GB2032653A

GB2032653A - Thermally actuating device

Info

Publication number: GB2032653A
Application number: GB7931058A
Authority: GB
Original assignee: Robertshaw Orient Ltd
Current assignee: Robertshaw Orient Ltd
Priority date: 1978-09-07
Filing date: 1979-09-07
Publication date: 1980-05-08
Also published as: DE2936311C2; IT1122534B; DE2936311A1; FR2435747A1; GB2032653B; IT7925546A0

Abstract

A thermally actuated radiator valve includes a positive temperature coefficient thermistor (22) and a thermally responsive unit (17) including a mass (20a) of thermally expansible semisolid material packed in a casing (18) and heated by the PTC thermistor to expand to drive a piston rod (20) inserted in the casing. The movement of the piston rod is transmitted to a valve member (8) to open the valve. A negative temperature coefficient thermistor (24) is mounted on the casing (18) in thermally conductive relation, to detect the displacement of the valve member in terms of the temperature of the thermal unit. A control circuit (Fig. 5 not shown) is responsive to the detected displacement of the valve member to energise the PTC thermistor so as to put the valve member at a desired position. The control circuit includes a further NTC thermistor (37) for sensing room temperature. <IMAGE>

Description

SPECIFICATION Thermally actuating device This invention relates to a thermally actuating and controlling device for displacing an object to be controlled. More specifically, the invention relates to a thermal device which is effective to position a valve member of a valve device finely and accurately.

There have been hitherto proposed a variety of centralised room temperature controlling systems using a thermal medium such as hot water or steam circulating between a boiler and radiators each disposed in one of a plurality of rooms. In such centralised room temperature controlling systems, however, it has been impossible or at least difficult to control the temperature of each room independently and precisely at a desired level.

This is attributable to the inability of adjusting the flow rate of thermal medium to each radiating unit precisely enough to maintain the room at a desired temperature. In this connection, there has been an ever-increasing demand for a valve device capable of freely and finely positioning its valve member with respect to its valve seat.

Accordingly, an object of the invention is to provide a control device for precisely positioning an object to be controlled, such as a valve member of a valve device.

According to one aspect of the invention, a thermally actuating and controlling device comprises heat generating means; means responsive to the so generated heat for displacing an object to be controlled; means to detect the displacement of the object; and means to control the heat generating means in terms of the detected displacement of the object so as to put the object at a desired position.

In a preferred embodiment the heat responsive means comprises a thermally expansible unit including a thermally conductive casing in thermally-conductive relation with the heat generating means; a thermally expansible semisolid material contained in the casing; and a movable member which is slidably inserted in the casing and is adapted to be driven by the expansible material. When the casing of the thermally expansible element is heated, the semisolid material in the casing expands to push the movable member outwardly. Since the displacement of the movable member increases with the thermal expansion of the expansible material, the displacement of the object can be determined in terms of the temperature of the thermally expansible material.Therefore, in a preferred embodiment of the invention a heatsensitive element is attached to the thermally expansible element and is used to determine the displacement of the object. Preferably the heat generator comprises a positive temperature coefficient thermistor and the temperature (displacement) sensor comprises a negative temperature coefficient thermistor.

Further, the movable member may be springbiased so that when the expansible material cools, the movable member automatically withdraws into the casing.

According to another aspect of the invention, a thermally-controlled valve device comprises a housing having an inlet, an outlet and a passage extending therebetween; a valve member movable in the passage between closed and open positions; a heat source; means responsive to a heat input from the heat source to drive the valve member towards the open position; means to detect the position of the valve member; and means to control the heat source in terms of the detected position of the valve member, thereby putting the valve member at a desired position. In a preferred embodiment, the heat responsive means comprises a thermally expansible unit as described above, and the means for detecting the position of the valve member comprises a temperature sensor for the thermally expansible unit.

According to a further aspect of the invention, a room-temperature controlling system comprises a thermal medium circulating path including a thermal medium supplying unit, and a heat exchanging unit disposed in a room; and valve means to regulate the flow rate of the thermal medium to the associated heat exchanging unit.

