehicle, an electric motor (11), a second pulse generator (25) the frequency of which depends on the speed of rotation of the drive shaft of the said motor (11), an eddy current speedometer (16) driven by the motor (11), and a microprocessor (4) which compares the signals generated by the pulse generators and supplies the motor (11) accordingly. Programming means (26) provides the microprocessor (4) with a set of electrical logic signals, the combination of which is proportional to the desired ratio between the speed of the vehicle and the speed of rotation of the shaft (12) of the said motor (11) to permit calibration of the speedometer for different tyre diameters. A conventional odometer 12-17 can be provided. <IMAGE>

Description

SPECIFICATION Electronically controlled tachometer device The present invention relates to a tachometer device and in particular to an electronically controlled tachometer device which can be mounted on a vehicle for the purpose of indicating the speed at which the vehicle is moving.

Known tachometer devices of the type on which the device in question is based, comprise in the simplest version, a pulse generator electrically coupled, in use, to a drive take-off connected for rotation with the wheels of the vehicle, and an indicator instrument connected to this generator. The value of the angular velocity at which the drive take-off rotates is converted by the generator into a corresponding frequency of the electrical pulses which it generates; this frequency value is then converted into an indication of the speed per hour of the vehicle by means of the said indicator instrument.

It is known that once such tachometer devices are made they can be mounted on vehicles which have very different characteristics from one another. For example, the diameters of the associated wheels could be different so that, at the same angular velocity of the drive take-off, the indicator instru ment would have to indicate two different values of the speed per hour, or else, for the same vehicle, it could be necessary to be able to provide an indication of the speed in kilometers per hour as well as in miles per hour.

For the purpose of reducing the costs to the minimum, the known tachometer devices have generators and indicator instruments with standar dised characteristics and are adapted as necessary to the characteristics of the vehicle on which they are to be mounted by introducing correction gear trains in a position intermediate between the drive take-off and the pulse generator. Such a modification requires first of all the availability of a space which is not always easily available and, moreover, is not always entirely satisfactory, especially when it is necessary to adapt from kilometers per hour to miles per hour. In this latter case, in fact, the conversion coefficient (multiplier or divisor) is not easily obtain able with precision by means of a simple ratio between the teeth of two or more gears.

The object of the present invention is that of providing a tachometer device which overcomes the above specified disadvantages of known devices.

The said object is achieved with the present invention in that it relates to a tachometer device for measuring the spseed of a vehicle, and of the type comprising a first electrical pulse generatrwhich can be coupled to a drive connection of the said vehicle so as to generate an electrical pulse signal the frequency of which is proportional to the instantaneous value of the speed of the said vehicle, an electric motor, a second pulse generator the fre quency of which depends on the speed of rotation of the shaft of the said electric motor, a speed indicat ing instrument driven by the said electric motor, and processor means which compare signals generated by the said first and second pulse generators and which generate an electrical signal utilised for supplying the said motor; the said device being characterised by the fact that it includes programmer means which provide the said processor means which a set of electricsal signals substantially of logic type the combination of which is related to the desired ratio between the speed of the said vehicle and the speed of rotation of the said motor shaft.

For a better understanding of the present invention a preferred embodiment will now be more particularly described, by way of example, with reference to the attached drawings, in which: Figure 1 is a schematic and simplified view of the mechanical and electronic part of the device in question; and Figure 2 is a block schematic diagram in more detail of the electronic part of Figure 1.

With particular reference to Figure 1 a tachometer device formed according to the principles of the present invention is generally indicated with the reference numeral 1, and this substantially comprises a pulse generator 2 driven from a drive take-off 3 and supplying by means of a processor circuit 4, an indicator instrument 5 of known type. In use, the drive take-off 3 is coupled to a wheel 6 of a vehicle the speed per hour of which it is desired to detect.

The pulse generator 2 is of known type and substantially comprises a wheel 7, conveniently made of ferro-magnetic material and having radial projections 8, and a detector circuit 9, conveniently an oscillator of the locked type. In use, upon the passage of each radial projection 8 in front of the locked oscillator 9, this latter emits an electric pulse the frequency of which is therefore proportional to the value of the angular velocity of the drive take-off 3 and therefore, via the wheel 6, to the value of the speed of movement of the vehicle.

