Short circuit detector for sensors

A short circuit detector circuit (10) for detecting when a sensor (2), such as a tachometer sensor in automotive applications, is operating normally and when the sensor is short circuited is disclosed. The detector circuit (10) comprises a reference signal input (16) for receiving a reference signal, and a closed loop (22, 24, 12, 14, 26, 34, 32) having a predetermined loop gain (K). The closed loop has a first input coupled to the reference signal input, a first output (12) coupled to drive the sensor with a signal dependent on the input signal and the predetermined loop gain, a second input (14) for receiving a feed back signal from the sensor and a second output (36) coupled to the first input for providing an output response signal dependent on the operation of the sensor. The value of the predetermined loop gain is selected so that the output response signal is an undamped oscillation for normal sensor operation. The detector circuit (10) further comprises means (4, 50, 52, 54, 56,. 58, 60) for analysing the output response signal and for detecting normal sensor operation when the output response signal is an undamped oscillation and for detecting short circuit sensor operation when the output response signal is a damped oscillation.

This invention relates to a short circuit detector for sensors. 
BACKGROUND OF THE INVENTION 
In many systems which have sensor networks for sensing system operating 
parameters, filters are used in the sensor networks in order to optimise 
the operation of the network: for example, to ensure good noise immunity 
and to limit the voltage supplied from the sensor. The latter is 
particularly important for tachometer (tacho) sensors in automotive 
applications. Typically, the time constants of the inductive and 
capacitive sensors of the networks are low compared to that of the 
filters. For these applications therefore, the sensors require close 
monitoring in order to ensure their proper operation. 
By monitoring the DC current flowing through the sensor, it is possible to 
determine whether the sensor is tied to ground, tied to the power supply 
or whether the sensor is disconnected. However, this type of monitoring 
does not allow for the detection of a short-circuited sensor due to the 
low internal inductance of the sensor compared to the other components in 
the network. 
One method of detecting sensor short-circuit is to apply a step or Dirac 
pulse to the network and observe the closed loop system response. However 
for tachometer sensors, such a method does not accurately or reliably 
discriminate between the short-circuited or not short-circuited sensor 
response. Furthermore, the time constant of the tachometer sensor, when 
combined with the passive filter components, becomes insignificant 
compared to the other time constants of the network. 
SUMMARY OF THE INVENTION 
Thus, it is an object of the invention to provide an improved short-circuit 
detector for a inductive or capacitive sensor. 
In accordance with the present invention there is provided a short circuit 
detector circuit for detecting when a sensor is operating normally and 
when the sensor is short circuited comprising: 
a reference signal input for receiving a reference signal; 
closed loop means having a predetermined loop gain, the closed loop means 
having a first input coupled to the reference signal input, a first output 
coupled to drive the sensor with a signal dependent on the input signal 
and the predetermined loop gain, a second input for receiving a feed back 
signal from the sensor and a second output coupled to the first input for 
providing an output response signal dependent on the operation of the 
sensor, the value of the predetermined loop gain being selected so that 
the output response signal is an undamped oscillation for normal sensor 
operation; 
means for analysing the output response signal and for detecting normal 
sensor operation when the output response signal is an undamped 
oscillation and for detecting short circuit sensor operation when the 
output response signal is a damped oscillation.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring firstly to FIG. 1, in a preferred automotive sensor network 1, 
the output signals from a tacho sensor 2 are coupled to a processor 4 via 
a twisted wire pair 6 and filter circuitry 8. The processor 4 analyses the 
signals from the tacho sensor and in response thereto, controls the 
operation of the engine accordingly. 
In order to determine whether the tacho sensor 2 is operating normally, the 
processor 4 must be able to detect whether one or both of the wires of the 
twisted wire pair 6 are short circuited to ground, short circuited to a 
power supply, open circuited or when the wires are short circuited 
together. As discussed above, the prior art methods cannot accurately and 
reliably determine when the wires are short circuited together. The 
invention attempts to overcome this problem by providing a short circuit 
detector circuit which reliably detects when the wires are short circuited 
together. 