The valve means includes a housing having an inlet, an outlet and a passage extending therebetween; a valve member movable in the passage between closed and open positions; a heat source; means responsive to the heat input from the heat source to drive the valve member towards the open position; and means to detect the position of the valve member.The system also comprises means to sense the room temperature; roomtemperature setting means; and control means constituting a feed-back control loop together with the valve member position detecting means, the room-temperature sensing means and the room-temperature setting means thereby controlling the heat source so that the heat input to the heat responsive means is adjusted for the valve member to allow the passage of the thermal medium to the heat exchanging unit at a precisely regulated flow rate to maintain the room at a set temperature.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a valve device incorporating a thermally actuating and controlling device in accordance with the invention; Fig. 2 is a graph showing the characteristics of a heat responsive-and-motive unit of the valve device in terms of temperature-to-displacement of an object; Fig. 3 is a graph showing the characteristics of a positive temperature coefficient thermistor used in the valve device in terms of temperature-toresistance; Fig. 4 is a graph showing the characteristics of the positive temperature coefficient thermistor used in the valve device in terms of temperature to-conduction time; Fig. 5 is a circuit diagram of the roomtemperature control circuit including the valve device shown in Fig. 1;; Fig. 6 is a graph showing the modified characteristics of the heat responsive-and-motive unit in terms of temperature-to-displacement of an object under the control by the circuit shown in Fig. 5; Fig. 7 is a graph showing the relation between the flow rate and the displacement of a piston rod in the heat responsive-and-motive unit when operated within the operation range shown in Fig.

6: Fig. 8 is a graph showing an ideal relationship between the displacement of the piston rod of the heat responsive-and-motive unit and the inverted input of the amplifier 34 shown in Fig. 5 when operated within the operation range shown in Fig.

6; and Fig. 9 shows diagrammatically a fluid circuit of centralized room-temperature control system.

Referring to Fig. 1, there is shown a valve device incorporating a thermally actuating and controlling device in accordance with this invention. As shown, the valve device includes a housing 3 having an inlet 1 and an outlet 2. The valve device shown can effectively be used in a centralized room-temperature control system as diagrammatically shown in Fig. 9. In such a case, the inlet 1 of the valve device V is connected through a flow pipe to an outlet of a thermal medium supplying unit such as a boiler 50, and the outlet 2 of the valve is connected to an inlet of an associated heat exchanging unit such as a radiator 51 located in a room to supply a thermal medium such as hot water or steam from the boiler to the associated radiator. An outlet of the radiator is connected through a return pipe to a recovery port of the boiler.One valve device is ordinarily provided for each radiator, but in the case that two or more valve devices are located in one room, one valve device may be provided common to all the radiators in one room.

The valve device has a passage 4 formed to extend between the inlet 1 and the outlet 2. The passage 4 is partitioned by a wall 6 having an opening 5 as shown in the drawing. The partition wall 6 has a valve seat 7 formed around the opening 5 and a valve member 8 is forced to be seated on the valve seat 7 by a coil spring 9 which is supported by a cover 10 liquid-tightly attached to the housing 3. Therefore, when the valve member 8 is forced against the valve seat 7 by the coil spring 9 as shown, the passage is interrupted between the inlet 1 and the outlet 2, so that the thermal medium from the thermal medium supplying unit is not supplied to the associated heat exchanging unit.

A valve driving device 11 is disposed on the upper portion of the valve housing. The valve driving device 11 has a casing 12 which is secured to the housing 3 by means of bolts 13.

A rod 14 is disposed in the casing and has a lower portion which extends through the wall of the housing 3 and contacts at its lower end with an upper portion of the valve member 8. The rod 14 is mounted liquid-tightly and slidably with respect to the housing by means of an O-ring 1 5.