The indicator instrument 5 is also of known type and substantially comprises a motor 11 supplied from the processor circuit 4 and having a shaft 12 coupled at one end for rotation with a four pole permanent magnet 13 of circular type, and at the opposite end with a worm screw 14. This latter meshes with a worm wheel 15 which is in turn connected to a shaft 16 driving a speedometer unit generally indicated 17.

The indicator instrument 5 further includes a movable component 18 provided with a shaft 19 rotatable about its axis against the action of a biasing spring 20, a needle which swings over a dial 22 and a disc 23 connected to the shaft 19 at the end opposite the needle 21. In particular, the disc 23 is mounted facing the permanent magnet 13 and is such as to have eddy currents generated therein (Foucault currents) when the magnet 13 is driven to rotate by the shaft 12 of the motor 11. This magnet 13 also drives, by means of the flux generated by its poles, a magneto-electric transducer 25 which substantially acts as a pulse generator. The outputs of such transducer 25 are respectively connected to a signal input of the processor circuit 4 and to a connection terminal 27.

The processor circuit 4 also has a plurality of programming inputs which can receive signals from a programming unit generally indicated 26 and better described with reference to Figure 2.

According to the present invention and with particular reference to Figure 2, the processor cicuit 4 includes a processor unit 30 of the micro-processor type, and a digital-to-analogue converter 31 connected to the outputs of the unit 30, which supplies the motor 11 via an amplifier 32 having an output stage 33 of the emitter-followertype. In more detail, the unit 30 has a clock input 34 to which is supplied a signal from an oscillator circuit 35, two signal inputs 36,37 at which arrive signals generated by the pulse generators 2 and 25 respectively, through filters 38 and 39, and a plurality of programming inputs 40 each of which is connected to a first terminal of a selector constituting the programming unit 26.A second terminal of this selector is connected to earth, and there are further provided several short circuiting bridges, generally indicated 41, provided in the said selector between some of the said terminals for the purpose of connecting the corresponding input 40 of the unit 30 to earth.

The digital-to-analogue converter 31 is of the weighted resistive network type and has in particular a first set of resistors 43 having one terminal connected to an associated output of the unit 30 and an opposite terminal connected to the junction points of a second set of series resistors each of which is indicated 44. An end one of the resistors 44 has a terminal connected to earth, whilst the resistor 44 at the opposite end has a terminal which is connected to the inverting input (-)of an operational amplifier 45 constituting the active element of the amplifier 32. This latter further includes a feedback resistor 46 and, in the emitter-follower stage 33, a biasing resistor 47 and a transistor 48.The operational amplifier 45 finally has a non-inverting input (+) which is connected to the cursor of a potentio meter 49 which has a terminal connected to earth and an opposite terminal connected to an output 50 of the unit 30 at which there appears a stabilised dc voltage signal. Finally, two inputs 51 and 52 are connected to the inputs of the filters 38 and 39 and as low suitable signals to be transmitted to the input of these filters.

The operation of the device 1 is in principle of known type. In fact, with reference to Figure 1,the rotation of the wheel 6 causes the pulse generator 2 to transmit a pulse signal to the processor circuit 4, which signal supplies the motor 11,acting in such a way that there is a correspondence between the signal provided by the generator 2 and that provided by the pulse generator 25 upon rotation of the shaft 12 of the motor 11. The inventive part of the device 1 lies in the particular operation of the processor circuit 4 which is now described with reference to Figure 2.

In particular, at the end of the manufacturing stage, and in any case, at any time during operation, it is possible to check the correct behaviour of the processor unit 30 as far as the correct operation of the outputs to the digital-to-analogue converter are concerned. Such testing is effected by short circuting all the terminals of the programming unit 26 and sending suitable logic signals to the inputs 36 and 37 via the terminals 51 and 52. In more detail, when both inputs 36 and 37 have a signal at the "0" logic level all the outputs of the unit 30 should be at the "0" logic level such that the output signal from the digital-to-analogue converter 31 will be zero.This output signal must be a maximum, on the other hand, when the inputs 36 and 37 both have a signal at the logic level "1" simultaneously; finally the output signal from the converter 31 must be, for example, 2/3 and 1/3 of the maximum value mentioned bove when the inputs 36 and 37 have a signal at the logic level "1" and "0" respectively and "0" and "1" respectively. In this way it is possible to verify both the exact behaviour of the output of the unit 30 and the correct operation of the digital-toanalogue converter 31.

