GB2225415A

GB2225415A - Fluid control valve

Info

Publication number: GB2225415A
Application number: GB8827186A
Authority: GB
Inventors: M S Pulzer; Toshiyuki Kondo; Katsuhiro Mori
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1988-11-21
Filing date: 1988-11-21
Publication date: 1990-05-30
Also published as: JPH02180388A; GB8827186D0

Abstract

A fluid flow control valve (10) is controlled by a stepping motor (20) which drives the valve member (13) relative to the valve seat (14) through a drive mechanism (30, 15). The drive mechanism (30, 15) converts the rotary output of the motor (20) into axial valve movement. An interlock is provided between limit switches (50) and the control circuitry for the stepping motor (20), to ensure that the motor does not attempt to drive the valve member (13) beyond the limits of its intended motion, the switches (50) being controlled by an element (51) which is axially movable with the valve member (13). <IMAGE>

Description

TITLE: Fluid Control Valve DESCRIPTION Field of the Invention The invention relates to fluid flow control valves, which may be proportional control valves or on/off valves.

The advantages provided by the valves of the invention are, however, most suited to proportional control valves.

Background Art Many fluid flow valves in commercial -use are pneumatic valves, utilizing a source of compressed air as the source of motive power. Such pneumatically controlled valves have the following disadvantages: (1) Pneumatic valves require an on-line source of compressed air. Preferably that air should be clean, dry and oil-free. Although such airlines are available in many industrial premises, when they have to be specially provided they give rise to a system that is expensive, bulky and expensive to maintain.

(2) Pneumatic valves do not eradicate the need for electical cabling, since control signals have to be received and transducer signals transmitted. The resultant system is mixed and complex.

(3) Pneumatic valves are intricate high precision devices which require careful maintenance and a large initial capital outlay.

(4) Pneumatic valves are analog devices which require an analog-to-digital converter interface before they can be integrated into a digital computer control system.

This increases the cost and complexity of the system.

(5) If the valve is to control a fluid medium at a substantially higher pressure than the pressure of the air supply, then a large area diaphragm must be used, which gives rise to bulky and heavy control valves.

An alternative to pneumatically controlled valves is an electrically controlled fluid flow control valve. Many have been proposed, and have the advantage that the force controlling the movement of the valve member in opposition to the pressure of the fluid being controlled can be quite freely selected by suitable choice of the size of the motor or of the reduction gearing provided between the motor and the valve member. An increase in this valve actuating force does, however, carry with it an added risk of substantial damage to the valve assembly if the valve member is driven onto its valve seat under the full operating force of the motor, or alternatively drawn back against a housing with that same full operating force.Furthermore, it is desirable for any electrically actuated fluid control valve to be capable of standing idle for prolonged periods without movement of the valve member, and during those periods it is important that the fluid flow itself does not effect movement of the valve member from its intended static position.

Summary of the Invention The invention provides a fluid flow control valve comprising an axially movable valve member engageable with a valve seat to control fluid flow, a stepping motor driving a rotary valve actuator, a drive mechanism between the rotary valve actuator and the valve member for controlling the two-way axial movement of the valve member in response to rotary movement of the valve actuator, limit switches assocaited with the valve member or with an axially movable part of the drive mechanism, for defining limits of permitted movement of the valve member, and means for preventing actuation of the stepping motor when that actuation would cause the said defined limits to be exceeded.The use of a stepping motor in the flow control valve of the invention is of significance because stepping motors have the characteristic that they can remain idle for prolonged periods without wandering from their rest position.

Indeed any mechanical bias on the valve member sufficient to exert a torque on the motor would be resisted up to a quite significant level, which makes the valve of the invention particularly robust and reliable.

If desired, the valve of the invention can be further improved by incorporating a feedback loop from a position sensor associated with the valve actuator or the valve member. The pulses controlling the stepping motor are then generated in response to a comparison between a command signal indicating the intended rest position of the valve member and the position feedback signal, with the effect that the valve member or its actuator are moved to and held in their precisely determined intended end position. Such a feedback loop avoids the possible problems that might be associated with the occasional pulse skipping of the stepping motor (that is to say, the failure of the motor to move one step for every single pulse supplied to it).

The stepping motor is preferably located fast in a side housing, protected by seals from the operating environment of the valve itself and of its drive mechanism. In those circumstances the drive mechanism incorporates a lost-motion mechanism whereby rotary movement of the axially fixed actuator is converted to rotary or non-rotary axial movement of the valve member.

The drive mechanism may also incorporate a lead screw for imparting the axial movement to the valve member.

The use of a lead screw has the dual benefit of enhanced mechanical advantage between the stepping motor and the valve member, and substantial isolation of the stepping motor from any reverse torque from the valve member, caused by fluid forces on the valve member when the valve is held in the same open or closed position for long periods.