An abutting member 1 6 is mounted on the upper end of the rod 14. On the upper portion of the casing 12, there is mounted a heat responsiveand-motive unit such as a thermally expansible unit 1 7 available under the tradename "Power Pill". For better understanding, description will now be made on the thermal expansible unit 17 taking the "Power Pill" as an example. The unit 1 7 includes a thermally-conductive metal container 1 8 which has a cylinder portion 1 9 for slidably receiving a movable member such as a piston rod 20 and a semisolid material 20a having a high coefficient of thermal expansion and encapsulated in the container 1 8 to engage and drive the piston rod 20.In the case of the embodiment as shown, the heat responsive-and-motive unit 1 7 is positioned so that the free end of the piston rod 20 engages with the abutting member 1 6.

Further, the cylinder portion 1 9 is inserted into a hole formed in the top wall of the casing 12 and then a ring 21 is fitted over a concaved portion of the cylinder portion 1 9 thereby fixing the heat responsive-and-motive unit 1 7 to the casing.

The piston rod 20 is in a retracted condition under ordinary temperature. When the metal container 18 is heated, the semisolid material in the container expands to force the piston rod 20 outwardly. When the container is cooled down, the semisolid material contracts whereby the piston rod is returned to the retracted position under the action of the coil spring 9 which acts through the rod 14 and the valve member 8. The movement of the free end of the piston rod in the case of the "Power Pill" is shown in Fig. 2. In Fig.

2, the mark "0" on the curved line designates the position of the piston rod 20 under ordinary temperature and the mark "x" designates the position of the same under 1 000C. Thus, with the increase of temperature from room temperature to 1 000C, the piston rod will be pushed outwardly by about 5-7 mm. As is apparent from Fig. 2, the projection of the piston rod is substantially in linear proportion with the increase in temperature of the "Power Pill".

A positive temperature coefficient (PTC) thermistor 22 is mounted on and is brought into contact with the upper portion of the metal container 1 8 of the heat responsive-and-motive unit 1 7 by a clamping member 23 so as to heat the thermal expansibie material in the metal container.

As well-known, the PTC thermistor 22 has the temperature characteristics as shown in Figs. 3 and 4. More specifically, there is a limit in increase of temperature as shown in Fig. 4. Accordingly, if the PTC thermistor having temperature limit of about 1000C is selected, the heat responsive-andmotive unit can be heated- until about 1000C but is in no way heated at a temperature beyond 1 000C. This will give a sufficient protection for the heat responsive-and-motive unit where the thermal expansible semisolid material in the unit is destroyed or resolved under a temperature exceeding a predetermined value.

A negative temperature coefficient thermistor 24 is mounted on the heat responsive-and-motive unit 17 by a clamping member 25 to measure the temperature of the unit 1 7 and hence the distance of the projection of the piston rod 20.

Operation of the valve device shown in Fig. 1 will be described. When the PTC thermistor 22 is not activated and is under ordinary temperature, the valve member 8, the rod 14 and the piston rod 20 are held at their positions as shown by the coil spring 9. Therefore, the passage between the inlet 1 and the outlet 2 is interrupted.

When the PTC thermistor 22 is energized, the thermal expansible material in the heat responsive-and-motive unit 1 7 expands to push the piston rod 20 outwardly. As the result, the rod 14 is pushed by the piston rod 20 to drive the valve member 8 against the spring 9, thereby opening the valve. Thus, the thermal medium flows from the inlet 1 through the opening 5 and the space between the valve member 8 and the valve seat 7 to the outlet 2. In this instance, the amount of displacement of the valve member 8, that is, the degree of valve opening is substantially proportional to the flow rate of the thermal medium flowing from the inlet 1 to the outlet, and the flow rate of the thermal medium supplied through the valve to the heat exchanger is in proportion to the amount of the heat radiated from the heat exchanger to increase the room temperature.Accordingly, the amount of the heat radiated from the heat exchanger to the room and hence the room temperature can be regulated by controlling the degree of valve opening. On the other hand, the degree of valve opening, that is, the amount of displacement of the valve member 8, is equal to the amount of displacement of the piston rod of the heat responsive-and-motive unit 1 7. Furthermore, as mentioned above, the amount of displacement of the piston rod is substantially in linear proportion to the change in temperature of the heat responsive-and-motive unit. Therefore, the amount of displacement of the piston rod, that is, the degree of valve opening, can be sensed by sensing the temperature of the heat responsiveand-motive unit by means of the negative characteristic thermistor 24.Therefore, if supply of an electric current to the PTC thermistor 22 is controlled, for example by intermittently supplying an electric current to it on the basis of the data detected by the thermistor 24, so that the heat responsiveand-motive unit is held at such a temperature as to give a desired amount of displacement, that is, a desired degree of valve opening, the supply of the thermal medium to the heat exchanger can then be maintained at such a level as to maintain the room at a desired temperature.