At the end of the self checking phase of the unit 30 the short circuiting to earth of all the inputs 40 is removed and the bridge connectors 41 are disposed as for example indicated in Figure 2 according to a predetermined configuration corresponding to the particular circumference of the wheel 6 of the motor vehicle the speed of which it is desired to measure.

In the example illustrated in Figure 2 there are 63 different possible combinations, there being in fact six sets of terminals and it being necessary to exclude the combination in which all the pairs of terminals are connected to earth since such combination is in fact utilised to perform the self checking described above.

Therefore by detecting the number and position of the inputs 40 connected to earth the unit 30 is able to correlate exactly the pulses coming from the generator 2 with the pulses coming from the generator 25 in such a way as to establish the exact voltage value which must be present at the output of the digital-toanalogue converter 31 and therefore, finally, the desired ratio between the speed of the vehicle and the speed of rotation of the shaft 12 of the electric motor 11. in fact, this latter speed is also proportional to the value of the induced current in the disc 23 and therefore to the rotation of the needle 21 on the dial 22.Given that the signal available at the output of the digital-to-analogue converter 31 varies incrementally, to each increment of which corres ponds a given speed variation, it is arranged that such minimum variation be rather small (in particular of a value less than one kilometer per hour).

When the speed at which the vehicle is moving lies between two values corresponding to two different adjacent supply voltage levels for the motor 11, the unit 30 emits alternately the first and then the second such value at a frequency of, preferably, 1 Hz in such a way that the movable assembly 18, because of its inertia, effectively integrates between the two such values.

From an examination of the characteristics of the device 1 formed according to the principles of the present invention it is possible to see how it allows the above specified objectes to be achieved. In fact, such device lends itself to be mounted without modification on any type of vehicle in that it is possible to choose between a very high number of combinations (63) corresponding to different cir cumferences of the vehicle wheels, simply by suit ably combining the six possible short circuiting bridge connectors 41 in the programming unit 26.

The device 1 is therefore easily adaptable and extremely flexible in use as well as being easily checked as far as the correct operation is concerned it being also pre-arranged for a self checking operation. The signals which the pulse generator 25 provides can finally also be used, by means of the terminal 27, for any other processing by other instruments on board the vehicle in question.

Finally, it is clear that the tachometer device 1 described above can be modified and varied without by this departing from the scope of the present invention.

Claims (6)

1. A tachometer device for measuring the speed of a vehicle, and of the type comprising a first electrical pulse generator which can be connected to a drive take-off of the said vehicle whereby, to generate an electrical pulse signal the frequency of which is proportional to the instantaneous value of the speed of the vehicle, an electric motor, a second pulse generator the frequency of which depends on the speed of rotation of the drive shaft of the electric motor, an indicator instrument for indicating the speed of rotation of the said electric motor, and processor means which compare the signals generated by the said first and second pulse generators and which generate an electric signal utilised for supplying the said motor; the said device being characterised by the fact that it includes prog ramming means which provide said processor means with a set of electrical signals substantially of logic type the combination of which is related to the desired ratio between the speed of the said vehicle and the speed of rotation of the shaft of the said motor.

2. A device according to Claim 1, characterised by the fact that the said processor means include a micro-processor unit, and by the fact that the said programming unit includes a plurality of pairs of terminals each of which is connected on one side to a respective input of the said unit and on the other side is connected with a terminal held at a reference potential; each of the terminals constituting the said pairs being connectable together in short circuit by means of a suitable bridging connector.

3. A device according to Claim 2, characterised by the fact that in a first predetermined electrical connection configuration between the said pairs of terminals of the said programming unit the said processor unit generates at its output electrical signals depending on a predetermined combination of signals at the inputs to which the said first and second pulse generators are connected.

4. A device according to any preceding Claim, characterised by the fact that the said processor means include a digital-to-analogue converter the inputs of which are connected to corresponding outputs of the said processor unit.

5. A device according to Claim 4, characterised by the fact that the said digital-to-analogue converter is of the weighted resistive network type.

6. A tachometer device for measuring the speed of a vehicle substantially as described with reference to the attached drawings.