Meter overdrive protection circuit

A meter overdrive protection circuit is provided using a variable duty cycle drive pulse. Such meters usually comprise speedometers and tachometers used in highway vehicles such as trucks. A programmable pulse generator is utilized to provide a series of pulses of a selected width depending on the truck model. A sensor input corresponding to the rate of rotation of the drive shaft of the truck is used to trigger the pulse generator to produce each output pulse. Such output pulses are supplied to a meter drive which, based on the time average value of the output pulses received, displays an output value related to the rate of rotation of the truck drive shaft. Upon the triggering inputs exceeding a preselected frequency, a microcomputer will change the pulse generator operation to a fixed duty cycle operation whereupon the meter will be held to a full scale deflection. When the triggering inputs decrease below the preselected frequency, the pulse generator will be returned to a triggering output mode. Such meter drive circuit protects electromagnetic meter needle movement from damage due to full scale pinning against a mechanical stop.

BACKGROUND OF THE INVENTION 
The present invention relates to a meter overdrive protection circuit, and 
more particularly, to a programmable meter drive circuit having an 
overdrive protection scheme. 
In developing a speedometer or tachometer for a highway vehicle such as a 
truck, it is desirable to utilize a programmable pulse source as a meter 
drive. Differences in tire size and gear ratios for the particular truck 
models can be programmed into the pulse source such that a particular 
width pulse is utilized for a particular truck. The width of the pulse is 
related to revolutions of the transmission shaft in the case of a 
speedometer or revolutions of the drive shaft in the case of a tachometer. 
A microcomputer is usually utilized to program the pulse generator to 
produce pulses of a width necessary to provide proper meter deflection. 
The programmable pulse generator usually comprises an integrated circuit 
chip having a pulse generation mode. The output pulses from the pulse 
generator are utilized to drive the meter. The meter drive typically 
comprises an electromechanical meter movement which performs a time 
averaging of the pulses received from the pulse generator and produces a 
corresponding deflection in the meter needle. 
The pulse generator is dependent on a trigger signal received from and 
generated by a reluctance sensor coil positioned with a rotating gear on 
the appropriate rotating shaft of the vehicle. In a tachometer 
application, such gear is usually the fly-wheel, and in a speedometer 
application, such shaft is usually the transmission output drive shaft. 
The rotating device contains teeth which induce a current in the 
reluctance coil sensor. Such current is of a sinusoidal nature, and this 
current is input to the pulse generator to typically cause the triggering 
of the pulse generator upon receipt of a positive waveform from the 
sensor. 
In the event of an overspeed condition of the rotating device, an excessive 
number of trigger signals will be received by the pulse generator. The 
width of the pulse generator output waves will not change, but rather the 
frequency of such output waves will increase due to the increased frequeny 
of trigger signals. Accordingly, the meter deflection will increase and 
correctly reflect the increased rate of rotation of the shaft being 
measured. However, if the frequency of such triggering signals exceeds a 
certain rate, the time average value of the pulse generator output would 
exceed the preselected value required for full scale meter deflection and 
cause the needle to be pinned against a mechanical stop. It is an object 
of the present invention to provide a scheme of avoiding the mechanical 
pinning and possible damage to the meter needle. 
Another problem which could occur in the event of an overspeed operation of 
the rotating device is that the triggering signals from the sensor device 
could increase in frequency to a point such that, due to the width of the 
pulse generator output pulses, a triggering signal could be received 
during a pulse generator on condition. Such a triggering signal would be 
ignored since the pulse generator was already in an on condition, and 
accordingly an incorrect reading of the rotation rate of the shaft would 
be reflected in the meter device. Accordingly, it is another object of the 
present invention to provide a method of avoiding such incorrect meter 
indication of an overspeed condition. 