Fig. 5 is a circuit diagram of the control circuit for a room temperature control system utilizing the valve means as shown in Fig. 1.

Referring to Fig. 5, a constant voltage direct current power source 30 is composed of, for example, an alternating current power source 31, a rectifier 32, a capacitor Ca, a resistor R1, and a zener diode ZD1, which are connected as shown.

The constant-voltage source 30 has output terminals connected across a variable resistor VR for setting a desired room temperature. A variable terminal of the variable resistor VR, is connected to an inverted input of a differential amplifier 33.

The differential amplifier 33 has a non-inverted input connected to a connecting point between a resistor R2 and a negative temperature coefficient thermistor 37 for sensing the room temperature.

The other terminal of the resistor R2 is connected to the positive line of the constant-voltage source 30, and the other terminal of the thermistor 34 is connected to the negative line of the constantvoltage source 30 through a variable resistor VR2.

Therefore, an analog voltage corresponding to the desired room temperature set by the variable resistor VR1 is applied to the inverted input of the differential amplifier 33, while an analog voltage corresponding to the room temperature sensed by the thermistor 34 is applied to the non-inverted input of the differential amplifier 33. As a result, the differential amplifer 33 outputs a voltage signal corresponding to the voltage difference between its two inputs, which expresses a difference between the desired room temperature and the actual room temperature.

Specifically, when a lower temperature is set by the variable resistorVR1, a larger voltage is applied to the inverted input of the amplifier 33, and when a high temperature is set, a smaller voltage is applied to the inverted input. On the other hand, the resistance of the thermistor 34 is small at high room temperature whereby a small voltage is applied to the non-inverted input of the differential amplifier 33. And, when the room temperature is low, the resistance of the thermistor 34 is large whereby a larger voltage is applied to the noninverted inverted input of the amplifier 33.Therefore, when the set room temperature is higher than the actua! room temperature, the voltage applied to the non-inverted input of the differential amplifier 33 is higher than the voltage at the inverted input so that a positive voltage difference is outputted from the amplifier 33, and then, as the actual room temperature approaches the set value, the output voltage become small.

The output of the differential amplifier 33 is connected through a resistor R3 to a circuit 40 for controlling the piston rod projection and hence the degree of valve opening. The circuit 40 includes a differential amplifier 34 having a non-inverted input connected through the resistor R3 to the output of the differential amplifier 33.The noninverted input is also connected to the negative line of the constant-voltage source 30 through a zener diode ZD2. The zener diode ZD2 functions to provide a protection against excess voltage which may otherwise be applied to the differential amplifier 34 and at the same time limit the voltage applied to the differential amplifier 34 below the determined value .The connection point between a variable resistor VR3 and a resistor R4 is connected to an inverted input of the differential amplifier 34, and the other terminal of the variable resistor VR3 is connected to the positive line of the constantvoltage source 30 through the negative temperature coefficient thermistor 24 of the valve device as shonw in Fig. 1. The other terminal of the resistor R4 is connected to the negative line of the constant-voltage source 30.

As mentioned above, the thermistor 24 senses the amount of displacement of the piston rod and hence the degree of valve opening by sensing the temperature of the heat responsive-and-motive unit 17. Specifically, when the temperature of the heat responsive-and-motive unit is low, that is, when the degree of valve opening is small, the resistance of the thermistor 24 is large so that the voltage applied to the inverted input of the differential amplifier 34 is small. On the other hand, when the temperature of the heat responsive-and-motive unit is high, that is, when the degree of valve opening is large, the resistance of the thermistor 24 is small, whereby a large voltage is applied to the inverted input of the differential amplifier 34.

