GB2197504A - Thermostat control - Google Patents

Abstract

A thermostat control has a temperature sensor (1) including a variable impedance connected to a measurement circuit (2) which has a proportional output of for example 100 mV per DEG C. The output is compared in voltage level detectors (5a, 5b) with preset voltages set on potentiometers (6a, 6b) and representing different temperatures. The output of one detector (depending on for example the time of day) is selected by relay (B) and output to relay (A) to switch a heater on or off. An LED bargraph array may be connected at (28) to display the measured temperature. <IMAGE>

Description

SPECIFICATION Thermostat control This invention relates to a thermostat control and, more particularly, to a means for accurately sensing a temperature, for adjustably setting one or more desired temperatures, and for controlling the operation of a heater on the basis thereof.

Thermostat controls are very common; however, associated with the known devices are a number of disadvantages.

With the known controls, in order to set accurately the desired temperature, it is necessary to use electronic components with very low tolerances, and the result of this is that the controls are expensive. In addition, the preset temperatures need to be "built in" to the control device as it is being manufactured and cannot easily be altered thereafter. This is a great disadvantage, as the air temperature within a room is rarely uniform over all parts of the room. Thus, if it is required to maintain a room at a certain temperature, the temperature sensor must be carefully, and possibly inconveniently, located, in order to ensure that it gives an accurate indication of the temperature in the area of the room which is of interest, or the control device must be adjusted in order to compensate for the temperature variation.As mentioned previously, however, the desired temperatures are built in to the device during manufacture, and thus their adjustment is a complicated and expensive task.

Of course, it is also known to provide a thermostat control in which the desired temperature may be easily increased or decreased by way of a simple adjustment. There are circumstances, however, such as when maintaining a workplace at a certain temperature, when such ease of adjustment is inappropriate, as it does not allow energy use to be controlled. Moreover, when the desired temperature is easily adjustable as mentioned above, then calibration of the control device is a problem, and so the desired temperature cannot be set with any great accuracy.

The present invention seeks to overcome the abovementioned disadvantages, by providing a thermostat control, in which the one or more desired temperature may be set accurately and adjustably, and which provides an indication of the temperature sensed.

According to the present invention there is provided a thermostat control comprising a temperature sensor adapted to deliver a signal representing the sensed temperature, adjustable signal generating means for generating a reference signal representing a desired temperature, and a comparator for comparing the sensed temperature signal end the desired temperature signal and for delivering a signal dependent upon the relative magnitudes of the sensed and desired temperatures.

In a preferred embodiment in accordance with the present invention the thermostat control comprises: a temperature sensor, including an impedance, the value of which corresponds to a sensed temperature; an amplifier; and a desired temperature setting means, the or each desired temperature setting means providing a first voltage corresponding to the desired temperature and being associated with a comparator, wherein the amplifier, in conjunction with the impedance, provides a second voltage corresponding to the sensed temperature, and wherein the second voltage is compared, by means of the comparator,with the first voltage to provide a control signal.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block schematic diagram illustrating a thermostat control in accordance with one embodiment of the present invention, with the provision for the setting of two desired temperatures; Figure 2 is a block schematic diagram of the electrical connections which may be used to provide a power supply and a clock input for the device of Figure 1; Figure 3 is a diagram illustrating a relay which is provided in the embodiment of the present invention illustrated in Figures 1 and 2; and Figure 4 illustrates a further relay provided in the device of Figure 1; Figure 5 is a circuit diagram of a temperature measurement circuit of the thermostat control shown in Figure 1; Figure 6 is a circuit diagram of a voltage level detector of the thermostat control shown in Figure 1; and Figure 7 is a schematic diagram of a temperature display, which may be used in conjunction with the thermostat control of Figure 1.

In Figure 1, the reference numeral 1 designates a calibrated temperature sensor connected to, but provided remote from, a measurement circuit 2. The temperature sensor is of a known sort which includes a resistance, the value of which increases generally linearly with increasing temperature. The measurement circuit 2 includes an operational amplifier (such as a CA3140E), and -the temperature-dependent resistor is connected in the biassing circuit thereof. As a result, by suitable calibration using a variable resistor 3, an amplifier output signal, directly proportional to the temperature in degrees celsius, may be produced on a line 4. The measurement circuit is shown in greater detail in, and described with reference to, Figure 5.

