GB2064168A

GB2064168A - A Pneumatic Power Amplifier

Info

Publication number: GB2064168A
Application number: GB8011616A
Authority: GB
Original assignee: SAUTER AG
Current assignee: SAUTER AG
Priority date: 1979-11-14
Filing date: 1980-04-08
Publication date: 1981-06-10
Also published as: FR2469588B1; GB2064168B; FR2469588A1; DE2946035C2; DE2946035A1

Abstract

A pneumatic power amplifier comprises a working pressure circuit 2-10 for the supply of compressed air to a load through a shut-off device 7, and a control pressure circuit 11, 18, 22 branched from the working pressure circuit through a fixed throttle 12 and connected to atmosphere through a variable throttle 19 for controlling the shut-off device 7 in accordance with a measuring force 20 acting on the variable throttle e.g. in response to temperature or humidity, and a non-return valve 14 is provided between the control pressure circuit and the load side of the working pressure circuit and pre- loaded towards the working pressure circuit, for ventilating the load under a condition of decreased measuring force. <IMAGE>

Description

SPECIFICATION A Pneumatic Power Amplifier This invention relates to a pneumatic power amplifier.

In pneumatic control circuits which are used, for example, for temperature control, humidity control or the like, compressed air can be supplied to the final control element of the control circuit through a pneumatic pressure distributor which includes a fixed throttle and a variable throttle, the line leading to the final control element being branched off from the connection between the two throttles. A very small measuring force derived from a temperature sensor, a humidity sensor or the like produces an adjustment of the variable throttle in such a manner that the pressure building up in front of it adjusts the control circuit in accordance with the measuring force. In a pneumatic control circuit, a pneumatic motor normally serves as a final control device and can have an air capacity of many litres.So that a rapid response of this pneumatic motor can be achieved with a variation of the measuring force, it is necessary to be able to supply a large quantity of air to the motor and to discharge the greatest possible quantity of air from the motor as rapidly as possible. A rapid discharge of air can only be obtained when the cross-section of the variable throttle is made substantially larger than that of the fixed throttle, but this does not assist rapid pressure increase to the pneumatic motor since the relatively small cross-section of the fixed throttle prevents this increase.

To overcome this disadvantage, pneumatic power amplifiers have been used in which a control pressure dependent on the measuring force is generated in a control pressure circuit and which opens a shut-off device in a working pressure circuit leading to the final control device representing the load, when it is desired to supply compressed air to the final control device, and opens a further shut-off device leading from the working pressure circuit to the surrounding atmosphere, when compressed air is to be bled off from the final control device. The use of the two shut-off devices for the air supply and the air discharge involves a complicated construction of the power amplifier.

In accordance with this invention, there is provided a pneumatic power amplifier comprising a working pressure circuit for the supply of compressed air to a load through a shut-off device, and a control pressure circuit branched off from the working pressure circuit through a fixed throttle and connected to the surrounding atmosphere through a variable throttle, the shutoff device being controlled in accordance with a measuring force acting on the variable throttle and a non-return valve being provided between the control pressure circuit and the load side of the working pressure circuit and pre-loaded towards the working pressure circuit. With this power amplifier, the ventilation of the load takes place through the non-return valve into the control pressure circuit and from the to the variable throttle to the surrounding atmosphere.

Thus, a separate shut-off device for the ventilation is not required, which substantially simplifies the internal construction of the power amplifier.

Moreover, with this simplification, the power amplifier is suitable for incorporation in control apparatus which must respond rapidly to variations in the measured parameters such as temperature, humidity and the like.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view of a power amplifier; Figure 2 is a partial sectional view of the power amplifier of Figure 1 with an alternative form of non-return valve between the control pressure circuit and the working pressure circuit; and Figure 3 is a plan view of the diaphragm used with the non-return valve of Figure 2.