Furthermore, the voltage signal applied to the non-inverted input of differential amplifier 34 indicates the difference between the set temperature and the actual room temperature. On the basis of the difference, the amount of change in the flow rate of the thermal medium is determined for the purpose of making the room temperature consistent with the set temperature.

The flow rate is dependent upon the degree of valve opening. Therefore, the differential amplifier 34 outputs a signal indicating the amount to be corrected in the degree of valve opening to make the temperature difference zero.

The output of the differential amplifier 34 is connected to a base of the transistor 35 through the resistor R5 and the base of the transistor is connected to the negative line of the constantvoltage source 30 through a resistor R6. The collector of the transistor is connected to the positive line of the constant-voltage source 30 through a relay coil Rc, and the emitter is connected to the negative line of the constantvoltage source 30. And, the PTC thermistor 22 is connected to a power source 36 through a normally opened relay contact Ra controlled by the relay coil Rc.When the voltage supplied to the non-inverted input of the differential amplifier 34 is higher than the voltage appearing at its inverted input, the output of the differential amplifier 34 is positive, so that transistor 35 is rendered conductive to energize the relay coil Rc so as to close the contact Ra thereby energizing the PTC thermistor 22. With the heating of the heat responsive-and-motive unit 1 7 by the PTC thermistor 22, the piston rod is projected to drive the valve member so that the degree of valve opening is increased.When the voltage appearing at the inverted input of the amplifier 34 becomes larger than the voltage appearing at its non-inverted input because of increase in the temperature of the heat responsive-and-motive unit 17, the amplifier 34 inverts its output into a negative voltage, so that the transistor 35 is rendered nonconductive to deenergize the relay coil Rc so as to open the relay contact Ra thereby deenergizing the PTC thermistor 22. Consequently, the heat responsive-and-motive element 1 7 is naturally cooled and the piston rod is retracted so that the valve member 8 is forced toward the valve seat 7 whereby the degree of valve opening is decreased.

The valve device as shown in Fig. 1 is controlled by the control circuit as mentioned above to regulate the flow rate of the thermal medium so as to control the room temperature.

However, in order to finely regulate the room temperature with high-accuracy, it is preferable that the amount sensed by the sensor is in linear relationship with the variable to be controlled.

First, for this purpose, the heat responsive-andmotive unit 1 7 is operated within a range wherein the displacement of the piston rod is in linear relationship to the temperature, for example, within the range from 400C to 600 C. Referring to the graph shown in Fig. 6, there is shown the relationship between the displacement of the piston rod 20 and the temperature with the position of the piston rod 20 at 400C taken as the reference position. On the other hand, referring to Fig. 7, there is shown the relationship between the displacement of the piston rod (the degree of valve opening) and the flow rate through the valve. As is apparent from Fig. 7, the displacement of the piston rod 20 is substantially in direct proportion to the flow rate insofar as the displacement is between 0.5 mm and 2.5 mm.With the displacement of the piston rod 20 less than 0.5 mm, namely with the flow rate less than 10%, the displacement of the piston rod is not in linear relation with the flow rate. But, this is negligible because the valve device is seldom operated at such a low flow rate. On the other hand, the characteristics of the thermistor 24 and the resistances of the thermistor 24, the resistor R4 and the variable resistor VR3 are selected so that the relationship between the displacement of the piston rod 20 and the voltage appearing on the inverted input of the differential amplifier 34 is made linear as shown in Fig. 8. For example, the relationship as shown in Fig. 8 can be accomplished by using a thermistor having a resistance which linearly decreases in response to an increase in temperature from 400C to 600C and by making the sum of the resistance of the thermistor at 500C and the resistance of the variable resistor VR3 substantially equal to the resistance of the resistor R4.

In this manner, the supply of the thermal medium can be precisely controlled in accordance with the temperature difference signal applied to the non-inverted input of the differential amplifier 34.