Preferably, the variable resistor 3 is adjusted to provide an amplifier signal which, at room temperature, is greater than 0.1 volt, and more preferably greater than 1 volt. The relationship between the voltage and the temperature is thus preferably such that each degree celsius change in temperature causes a voltage change of at least 10 millivolts, and more preferably 100 millivolts.

The amplifier output signal is fed, in this case, to two contact points 31a,31b, and then to two voltage level detectors 5a,5b, in each of which it is compared, by means of a comparator such as an LM324, with a preset voltage corresponding to a respective desired temperature. The preset voltages are determined by suitable adjustment of potentiometers 6a,6b, and are then input to the respective voltage level detectors 5a,5b along lines 7a,7b. A circuit diagram of one voltage level detector is provided as Figure 6, and the function of the circuit is described in more detail with reference to that Figure.

The outputs from the voltage level detectors 5a,5b, at points 32a, 32b, are then fed to a relay B and, depending upon which contact is made by that relay, one of them is fed to a contact 8 of the switch 9, which can allow a relay operator 10 to be connected to the contact 8, to an "OFF" point 11 or to an "ON" point 12. The signal passing to a relay operator 10 then determines the operation of a relay A and whether or not current flows through an LED 13.

Figure 2 shows the electrical connections which may be used to provide a power supply for the control device, and to control the operation of the relay B. Input lines 14,15,16 are connected to a 240 V a.c. power supply, the letters L, N and E indicating the Live, Neutral and Earth wires respectively. Lines 14,15 are connected to lines 17,18 which are the power supply lines for a time clock (not illustrated), and to a known power supply unit 19, which has outputs 20,21. Connected between the output line 21 and the Earth line 16 is an LED 22.

The time clock mentioned above controls the switched 240V a.c. signal which flows along a line 23 to an optical isolator 24, the output of which passes to a relay operator 25. The relay operator 25 is then connected to the relays B,C, which are connected in parallel.

Figure 3 shows the relay C, with contacts 26a,26b. The contact is made or broken, depending upon the output from the relay operator 25.

Figure 4 shows the relay A, which is connected to a heater (not shown), and causes the heater to be brought into operation when the sensed temperature is less than the appropriate desired temperature which has been set, depending upon the output from the relay operator 10.

Figure 5 shows the temperature measurement circuit 2 and associated temperature sensor 1 and adjustment resistor RV1, designated in Figure 1 by the reference numeral 3.

The circuit is provided with a stabilised 5V supply to reduce inaccuracies in the output which might result from self-heating of the sensor. The temperature sensitive resistor 1 is standardised by means of series or parallel resistance (not shown), so that, at 25"C, it has a resistance of 1.02kQ + 2Q. The use of the standardising resistance enables this accuracy to be achieved using a temperature sensitive resistor with a basic tolerance of 2%.

The resistor R, linearises the sensor voltage slope to an accuracy of better than + 0.2%, and using the value of R, shown in the Figure gives a sensor voltage slope of 7.68 mV.K-'.

It is desirable to obtain a sensed temperature signal which can easily be converted to a temperature measurement, and the component values have been chosen such that the output voltage on line 4 increases by 100mV for every degree celsius change in sensed temperature, and is equal to zero at 0 C. This allows required values of R2 and R3, and a ratio R6/R3, to be chosen. The sensor output voltage changes by 7.68mV.K-l, and therefore the gain of the amplifier Al, must be adjusted to be equal to 100/7.68 = 13. This fixes the ratio R3/R4 at 12; in this case, R6 has been chosen as 360kin and R4 as 30kQ.

To allow for possible inaccuracies in component values, the resistor Rvi may be adjusted so that the desired output voltage of 2.5V appears on the line 4 when the sensor resistance is equal to 1020Q, i.e. at 25to.

Figure 6 shows one of the two voltage level detectors 5 of Figure 1, plus associated circuitry. The potentiometer 6 may be adjusted to give an output voltage at point 29 anywhere in the range between V, and Vu. while the amplifier A3 is provided merely as a buffer to present a low impedance voltage source to the comparator A2. The output from the temperature measurement circuit is input at point 31.

Rg is provided to ensure that there is some hysteresis, and prevent oscillation of the output as the two inputs become approximately equal, and the comparator output is provided at point 32.

For the or each voltage level detector, Vu and V, are chosen so that any required desired temperature may be set.