SUMMARY OF THE INVENTION 
The present invention provides a programmable meter drive circuit having a 
meter overdrive protection scheme. Such meters are typically speedometers 
or tachometers in highway vehicles such as trucks. In the case of a 
speedometer, the particular vehicle tire size and gear ratio effect the 
speedometer reading. Accordingly, a microcomputer is programmed to 
identify the particular vehicle model and in turn to program a pulse 
generator to produce pulses of a width necessary to provide proper meter 
deflection. The programmable pulse generator usually comprises an 
integrated circuit chip having a pulse generation mode. The pulse 
generator output is a series of square waves the width of each of which is 
dependent on the vehicle model. The pulse generator is triggered to 
produce such an output wave upon receipt of a triggering pulse from a 
sensor circuit. The sensor circuit is typically a reluctance coil 
positioned with a rotating gear on the appropriate rotating shaft of the 
vehicle. The rotating device contains teeth which induce a current in the 
reluctance coil. Such current is of a sinusoidal nature, and this current 
is input to the pulse generator to typically cause the triggering of the 
pulse generator upon receipt of a positive wave form from the sensor. The 
output pulses from the pulse generator are utilized to drive a meter 
usually comprising an electromagnetic needle movement. Such meter drive 
performs a time averaging of the pulses received from the pulse generator 
and produces a corresponding deflection in the meter. 
As the rate of rotation of the shaft on which the sensor device is located 
increases, the triggering signals from the sensor device will increase in 
frequency. Such triggering signals can be thought of as spike pulses. 
Because the width of the pulse generator output waves does not change once 
the particular vehicle model is identified to the microcomputer, the 
frequency of the output waves increases with the increased frequency of 
the triggering signals from the sensor device. Accordingly, the meter 
deflection increases to correctly reflect the increased rate of rotation 
of the shaft being measured. 
One problem which can occur upon the rate of rotation of the shaft 
increasing beyond a preselected maximum value is that the meter needle 
will be deflected to a maximum point usually encountering a mechanical 
stop. This is undesirable as such pinning of the needle against a 
mechanical stop can damage the meter movement and introduce error into 
future readings. Another problem which can occur upon the increasing of 
the triggering signals from the sensor circuit beyond a preselected 
frequency is that a triggering signal could be received by the pulse 
generator while the pulse generator is in an on or pulse generating 
condition. It is the nature of such pulse generator circuits to ignore 
such trigger pulse and continue through the normal width of the pulse 
being generated. Accordingly, the trigger pulse will be ignored and the 
next or following trigger pulse will be required to initiate an output 
pulse from the pulse generator. Accordingly, a fold back condition is 
developed wherein the pulse generator ignores every other or alternating 
trigger signals and accordingly provides fewer than the required output 
pulses to properly drive the meter to reflect the proper rate of rotation 
of the shaft being measured. In effect, the period of the trigger pulses 
has become shorter than the pulse generator output width. 
The meter overdrive protection circuit of the present invention solves 
these problems. The microcomputer is utilized to operate the pulse 
generator in two modes. The microcomputer receives an output from the 
sensor circuit whereupon it is aware of the frequency of the trigger 
pulses. As the microcomputer has also preselected the pulse width of the 
output pulses to be supplied by the pulse generator it is able to compare 
the pulse width with the period of the trigger input. The computer, pulse 
generator and meter drive circuit are selected such that a 90% (or other 
preselected value) on cycle condition of the pulse generator will provide 
a full scale meter deflection. The meter is so designed that a mechanical 
stop is provided for a two or three percent above full scale needle 
deflection. The circuitry of the present invention is designed to permit a 
maximum of 100% full scale deflection. When the microcomputer senses that 
the triggering inputs have required the pulse generator to provide on 
pulses for 90% of the time period, the computer will change the mode of 
the pulse generator operation from a normal or triggered pulse mode to a 
constant duty cycle mode. In such constant duty cycle mode, the pulse 
generator will provide a series of pulses to the meter drive corresponding 
to a 90% on duty cycle. Further, the computer will instruct the pulse 
generator to ignore triggering pulse inputs from the sensor circuit. 