The power amplifier 1 illustrated in Figure 1 has an input 2 to which is connected a compressed air source. From the input 2, a duct 3 leads to a valve 4 which is formed by a ball 7 urged against a sealing seat 6 by a spring 5. The valve 4 issues into a pressure chamber 8 which is connected through a duct 9 to the outlet 10 from the power amplifier. The load, which it is desired to actuate with the aid of the compressed air, can be connected to the said outlet 1 0.

The path leading from the input 2 through the duct 3, the valve 4, the pressure chamber 8 and the duct 9 to the outlet 10 forms the working pressure circuit leading from the pressure medium source to the load.

A further duct 11 is branched from the duct 3 and issues, through a fixed throttle 12 and a further duct 13 as well as through a non-return valve 14, into the duct 9 leading to the outlet 10.

The non-return valve 14 includes a ball 1 5 which is forced by a spring 1 6 against a sealing seat 1 7.

Between the fixed throttle 12 and the nonreturn valve 14, a branch duct 1 8 leads from the duct 13 to the surrounding atmosphere. The outlet from the branch duct 1 8 to the surrounding atmosphere is formed as a sealing seat against which a ball 19 is held by a measuring force which is illustrated by the arrow 20. Together with the sealing seat at the outlet from the branch duct 18, the ball 1 9 forms a variable throttle which, according to the value of the measuring force, varies the flow cross-section which is available to the compressed air arriving through the fixed throttle 12 into the duct 13 and into the branch duct 1 8 for discharge to the surrounding atmosphere.

From the duct 13, a further branch duct 21 leads into a pressure chamber 22 which is hermetically separated from the pressure chamber 8 by means of an elastic diaphragm 23.

A plate 24 which carries a pin 25 acting on the ball 7 of the valve 4 is fixed to the said diaphragm 23 in the pressure chamber 8.

The variable throttle formed by the opening from the branch duct 1 8 and the ball 19, together with the fixed throttle 12, forms a system with the aid of which the measuring force in the control pressure circuit formed by the duct 13, the branch ducts 18 and 21 as well as the pressure chamber 22, is converted into a control pressure. The greater the measuring force, the higher becomes the control pressure which is built up in the control pressure circuit at fixed throttle 12 and the variable throttle due to the pressure drop produced by the quantity of air flowing therethrough.

It must be pointed out that, instead of the ball 19 against the opening from the branch duct 1 8 issuing to the surrounding atmosphere, a flapper can be applied which is moved against the opening, then formed as a nozzle, under the influence of the measuring force. Such a variable throttle formed from a flapper and a nozzle would also facilitate the conversion of the measuring force into a control pressure in the control pressure circuit.

In describing the method of operation of the power amplifier illustrated in Figure 1, it is assumed that the measuring force is generated bs a temperature sensor which is formed by a bimetal and that a pneumatic motor for actuating a flap is connected as a load to the outlet 10 from the power amplifier 1. Moreover, the power amplifier is included in a temperature control circuit and with a temperature rise an increase in the measuring force acting on the ball 1 9 occurs, which leads to a supply of compressed air to the pneumatic motor, so that, for example, the warm air supply is throttled by means of the flap actuated by the pneumatic motor.

When, in the opposite case, the temperature sensor indicates a fall in temperature, the pneumatic motor must be ventilated so that it opens the warm air flap further.

In detail, the power amplifier 1 illustrated in Figure 1 operates in the following manner: A) Air Supply to the Pneumatic Motor It is assumed that the temperature sensor which generates the measuring force senses a temperature increase which manifests itself correspondingly by an increased measuring force.