Figure 7 shows a temperature display means suitable for use with the temperature control device of the invention. The voltage corresponding to the sensed temperature is applied to point 28, while resistors Rlo R11, R,2 R,3 are chosen in conjunction with amplifier A3 to provide a suitable input to the known bargraph driver integrated circuits 34 which power the bargraph 35. The bargraph 35 consists of a row of LEDs 36a, 36b,...

36n, which light successively as the applied voltage at point 28 increases. In the embodiment shown, each LED 36 corresponds to a range of 1"C, and the 20 LEDs of the illustrated bargraph allow temperatures in the range of 5"C to 24"C to be indicated, with LED 36a corresponding to 5"C, LED 36b to 6"C and so on.

There is also a set point 33 to which a known voltage may be applied for calibration purposes.

The operation of one preferred embodiment of the present invention will now be explained, in greater detail, with particular reference to Figure 1. By means of the variable resistor 3, the measurement circuit 2 has been adjusted such that the amplifier output signal is directly proportional to the sensed temperature. As described with reference to Figure 5, in fact, the amplifier output signal (measured in volts) may be converted to the sensed temperature value (in degrees celsius) simply by multiplying by a factor of ten. That is, for example, an amplifier output signal of 2.55V corresponds to a temperature of 25.5"C.

The amplifier output signal is fed to a printed circuit board 27 which has connections to earth and to a stabilised 12V source. The amplifier output signal is fed to the pin 28 of the printed circuit board 27 and may easily be measured at that point. This allows the sensed temperature to be measured simply and accurately, since the signal is very sensitive to changes in temperature, increasing at 100mV.K-'. Measurement of indicated temperature from the pin 28 also allows the calibration of the measurement circuit to be checked.

In addition, the output from the pin 28, providing an accurate measure of the sensed temperature, may be used to provide a continuous reading of the sensed temperature, by connection of pin 28 to a suitable display means, which may have a digital display or may be a bargraph 35 as described with reference to Figure 7. Moreover, pin 28 may be connected to an energy management system, which could be connected to several other similar devices, in order to provide an overall picture of the temperature throughout a factory, for example. The large 100mV.K-' gradient means that the voltage at the pin 28 will be significant, and consequently the amplifier output signal can easily be converted to a pulse train and passed over long distances without degradation.

This also allows a continuous reading of the sensed temperature to be fed to a fire alarm system, which may be programmed to provide an alarm when either the sensed temperature, or the rate of increase of the sensed temperature, exceeds a respective threshold value.

The voltage at pin 28 may also be used, in conjunction with some voltage corresponding to a desired temperature, for example that produced at test points 29a, 29b, to introduce some degree of proportional control of the heating system. This might be achieved by applying the signal to a motorised gas valve, possibly using positional feedback from the valve to improve control, to reduce the fuel flow as the desired temperature is approached. This technique allows the steadystate desired temperature to be reached more quickly and using less fuel.

The linear dependence of the amplifier output signal on the sensed temperature also means that the desired temperatures may be set on site, as they correspond to the voltages fed along the lines 7a,7b in the same way as the sensed temperature corresponds to the amplifier output signal on line 4. In this embodiment, the device is provided with two voltage level detectors 5a,5b, and the relay B causes the voltage level detectors to be brought into and out of operation at desired preset times, as shown in Figure 2. The preset times, in conjunction with the time clock signal on the line 23, cause the relays B and C to switch at those preset times. Switching of the relay B causes voltage level detectors 5a,5b to be brought into and out of operation successively.Two voltage level detectors are provided to allow the setting of two different desired temperatures, which may be required to be maintained at different times of day. For example, a desired temperature of 20"C may be set on voltage level detector 5a while a desired temperature of 5"C is set on voltage level detector 5b. Thus, by suitable adjustment of the time clock circuit, the device may maintain a minimum temperature of 20"C while a building is occupied, and a minimum temperature of 5"C, for frost protection, while a building is unoccupied.

Although, in this embodiment, the control device has two voltage level detectors, this is by no means necessary. If only one desired temperature is required to be set, then only one voltage level detector need be provided, and relay B becomes unnecessary. Conversely, if it is required to set more than two desired temperatures for different time periods, then an appropriate number of voltage level detectors, and a suitable switching system, may be provided.

The relay C is provided in parallel with the relay B so that it switches at the same time, and thus allows other circuits, for example a lighting system, to be brought into operation at the same times as the voltage level detectors.