Accordingly, if the rate of rotation increases beyond such preselected 90% 
and an increased number of triggering pulses is output by the sensor 
circuit to the pulse generator, the pulse generator remains in the 
constant duty cycle mode. Accordingly, the meter remains at 100% full 
scale deflection, without contacting the mechanical stop which is at 102 
or 103% of full scale deflection. Further, the pulse generator cannot miss 
every other or alternating trigger signals from the sensor circuit and 
accordingly, incorrectly provide only one-half the necessary output to the 
meter drive, as the pulse generator is ignoring the sensor circuit output. 
Upon a decrease in the rotation of the shaft being measured, the sensor 
circuit will provide a frequency of trigger signals less than the 
frequency of trigger signals required to provide full scale meter 
deflection, and the computer will sense such decreased trigger signal 
frequency. The computer will return the pulse generator to a normal or 
triggered pulse mode whereupon the meter can correctly reflect the 
decreasing number of revolutions of the measured shaft. 
In particular, the present invention provides a meter drive circuit 
comprising circuit means adapted to produce an output signal having a 
frequency related to the rate of rotation of a shaft, a sensor circuit 
receiving the output signal from said circuit means and providing trigger 
pulses related to the rate of shaft rotation, a computer means receiving 
the trigger pulses from said sensor circuit, a pulse generator also 
receiving the trigger pulses from said sensor circuit and providing a 
meter drive pulse of a selected width upon the receipt of each trigger 
pulse, a meter circuit receiving said series of meter drive pulses from 
said pulse generator, said computer means providing an output signal to 
said pulse generator whereby the width of the meter drive pulses is 
established, said computer means also adapted to analyze the frequency of 
trigger pulses from said sensor circuit, and if the time interval between 
said trigger pulses is less than a predetermined time interval, said 
computer means will provide a signal to said pulse generator whereby said 
pulse generator will ignore the trigger pulses from the sensor circuit and 
provide meter drive pulses at a preselected frequency and pulse length. 
The present invention also provides a meter drive circuit comprising a 
source of trigger pulses having a frequency related to the rate of 
rotation of a shaft, a computer receiving said trigger pulses, a pulse 
generator receiving said trigger pulses and receiving an output from said 
computer, said computer output to said pulse generator adjusting the width 
of the drive pulses to be output by said pulse generator to correspond to 
certain parameters of said rotating shaft, the frequency of said drive 
pulses being related to the frequency of said trigger pulses, upon the 
frequency of said trigger pulses exceeding a predetermined value, said 
computer sensing such condition and outputting a signal to said pulse 
generator such that said pulse generator output drive pulses are provided 
at a predetermined frequency independent of said trigger pulses, and a 
meter circuit receiving the output drive pulses from said pulse generator.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a meter overdrive protection circuit in accordance 
with the present invention is shown. A rotating shaft is shown at 18 
surrounded by a wheel 20 having protruding teeth 22. When the meter drive 
of the present invention is being utilized in a tachometer circuit, wheel 
20 will correspond to the flywheel of the vehicle engine whose revolutions 
are to be metered by the tachometer. If the meter drive circuit of the 
present invention were being utilized in a speedometer, drive shaft 18 
would correspond to the output shaft of the vehicle transmission. As wheel 
20 is rotated by shaft 18, protruding gear teeth 22 would induce a current 
in magnetic coil 24. This current would be of a sinusoidal nature and 
would be output over line 28 to sensor circuit 30. Sensor circuit 30 
comprises a well known trigger circuit, which would provide a spike-like 
output pulse over line 32 corresponding with the positive sinewave input 
from coil 24. A computer 16 usually comprising a microcomputer having 
modest storage and retrieval capabilities also receives an output from 
sensor circuit 30 over line 33. A pulse generator circuit 34 usually 
comprising a programmable integrated circuit chip receives a programming 
input 36 from computer 16. Pulse generator 34 also receives a triggering 
input from sensor 30 via input line 32. The input from computer 16 over 
line 36 to pulse generator 34 comprises serial binary coded data from the 
computer and is placed in internal registers within the pulse generator 
34. Since pulse generator 34 is typically a standard integrated circuit 
capable of multiple functions, pulse generator 34 will also store 
information from computer 16 telling it to operate in a pulse generating 
mode. The programming input from computer 16 instructs pulse generator 34 
to produce a pulse of a selected width which is output whenever the pulse 
generator receives a triggering input from sensor 30. The width of such 
output pulse supplied to meter drive 40 is dependent upon the vehicle 
characteristics for which computer 16 has been programmed. Once such 
vehicle characteristics are entered in computer 16, the width of output 
pulses from pulse generator 34 becomes fixed. Such fixed length pulses 
will be output by pulse generator 34 upon receipt of each triggering input 
from sensor 30 as long as the pulse generator 34 is instructed by computer 
16 to operate in the pulse generating mode. 