The compressed air supplied to the inlet 2 arrives through the duct 11 and the fixed throttle 12 in the control pressure circuit from which it flows away through the branch duct 1 8 over the periphery of the ball 1 9 to the surrounding atmosphere. Moreover, such a pressure is established in the control pressure circuit that equilibrium with the measuring force acting on the ball is produced. The control pressure in the control pressure circuit acts in the pressure chamber 22 against the diaphragm 23.Since, on the basis of the sensed temperature increase and the increase in the measuring force resulting therefrom, the control pressure is higher than the pressure prevailing in the pressure chamber 8, which is equal to the pressure in the pneumatic motor, a downwards displacement of the diaphragm 23 takes place into the position shown in Figure 1. This displacement has the result that the valve 4 is opened by means of the pin 25 arranged on the plate 24. The compressed air supplied to the inlet 2 can then flow to the pneumatic motor unhindered and the desired adjustment of the blade actuated thereby take place. However, with the supply of compressed air to the pneumatic motor, the pressure in the pressure chamber 8 and in the duct 9 also rises until it is finally equal to the control pressure in the pressure chamber 22.When the pressures are equal on both sides of the diaphragm 23, the valve 4 closes once again under the action of the spring 5 so that the supply of compressed air to the pneumatic motor ceases.

B) Steady State Condition In the steady state condition, a balance exists between the measuring force acting on the ball 1 9 and the control pressure in the control pressure circuit, and the pressure in the pressure chamber 8, which is equal to the pressure in the pneumatic motor, is equal to the control pressure.

The valve 4 is closed so that no supply of compressed air to the pneumatic motor takes place and consequently the pressure in the pressure chamber 8 remains constant. On the other hand, the continuous flow of compressed air to the surrounding atmosphere takes place through the fixed throttle 12, through the branch duct 1 8 and through the variable throttle.

C) Ventilation of the Pneumatic Motor When the temperature sensor senses a drop in temperature and consequently a reduction is produced in the measuring force acting on the ball 1 9, more air can flow out of the control pressure circuit over the ball periphery to the surrounding atmosphere so that the control pressure falls.

Since the pressure in the pressure chamber 8 and in the duct 9 is then higher than the control pressure, the non-return valve 14 opens so that the compressed air can flow away from the pneumatic motor through the said non-return valve, the duct 13 and the branch duct 18 to the surrounding atmosphere. In so doing, the nonreturn valve 14 remains open until the control pressure is equal to the pressure in the duct 9, thus is equal to the pressure in the pneumatic motor. As soon as the pressure in the duct 9 becomes lower than the control pressure, the non-return valve 14 is closed whereupon the described steady state condition occurs once again.

According to Figure 2, the non-return valve 14 can instead be formed by using the diaphragm 23. As can be appreciated from Figure 2 and the plan view of Figure 3, the duct section 27 leading from the duct 13 of the control pressure circuit into the duct 9 is surrounded, in this case, by a sealing rib 28 over which the diaphragm 23 is stretched. Outside the sealing rib, two arcuate cut-outs 29 are formed in the diaphragm which provide an air passage as soon ass the diaphragm is raised from the sealing rib 27 as a result of a pressure difference.

Thus, the described power amplifier 1 not only provides a rapid increase in the pressure supplied to a load but also a rapid ventilation of the load with a simple construction so that it is very suitable for incorporation in control systems which must respond rapidly to varying measuring parameters and which must also function perfectly with low leakage at the outlet.

Claims

1. A pneumatic power amplifier comprising a working pressure circuit for the supply of compressed air to a load through a shut-off device, and a control pressure circuit branched off from the working pressure circuit through a fixed throttle and connected to the surrounding atmosphere through a variable throttle, the shutoff device being controlled in accordance with a measuring force acting on the variable throttle and a non-return valve being provided between the control pressure circuit and the load side of the working pressure circuit and pre-loaded towards the working pressure circuit.

2. A power amplifier according to claim 1, in which the ndn-return valve includes a sealing ball held against a sealing seat by a spring.

3. A power amplifier according to claim 1, in which the non-return valve comprises a diaphragm in which slots are formed which establish a flow connection between the working pressure circuit and the control pressure circuit when the working pressure is higher than the control pressure.

4. A pneumatic power amplifier substantially as herein described with reference to Figure 1, optionally modified as shown in Figures 2 and 3, of e aii ianying drawings.