Referring now also to FIG. 2, a short circuit detector circuit 10 in 
accordance with a preferred embodiment of the present invention for 
monitoring the output signals from the tacho sensor 2 is preferably 
incorporated in the processor 4. Like components to those of FIG. 1 are 
referred to by the same reference numerals. Alternatively, the short 
circuit detector circuit may be implemented in an interface circuit which 
acts as an interface between the processor and tacho sensor. It will be 
appreciated that it is not intended that the detector circuit in 
accordance with the invention be implemented exclusively with tacho 
sensors. The invention can be used with any inductive or capacitive 
sensors or actuators. 
The circuit 10 is a closed loop arrangement and comprises two terminals 12, 
14 coupled to respective wires of the twisted wire pair 6 via filter 
circuitry 8 and an input 16 coupled to receive a step pulse signal from a 
step signal generator (not shown). The step pulse signal at the input 16 
is coupled to a reference voltage amplifier 18 which comprises an 
inverting HCMOS transistor gate 19 coupled in a resistive feedback loop 
having a resistance 20. Such an arrangement provides an output voltage 
signal having a level of approximately half the voltage level supplied at 
the input. The output signal from the reference voltage amplifier 18 is 
fed to the terminal 12 via two inverters 22, 24. This output signal is 
coupled to the tacho sensor 2 and fed back to terminal 14 via the twisted 
wire pair 6. A comparator 32 which comprises resistors 30 and 28 compares 
the output signal from the reference voltage amplifier 18 with the feed 
back signal coupled thereto from terminal 14 via inverter 26. Inverters 
22, 24 and 26 determine the loop gain. A resistor 34 is coupled across the 
terminals 12 and 14 in order to prevent oscillation due to external 
capacitors, resistors and the tacho sensor 2 which occurs when the sensor 
network is open. 
FIG. 3 shows the equivalent electrical block diagram for the circuit of 
FIG. 2, where Sin is the output of the reference voltage amplifier 18, 
Sout is the feedback signal from terminal 14, K is the gain of the loop 
and G(s) represents the transfer function of the sensor network. For 
normal operation, the transfer function has three poles. When the tacho 
sensor 2 is short circuited, the number of poles is reduced to two. This 
produces a difference in the dynamic response of the network. 
If one considers the mathematical model of the sensor network of FIGS. 2 
and 3, the characteristic equation of the network is 
##EQU1## 
The difference in the dynamic response of the network is apparent when the 
root locus in the S-plane is plotted, with varying loop gain, from the 
characteristic equation (1). FIGS. 4 and 5 show the root locus for the 
closed loop network for normal operation having three poles A, B, C and 
short circuit operation having two poles D, E respectively. Thus, the 
roots of the characteristic equation determine the character of the 
dynamic response of the network. 
As can be seen in FIG. 4, the dynamic response of the sensor network 
depends on the loop gain K and is either convergent (stable), on the left 
of the imaginary axis Jw or divergent (unstable), on the right of the 
imaginary axis Jw. By selecting a predetermined loop gain K, it can be 
arranged for the root locus to intercept the imaginary axis. At this 
predetermined loop gain, the dynamic response is a relaxed oscillation. 
Thus, the loop gain is computed in order to ensure that the dynamic 
response for normal operation is a relaxed oscillation. In this case, when 
the processor 4 analyses the feedback signal, at for example terminal 36, 
it will detect a signal such as that shown in FIG. 6. FIG. 6 shows the 
step pulse signal 38 applied at the input terminal 16 and the feedback 
signal 39 at terminal 36. 
FIG. 5 shows the root locus when the tacho sensor 2 is short circuited. As 
discussed above the number of poles is reduced to two (D, E). Since the 
root locus will always be parallel to the imaginary axis Jw, the dynamic 
response of the sensor network will always be damped (stable) irrespective 
of the loop gain. Thus, when the tacho sensor 2 is short circuited, the 
dynamic response is a damped oscillation. The feedback signal at terminal 
36 will therefore appear such as that shown in FIG. 7. FIG. 7 shows the 
step pulse signal 38 applied at the input 16 and the damped dynamic 
response 41 at the terminal 36. 