Meter drive 40 receives such output pulses from pulse generator 34 over 
input line 38. Meter drive circuit 40 typically comprises a transistor 
which is triggered by output pulses from pulse generator 34. Referring to 
FIG. 2, once the particular vehicle characteristics are input to the 
computer, the width of output pulses from pulse generator 34 is set. 
Output from meter drive 40 is via line 41 to meter 42. Typically meter 42 
is of a deflecting needle or electromagnetic type. While the width of 
pulses from pulse generator 34 is fixed, once the vehicle characteristics 
are entered in the computer, the frequency of such pulses is dependent 
upon the frequency of triggering inputs from sensor 30. In FIG. 2, 
waveform A indicates triggering inputs from sensor 30, and waveform B 
indicates corresponding meter drive pulses from pulse generator 34. As 
trigger inputs occur at times t.sub.1 and t.sub.2, output pulses of 
preselected width t.sub.p are generated by pulse generator 34. Referring 
now to FIG. 3, an increased rate of rotation of shaft 18 is shown 
resulting in an increased frequency of trigger pulses from sensor 30 shown 
at waveform A. Waveform B shows the resulting outputs from pulse generator 
34. As the first triggering input occurs at time t.sub.3, a pulse 
generator 34 output wave of wave length t.sub.p would be initiated at such 
time t.sub.3. However, the length (or, interchangably for the present 
explanation, width) of pulse generator 34 output pulse t.sub.p exceeds the 
period or time between trigger pulses shown in waveform A as the next 
trigger pulses occurs at time t.sub.4. Accordingly, pulse generator 34 
would ignore such trigger input at time t.sub.4 and continue its output 
for time t.sub.p. The next triggering input from sensor 30 occurs at time 
t.sub.5 whereupon pulse generator 34 would again initiate an output pulse 
of length t.sub.p. It is easily seen from FIG. 3 how pulse generator 34 
could provide an incorrect indication of the increased rate of rotation of 
shaft 18. This is what is known as a fold back condition where in effect 
meter 42 would show less than the correct rate of rotation of shaft 18. 
By having computer 16 change the mode of operation of pulse generator 34 
from a normal or triggered pulse generation mode to a fixed duty cycle 
mode, the overdriving of the meter needle and the possible fold back 
incorrect indication of the rate of rotation are both eliminated. 
Referring to FIG. 2, as the on duty cycle time of waveform B approaches 
90% (or other preselected maximum desired value) of the total time, 
computer 16 would be programmed to sense such condition by measuring the 
time between the trigger pulses t.sub.1 and t.sub.2. As the time interval 
between trigger pulses equals about 1.1 times the selected wave length 
t.sub.p of the meter drive pulses from pulse generator 34, the computer 
will change the mode of operation of pulse generator 34 from a normal or 
triggered pulse output to a fixed duty cycle output. Another way of 
stating this is that the computer will sense from the pulse generator when 
the preselected frequency and pulse length of outputs from pulse generator 
34 are such that output drive pulses are being provided for 90% (or other 
preselected maximum desired values) of the time, and then the computer 
will instruct pulse generator 34 to operate in a fixed duty cycle mode 
rather than a normal or triggered pulse mode.