The limit switches associated with the valve member or with an axially movable part of the drive mechanism are important in that they avoid the problems outlined above associated with driving the valve member under maximum operating torque against either the valve seat or the valve housing. The limit switches are preferably connected to a logic circuit so that they inhibit any command signal sent to the stepping motor that would result in continued movement of the valve member beyond the respective limits. It can readily be arranged that the inhibition is effective to prevent movement of the valve member beyond the limits, without interfering with reverse movement of the valve member within the limits.

DRAWINGS: Figure 1 is an axial section through a fluid flow control valve according to the invention, in the valve closed position; Figure 2 is a section similar to that of Figure 1 but through the valve in the open position; Figure 3 is a cross section taken along the line 3-3, in a plane transverse to that of Figure 1, through the lost motion components of the drive mechanism of Figure 1; Figure 4 is a logic diagram illustrating the operation of the limit switches of Figures 1 and 2; Figure 5 is a block diagram illustrating the motor control of the stepping motor of the valve of Figures 1 and 2; and Figure 6 is a block diagram illustrating the control and operation of a fluid flow control valve according to Figure 1 or Figure 2 when used to regulate the flow of water through a heat exchanger.

Best Node of Carrying Out the Invention Referring first to Figures 1 and 2, there is illustrated a liquid flow control valve 10 according to the invention.

The valve 10 comprises a valve body 11 through which is formed a flow passage. A valve member 13 is movable axially against a valve seat 14 in this flow passage, to restrict or interrupt fluid flow. The valve member 13 is provided with a screw-threaded mounting stem 15 engageable in a screw threaded portion of the valve body 11, so that angular rotation of the valve member 13 is sufficient to cause the valve member to be moved in the axial direction towards or away from the valve seat 14.

The angular rotation is induced by a drive stem 12 of an actuator assembly 40 mounted on the side of the valve body 11. A packing 16 is provided between the drive stem 12 and the valve body 11, to maintain the fluid integrity of the valve.

The actuator mechanism 40 includes a stepping motor 20 comprising a rotor 21 and a stator 22 in a housing 23.

The rotor 21 drives a drive shaft 24 which extends axially out of the housing 23 and terminates in a transverse drive pin 25. The drive pin 25 transmits the rotary motion of the rotor 21 to a lost-motion linkage 30 which is shown in isolation in Figure 3. The linkage 30 comprises an open-ended cylindrical portion 31 integrally connected to the drive stem 12. Between the cylindrical portion 31 and the drive stem 12 is an integrally formed flange portion 51 the purpose of which is described below. Two axial slots 32 formed in the cylindrical wall of the portion 31 at diametrically opposed locations receive the opposite ends of the transverse drive pin 25, to provide the rotary coupling between the drive shaft 24 and the lost motion linkage 30.The lost-motion facility is provided by the ability of the linkage 30 to move between the positions shown in Figures 1 and 2 without any interruption to the rotary coupling.

Because the screw-threaded mounting stem 15 is an integral part of the valve member 13, the movement of the valve head of valve member 13 relative to the valve seat 14 is both rotary and axial. The valve head therefore seats and unseats in a sliding motion relative to the valve seat 14. This can be beneficial in ensuring good seating of the valve member 13, although by providing a rotary coupling between the valve head and its screw-threaded mounting stem 15 a simple lift valve can be obtained, with no rotary component of relative movement between the valve head and the valve seat.

Two limit switches, shown schematically in Figures 1 and 2 as 50, are mounted on or through the side wall of the actuator mechanism 40 and are actuated by contact with the flange portion 51 when the linkage 30 reaches the valve fully closed and valve fully opened positions of Figures 1 and 2 respectively.

The stepping motor 20 is driven by a power electronics drive circuit, the number and steps through which the motor moves being determined by the digital output of that drive circuit.

Stepping motors are available which have a resolution of up to one thousand steps per revolution. Selection of the required resolution of the stepping motor, coupled with selection of the pitch of the screw thread between the stem 15 of the valve member and the valve body, determines the sensitivity of the flow control valve and the time required to move it from its fully open to its fully closed condition. Other factors controlling the range, resolution and response rate of the valve are the word length of the digital signal produced by the power electronics drive circuitry and the stepping rate of the motor.

The use of a fine pitch screw thread on the valve stem 15 provides a very substantial mechanical advantage as the valve member is moved axially against the controlled fluid pressure. It is therefore particularly important that the control valve 10 should include some safeguard to prevent the valve member 13 from being driven at full operating thrust onto the valve seat 14, and to prevent it from being withdrawn at full thrust against the valve body 11. The limit switches 50 achieve this element of control, in that the lower limit switch 50 illustrated in Figures 1 and 2 is effective to inhibit the continued action of the stepping motor 20 in driving the valve member 13 down onto the valve seat 14; and actuation of the upper limit switch 50 is effective to inhibit the further actuation of the stepping motor in the reverse direction.Figure 4 illustrates an example of one circuit that can be used to implement this interlocking of the limit switches and the stepping motor.

