GB2206211A - Sensor circuit - Google Patents

Abstract

A sensor interface arrangement comprises a first reference sensor R1 and a second sensor R2 for detecting a physical parameter (e.g. humidity) to be measured, first and second resistors R3 and R4 connected in parallel with the sensors via a normally closed switch 1, a capacitor C being connected across resistor R3, it being arranged that the voltage across the capacitor C is one or two percent greater than the voltage across the resistor R2, a comparator for comparing the voltage at the junction of the sensors R1 and R2 with the voltage at the junction of the resistors R3 and R4, the comparator affording an output to a microcomputer 4 which is effective for operating the normally closed switch 1. When a physical parameter is to be measured, the switch 1 is opened by the microcomputer 4 whereby the voltage on the capacitor C begins to decay. When the voltage across the capacitor C equals the voltage at the junction of the sensors the comparator operates and affords an output to the microcomputer. The time interval between the time the switch 1 is operated and the time the comparator operates is indicative of the physical parameter being sensed. Applications to strain gauges and to temperature sensing are also envisaged. <IMAGE>

Description

Improvements in or relating to Sensors This invention relates to sensors and relates more particularly to the interfacing of sensors with computer (e.g. microcomputer) systems. The invention has particular application to one form of humidity sensor but may well have application to other sensors.

One form of humidity sensor that has previously been proposed makes use of a matched pair of thermistors, each having a negative temperature coefficient of resistance, one of which is sealed in an environment of dry air and the other of which is subjected to the atmosphere under test. The thermistors are conventionally connected in series and have an electric current passed through them to cause them to be heated. When water vapour is present around the exposed thermistor, heat is conducted or convected away from it causing a drop in temperature of the thermistor and therefore a change in its resistance.

The result of this is that the voltage ratio across the pair of thermistors changes, and this change can be measured in order to afford an indication of the degree of humidity that the exposed thermistor is exposed to.

The change in voltage ratio is small, however, typically being about one part per thousand.

In order to measure the change in voltage ratio, two fixed resistors, connected in series and acting as a fixed potential divider are connected in parallel with the thermistor pair. The result is a bridge circuit, in which the voltage difference between the midpoint of the thermistors is compared against the midpoint of the resistor pair. A differential amplifier multiplies the difference by a typical gain of two hundred, and the resulting change of about one volt is measured by an analogue-to-digital convertor.

Great care must be taken to balance the bridge to ensure that the amplified voltage is within the range of the convertor.

It is an object of the present invention to provide a sensor interface arrangement which avoids the need for an analogue convertor, and which allows for greater tolerance.

According to the present invention there is provided an interface arrangement comprising a first sensor adapted to be used as a reference, a second sensor for detecting a physical parameter to be measured, said first and second sensors being connected in series and having a voltage supply connected thereacross, first and second resistor means connected in series, normally closed switch means for connecting said first and second resistor means to said voltage supply, comparator means for comparing the voltage at the junction of said sensors with the voltage at the junction of said resistor means and for affording an output to microcomputer means in dependence thereon, capacitor means connected in parallel with one of said resistor means, said sensors and said resistor means being chosen such that the voltage across said capacitor means is greater than the corresponding voltage at the junction of said sensors, the arrangement being such that when a physical parameter is to be measured said switch means is caused to be operated by said microcomputer means whereby the voltage on said capacitor means is caused to decay, said microcomputer means being effective for affording an output in dependence upon the time said switch means is operated and the time said comparator means is caused to operate.

In carrying out the invention it- may be arranged that the comparator means is caused to operate when the voltage across said capacitor means substantially equals the corresponding voltage at the junction of said sensors.

In a preferred arrangement in accordance with the present invention, the second sensor will be effective for measuring humidity, in which case said sensors may take the form of a matched pair of thermistors, each of said sensors preferably having a negative temperature coefficient of resistance.

In carrying out the invention according to the preferred arrangement, it may be arranged that the first sensor is sealed in an environment of dry air and the second sensor is subjected to the humidity to be sensed.

In addition to being applicable to humidity sensing, said sensors may be arranged to sense other physical parameters, and may, for example, constitute a strain gauge or may be effective for measuring temperature.

An exemplary embodiment of the invention will now be described reference being made to the accompanying single figure drawing which is a partially block schematic circuit diagram of a sensor interface arrangement in accordance with the present invention.

In the arrangement shown in the drawing, a pair of sensor resistors R1 and R2, which may be thermistors having a negative temperature resistance, are connected in series and are also connected in series with a supply resistor Rs between a voltage supply Vs, which may typically be 15 volts, and a system ground.

From the junction of the supply resistor Rs and the sensor resistor R1, an analogue switch 1 (which is normally closed) connects to a pair of resistors R3 and R4 connected in series as a potential divider, the other end of the pair of resistors R3, R4 being connected to the system ground.