In the preferred embodiment, the set desired temperatures are directly proportional to the voltages on lines 7a,7b. Thus, they may be altered by suitable adjustments to potentiometers 6a,6b, and the desired temperatures which have been set can be obtained by measuring the voltages at test points 29a,29b and multiplying by the appropriate conversion factor. For example, in the example mentioned above, using the preferred embodiment of the invention, the voltages on lines 7a,7b are set to 2.00V and 0.50V respectively.

In order to allow the desired temperatures to be set, suitable choices for V, and Vu in each voltage level detector must be made.

Thus, to allow a first temperature to be selected in the range of 10 C to 30"C, V, and Vu in one voltage level detector must be chosen to be equal to 1.00V and 3.00V respectively. To allow a second preset temperature to be adjusted in the range of 0 C to 200C, V, and Vu must be equal to OV and 2.00V respectively, in the preferred embodiment. Thus, it will be appreciated that the temperature setting can be adjusted to take account, for example, of the possibility that the sensor 1 is installed at a location which is somewhat cooler or warmer than the rest of the room.

Nevertheless, since adjustment requires measurement of the voltages at the text points 29a and 29b, it will not normally be possible for adjustment to be made by unqualified personnel. Thus the temperature settings are substantially tamperproof.

When a temperature display, such as that illustrated in Figure 7 is used in conjunction with the device according to the invention, such measurement may be made by applying the voltages at points 29 to the set point 33.

Using the bargraph device 35, quite accurate calibration is possible as there are small overlaps in the ranges of the LEDs and so the input voltage is known quite accurately when two adjacent LEDs are lit.

Switch 9 is also provided; although in normal operation the contact 8 is made, the contact 11 may be made if the system is to be switched off altogether, and contact 12 may be made if the heating system is to be switched on permanently, for testing purposes, for example.

When contact 8 of switch 9 is made then, in the voltage level detector determined by relay B, the amplifier output signal is compared with the voltage corresponding to the desired temperature. When this comparison indicates that the sensed temperature is less than the desired temperature, a signal to relay operator 10 is provided, which causes operation of relay A such that the heating system (not shown) is switched on. In this event, current also flows through the LED 13, which provides a visual indication that the heater is on.

With reference to Figure 2, the LED 22 connected between output lines 16, 21 passes a current when there is a voltage between these lines. Thus, LED 22 provides a visual indication when the power supply 19 is connected to the mains supply and is switched on. The stabilised 12V supply output on line 20 from power supply 19 may be used for connection to the set points 12,30 shown in Figure 1.

Thus, there is provided a thermostat control which accurately measures the ambient temperature, and allows accurate and adjustable setting of one or more desired temperatures.

Claims (11)

1. A thermostat control comprising a temperature sensor adapted to deliver a signal representing the sensed temperature, adjustable signal generating means for generating a reference signal representing a desired temperature, and a comparator for comparing the sensed temperature signal and the desired temperature signal and for delivering a signal dependent upon the relative magnitudes of the sensed and desired temperatures.

2. A thermostat control as claimed in claim 1, wherein the temperature sensor includes an impedance, the value of which corresponds to a sensed temperature, the adjustable signal generating means comprising one or more desired temperature setting means, the or each desired temperature setting means providing a respective first voltage corresponding to a respective desired temperature and being associated with a comparator.

3. A thermostat control as claimed in claim 2, wherein the temperature sensor also includes an amplifier means, in conjunction with the impedance, and provides a second voltage corresponding to the sensed temperature.

4. A thermostat control as claimed in claim 3, wherein the second voltage is directly proportional to the sensed temperature.

5. A thermostat control as claimed in claim 4, wherein the second voltage increases by approximately 100mV for every 1"C change in the sensed temperature.

6. A thermostat control as claimed in any one of claims 2 to 5, further comprising switching means, the switching means allowing a selected desired temperature setting means, providing a selected first voltage, to be brought into operation.

7. A thermostat control as claimed in claim 6, wherein the second voltage is compared, by means of the comparator, with the selected first voltage to provide a control signal is provided on the basis of the comparison.

8. A thermostat control as claimed in any one of claims 2 to 7, wherein a desired temperature may be adjusted by varying a first voltage, as measured at a test point.

9. A thermostat control as claimed in any preceding claim, wherein the temperature sensor is located remote from the adjustable signal generating means.

10. A thermostat control as claimed in any preceding claim, further comprising display means, for indicating the value of the sensed temperature.

11. A thermostat control, substantially as herein described with reference to the accompanying drawings.