Now, the operation of the room temperature controlling system as shown in Fig. 9 utilizing the valve device shown in Fig. 1 and the control circuit shown in Fig 5 will be described.

First, a desired room temperature is set by adjusting the variable resistor VRa and the control device is turned on. At this time, if the room temperature is greatly lower than the set temperature, the resistance of the thermistor 34 for sensing the room temperature is large and the voltage difference between the two inputs of the differential amplifier 33 is also considerably large.

Accordingly, the amplifier 33 outputs a large voltage signal which is in turn limited by the zener diode ZD2 and is applied to the non-inverted input of the differential amplifier 34. On the other hand, the heat responsive-and-motive unit 1 7 is under ordinary temperature, and a small voltage appears at the inverted input of the differential amplifier 34. As a result, the differential amplifier 34 outputs a positive voltage signal to turn the transistor 35 on so that the PTC thermistor 22 is energized to heat the heat responsive-and-motive unit 1 7 whereby the valve member 8 is depressed downwardly against the function of the coil spring 9 by the projected piston rod 20 through the rod member 14. Thus, the termal medium is supplied through the valve device to the heat exchanger.

The valve device is perfectly opened with continuous energization of the PTC thermistor. As long as the voltage appearing at the inverted input of the differential amplifier 34 is considerably lower than the large voltage appearing at its noninverted input, the PTC thermistor continues to be energized. This state continues until the room temperature sensed by the thermistor 34 approaches the set temperature so that the output voltage of the differential amplifier 33 becomes lower than the voltage appearing at the inverted input of the amplifier 34 when the degree of valve opening is at 100%.

As the room temperature approaches the set temperature, the voltage of the non-inverted input of the differential amplifier 34 becomes lower than the voltage appearing at its inverted input under 100% of valve opening. Namely, at this time, the degree of valve opening required for making the temperature difference zero becomes lower than 100%. But, since the degree of valve opening is still maintained at 100%, the output of the differential amplifier 34 becomes negative to turn the transistor 35 off thereby deenergizing the relay coil and further the PTC thermistor. As the result, the heat responsive-and-motive unit 1 7 is cooled by natural radiation, so that the piston rod 20 is retracted upwardly by the coil sprng 9 whereby the degree of valve opening is decreased.

When the degree of valve opening is decreased and the voltage applied to the inverted input of the differential amplifier 34 becomes lower than the voltage appearing at its non-inverted input, the output of the differential amplifier 34 becomes positive again so that the PTC thermistor 22 is energized. In this manner, the valve device is controlled and maintained at the required degree of valve opening indicated by the output of the differential amplifier 33, and also, as the room temperature approaches the set value, the output voltage of the amplifier 33 indicating the required degree of valve opening becomes low so that the actual valve opening of the valve device is correspondingly made small.

Once the room has been brought to the set temperature in this manner, if the room temperature becomes slightly lower than the set temperature by, for example, 1 or 20C, the differential amplifier 33 outputs a small signal, for example a signal requiring 70% of valve opening which is in turn applied to the non-inverted input of the differential amplifier 34. As a result, the valve controlling circuit 40 which includes the differential amplifier 34, the transistor 35, the relay, the PTC thermistor 22, and the negative characteristic thermistor 24, operates so as to hold the degree of valve opening at 70%.

Accordingly, it is possible to restore the room to the set temperature without the excessive increase of the room temperature which would occur if the valve should be opened 100%, that is, without an overshoot in control. This same effect is also obtained when the room temperature control system is first turned on at a temperature lower than the set temperature or when the temperature setting is changed, either upwardly or downwardly, after the system is in operation.

As apparent from above, if the valve as shown in Fig. 1 is controlled by the control circuit as shown Fig. 5, the room temperature can be finely controlled with high-accuracy.

Claims

1. A thermally actuating and controlling device, comprising heat generating means; means responsive to the so generated heat for displacing an object to be controlled; means to detect the displacement of the object; and means to control the heat generating means in terms of the detected displacement of the object so as to put the object at a desired position.