In Figure 4, the two limit switches 50 of Figures 1 and 2 are identified as 50a and 50b. Input signals to the interlock circuit comprise command signals A and B from a control computer, together with the signals from the two limit switches. A single positive-going pulse on signal A instructs the stepping motor to move one step in a sense such as to move the valve member 13 down in the valve closing direction, and a single positive-going pulse of signal B is sufficient to move the motor one step in the valve opening direction. The simultaneous generation of pulses in command signals A and B is not permitted.

When neither of the limit switches 50 and 50b is actuated, the command signals on lines A and B are transmitted as output signals on lines C and D as follows.

The signal on line C indicates that the stepping motor should run when the output pulse is high, and should stop when the output pulse is low. The signal on line D is the directional signal, indicating that the motor should move in the valve closing direction when the signal D is high and in the valve opening direction when the signal D is low.

It will be readily understood how the logic elements of the interlock circuit apply a selective inhibit to one input of an AND gate AND2, to inhibit the generation of a RUN signal at the output C when the directional signal on output D is such that the movement would overrun that particular limit. However if movement in the opposite direction, away from the limit switch, is instructed then the RUN signal is permitted to pass through the AND gate AND2 unhindered.

Figure 5 illustrates in block diagram form the general arrangement of control commands for the valve of Figures 1 and 2. In Figure 5, a control signal is input to a computer 80 to define the valve movement required. The output from the computer 80 is converted at 81 to a digital instruction to the stepping motor 82 which controls the valve 83 as set out in Figures 1 and 2. A position sensor, such as an optical shaft encoder between the stepping motor 82 and the valve 83, may if present feed the current valve position back to the computer through a feedback circuit 84, so that if any command pulses have for any reason been skipped or ignored the computer 80 can generate additional compensating pulses.

Figure 6 is a logic flow block diagram ilustrating the connection of a valve according to the invention in a system for regulating the flow of cooling water through a heat exchanger to control the exit temperature of hot air passing through the other section of the heat exchanger. The system feedback is derived from a thermocouple 100 measuring the air outlet temperature.

The analog output of the thermocouple is applied to the negative input of a summing junction 101, and a reference signal indicating the desired exit temperature of the air from the heat exchanger is applied to a positive input of the summing junction 101.

The output of the summing junction 101 is applied to an analog-to-digital converter 102 the digital output of which is processed by a digital computer 103. The computer provides, in block 104, command signals for a stepping motor 105 which opens and closes the fluid flow control valve 106 as illustrated in Figures 1 and 2.

A position feedback 107, which is an optional feature, comprises a sensor such as an optical shaft encoder between the stepping motor 105 and the valve 106, to sense the actual movement of the valve in response to the drive signals generated in the block 104. The feedback 107 relays that sensed valve movement back to the computer 103, so that if the motor were to skip command pulses, then this would be sensed by the computer the program of which could ensure that additional compensating pulses were generated.

In the embodiment illustrated in Figure 6, the valve 106 controls the flow of cooling water through a heat exchanger 107, so as to achieved the desired control of the exit temperature of the air that is cooled by that heat exchanger.

Claims

CLAIMS:

1. A fluid flow control valve comprising an axially movable valve member engageable with a valve seat to control fluid flow, a stepping motor driving a rotary valve actuator, a drive mechanism between the rotary valve actuator and the valve member for controlling the two-way axial movement of the valve member in response to rotary movement of the valve actuator, limit switches associated with the valve member or with an axially movable part of the drive mechanism, for defining limits of permitted movement of the valve member, and means for preventing actuation of the stepping motor when that actuation would cause the said defined limits to be exceeded.

2. A valve according to claim 1, wherein the drive mechanism incorporates a lost-motion mechanism whereby axially fixed rotary movement of the actuator is converted to rotary and axial movement of the valve member.

3. A valve according to claim 1, wherein the drive mechanism incorporates a lost-motion mechanism whereby axially fixed rotary movement of the actuator is converted to non-rotary axial movement of the valve member.

4. A valve according to any preceding claim, wherein the drive mechanism comprises a lead screw for imparting axial movement to the valve member.

5. A valve according to any preceding claim, wherein the stepping motor is controlled in response to a comparison between a command signal and a position feedback signal from a position sensor associated with the valve actuator or the valve member.

6. A valve according to claim 5, wherein the position sensor is an optical shaft encoder.

7. A valve according to claim 5, wherein the position sensor is a rotary potentiometer.

8. A valve according to any preceding claim, wherein the limit switches are connected to a logic circuit for inhibiting any command signal to the stepping motor that would result in continued movement of the valve member beyond the respective limits when those limits have been reached.

9. A valve according to any preceding claim, being a liquid flow control valve.

10. A valve according to any of claims 1 to 8 being a gas flow control valve.

11. A fluid flow control valve substantially as described herein with reference to the drawings.