Thus, by selecting a predetermined loop gain, and by observing the feedback 
signal at terminal 36, the processor 4 can determine whether the sensor is 
working normally or is short circuited. 
The relaxed or damped oscillation response feedback signal may be observed 
using, for example, a counter and schmitt trigger to analyse the voltage 
signal at terminal 36 during the second half of the pulse period. An 
example of such a circuit is shown in FIG. 13. 
A schmitt trigger 50 is formed by two inverting HCMOS transistor gates 52 
coupled in a resistive feedback loop having a resistance 54 and a 
resistance 56. The hysteresis factor is determined by the ratio of the 
resistances 54 and 56. The output of the schmitt trigger 50 is coupled to 
a first input of an AND gate 60. A second input of the AND gate 60 is 
coupled to receive a signal CLK/2CLK which is dependent on the signal 
provided at input 16. The output from the schmitt trigger 50 is therefore 
arranged to be sampled in a window spanning part of the step signal 38. An 
output of AND gate 60 clocks a counter 58 which has an overflow output 
coupled to a check flag (CF). When the tacho sensor 2 is operating 
normally, the output of the AND gate 60 increments the counter until a 
predetermined count is reached whereby the overflow signal (OVR) is 
generated. The overflow signal (OVR) sets the check flag (CF). When the 
tacho sensor 2 is short circuited, the schmitt trigger output signal is 
filtered by the counter 58. Thus, no overflow signal (OVR) is generated 
and the check flag (CF) is not set. The processor 4 reads the state of the 
check flag (CF) so as to determine the operating state of the tacho sensor 
2. The check flag can be reset by an acknowledge flag (ACK) generated by 
the processor 4. Thus, the state of the check flag (CF) indicates whether 
the sensor is operating normally (i.e. relaxed oscillation) or short 
circuited (damped oscillation). 
The loop gain depends on the passive components used in the circuit (the 
inverters 22, 24 and 26) and remains constant irrespective of the tacho 
sensor's specification, temperature and voltage variations. This is due in 
part to the CMOS inverter implementation of the reference voltage 
amplifier 18, since reference voltage amplifier 18 will always provide an 
output voltage at a point midway between the two switching states of the 
gate irrespective of voltage drift and temperature drift (see FIGS. 8, 9, 
10, 11 and 12). 
In summary, the short circuit detector circuit in accordance with the 
present invention provides a simple and reliable means to detect a short 
circuit in a capacitive or inductive sensor or actuator. An advantage of 
the invention is that the behaviour of the sensor can still be determined 
even when the dynamic response of the sensor is much less than that of the 
network as a whole. Furthermore, by using a CMOS inverter as an amplifier, 
the invention can be simply implemented in a MCU at very low cost. 
Further, the use of CMOS inverters having a transfer function in the 
S-plane, allows for the self-tuning of the loop gain so as to ensure 
relaxed oscillation. 
The monitoring circuit in accordance with the invention also allows for the 
detection of a short circuit to ground or to the power supply or open 
circuit without a significant hardware modification of the network. An 
example of such a circuit is shown in FIG. 14. To test whether the tacho 
sensor 2 is short circuited to ground or to the power supply or open, the 
processor 4 for a predetermined period provides a `1` or `0` signal to the 
input of the reference voltage amplifier 18. In dependence on the setting 
values, such as the loop gain, and by monitoring the state of the check 
flag (CF), the processor 4 can monitor the operation using an exclusive OR 
function to determine whether a malfunction has occurred and the kind of 
malfunction. For example, the sensor is operating normally if the check 
flag has the `1` state or the `0` state for an input signal of `1` or `0` 
respectively. The sensor has malfunctioned, if the check flag has the `1` 
state or the `0` state for an input signal of `0` or `1` respectively. The 
switches SW1 and SW2 are activated by the processor 4 in order to enter 
this monitoring sequence. 
This invention can be used in networks using either inductive sensors or 
capacitive sensors and also for sensors having a very low time constant 
compared to the components in the rest of the network. This invention can 
also be used with a network having two poles in normal operation and one 
pole when the sensor is short circuited. 
The invention may be used in ABS systems, engine management systems and 
gear box control systems.