2. A device as claimed in Claim 1, wherein the heat responsive means comprises a thermally expansible unit including a thermally-conductive casing in thermally conductive relation with the heat generating means; a thermally expansible semisolid material contained in the casing; and a movable member which is slidably inserted in the casing and is adapted to be driven by the expansible material to displace the object, whereby when the casing is heated by the heat generating means, the material in the casing expands to displace the object through said movable member.

3. A device as claimed in claim 2, wherein the detecting means is a heat sensitive element attached to the thermally expansible unit to determine the displacement of the object in terms of the temperature of the thermally expansible material.

4. A device as claimed in claim 3, wherein the heat generating means is a positive temperature coefficient thermistor and the heat sensitive element is a negative temperature coefficient thermistor.

5. A thermally-controlled valve device, comprising a housing having an inlet, an outlet and a passage extending therebetween; a valve member movable in the passage between closed and open positions; a heat source: means responsive to a heat input from the heat source; means responsive to a heat input from the heat source to drive the valve member towards the open position; means to detect the position of the valve member; and means to control the heat source in terms of the detected position of the valve member, thereby putting the valve member at a desired position.

6. A thermally-controlled valve device as claimed in claim 5, wherein the heat responsive means is a thermally expansible unit including a thermally conductive casing adapted to be heated by the heat source, a thermally expansible material contained in the casing, and a movable member slidably inserted in the casing and adapted to be driven by the expansible material to displace the valve member, whereby when the casing is heated by the heat source, the material in the casing expands to displace the valve member through the movable member.

7. A device as claimed in claim 6, wherein the detecting means is a heat sensitive element attached to the thermally expansible unit to determine the displacement of the valve member in terms of the temperature of the thermally expansible material.

8. A device as claimed in claim 7, wherein the heat source is a positive temperature coefficient thermistor and the heat sensitive element is a negative temperature coefficient thermistor

9. A device as claimed in any one of claims 5-8, wherein the controlling means is adapted to receive a reference signal from a position setting means and a position signal from the position detecting means to control the heat source so that the heat input to the heat responsive means is adjusted for the valve member to allow the degree of valve opening at a desired value.

1 0. a room-temperature controlling system, comprising a thermal medium circulating path including a thermal medium supplying unit, and a heat exchanging unit disposed in a room; valve means for regulating the flow rate of the thermal medium to an associated heat exchanging unit, the valve means including a housing having an inlet, and outlet and a passage extending therebetween, a valve member movable in the passage between closed and open positions, a heat source, means responsive to the heat input from the heat source to drive the valve member towards the open position, and means to detect the position of the valve member; means to sense the room temperature; room-temperature setting means; and control means connected to the valve member position detecting means, the roomtemperature sensing means and the roomtemperature setting means to control the heat source so that the heat input to the heatresponsive means is adjusted for the valve member to allow the passage of the thermal medium to the heat exchanging unit at a precisely regulated flow rate to maintain the room at a set temperature.

11. A system as claimed in claim 10, wherein the heat responsive means is a thermally expansible unit including a thermally conductive casing adapted to be heated by the heat source, a thermally expansible material contained in the casing, and a movable member slidably inserted in the casing and adapted to be driven by the expansible material to displace the valve member, so that when the casing is heated by the heat source, the thermally expansible material expands to displace the valve member via the movable member.

12. A system as claimed in claim 11, wherein the position detecting means is a heat sensitive element attached to the thermally expansible unit to determine the displacement of the valve member in terms of the temperature of the thermally expansible material.

13. A system as claimed in claim 12, wherein the heat source is a positive temperature coefficient thermistor and the heat sensitive element is a negative temperature coefficient thermistor.

14. A system as claimed in any one of claims 10-1 3, wherein the control means comprises a first differential amplifier to receive a reference temperature signal from the room-temperature setting means and an actual. temperature signal from the room temperature sensing means to generate a temperature difference signal, a second differential amplifier for receiving a position signal from the valve member position detecting means and the temperature difference signal to generate a valve member position correcting signal, and a driving circuit to receive the valve member position correcting signal to drive the heat source.

1 5. A device as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.