The ratio of the divider resistor pair R3, R4 is chosen to be between one and two percent less than the normal ratio of the sensor resistor pair R1, R2, the resistors R2, R4 being regarded as the denominator of each ratio. A capacitor C is connected in parallel with the divider resistor R4.

The midpoint of the sensor resistor pair R1, R2 and the midpoint of the divider resistor pair R3, R4 are each connected to a respective input of a comparator 2 which affords an output at logic level according to the sign of the difference in voltage between the two midpoints. The output from the comparator 2 is connected to a logic input 3 of a microcomputer 4 to which the sensor arrangement described is to be interfaced, and a logic output 5 of the microcomputer 4 is also connected to the analogue switch 1.

In operation of the sensor interface arrangement thus far described, it will be arranged that one of the sensor resistors R1, R2 will be sealed in an environment of dry air, and the other will be subjected to the atmosphere being tested. The analogue switch 1 is normally closed so that the voltage across the capacitor C would normally correspond to the voltage across the divider resistor R4 to which it is connected in parallel. This voltage will exceed the voltage of the midpoint of the sensor resistor pair R1, R2 by between one and two percent.

When a reading of humidity is to be taken, the microcomputer 4 affords a signal on its output 5 to cause the analogue switch 1 to open. The voltage across the capacitor C will immediately start to decay at a rate, the time constant of which is accurately determined by the product of the values of the capacitor C and the divider resistor R4. After a time, which by design may be in the order of two milliseconds, the voltage across the capacitor C will fall to equal the voltage at the midpoint of the sensor resistors R1, R2, at which point the comparator 2 will change the output logic state of the signal applied to input 3 of the microcomputer 4.The time interval between the microcomputer 4 causing the analogue switch 1 to be opened and the time that the comparator output changes state will accurately depend on the ratio of the initial midpoint voltages, which will also accurately depend upon the level of humidity being sensed. It should be noted that this time interval will be insensitive to variation in the bridge excitation voltage. In order to afford an indication of humidity the duration of the time interval is accurately measured by the microcomputer 4, e.g. by means of a counter or by means of counting software.

By arranging that the expected time interval will be in the region of two milliseconds, an eight-bit counter may be used, which might typically afford a count of ten when vapour is detected.

The arrangement described is advantageous in that typically changes of less than one part per thousand can be detected. Because the system can be arranged to check its own reference point each time the arrangement is used, there is no need to use precision components in the circuit. Ordinary low cost, wide tolerance components may be used. Typically the divider resistors R3, R4 will have ratio accurate to one percent, the other components having a tolerance of perhaps five percent or more. The arrangement described is particularly advantageous in that it only requires one logic input pin and one logic output pin to effect the interface with the microcomputer 4.

Although especially applicable to humidity sensors, the present invention could be applied to any system of measurement where the signal is potentiometric, i.e. the signal is a difference voltage caused by a change of resistance such as strain gauges, temperature sensitive devices, etc.

Claims (9)

1. A sensor interface arrangement comprising a first sensor adapted to be used as a reference, a second sensor for detecting a physical parameter to be measured, said first and second sensors being connected in series and having a voltage supply connected thereacross, first and second resistor means connected in series, normally closed switch means for connecting said first and second resistor means to said voltage supply, comparator means for comparing the voltage at the junction of said sensors with the voltage at the junction of said resi'stor means and for affording an output to microcomputer means in dependence thereon, capacitor means connected in parallel with one of said resistor means, said sensors and said resistor means being chosen such that the voltage across said capacitor means is greater than the corresponding voltage at the junction of said sensors, the arrangement being such that when a physical parameter is to be measured said switch means is caused to be operated by said microcomputer means whereby the voltage on said capacitor means is caused to decay, said microcomputer means being effective for affording an output in dependence upon the time said switch means is operated and the time said comparator means is caused to operate.

2. An arrangement as claimed in claim 1, in which the comparator means is caused to operate when the voltage across said capacitor means substantially equals the corresponding voltage at the junction of said sensors.

3. An arrangement as claimed in claim 1 or claim 2, in which the second sensor is effective for measuring humidity.

4. An arrangement as claimed in claim 3, in which said sensors take the form of a matched pair of thermistors.

5. An arrangement as claimed in claim 4, in which said sensors have a negative temperature coefficient of resistance.

6. An arrangement as claimed in claim 4 or claim 5, in which the first sensor is sealed in an environment of dry air and the second sensor is subjected to the humidity to be sensed.

7. An arrangement as claimed in claim 1 or claim 2, in which said sensors constitute a strain gauge.

8. An arrangement as claimed in claim 1 or claim 2, in which said sensors are effective for measuring temperature.

9. An arrangement substantially as hereinbefore described with reference to the accompanying drawings.