GB2230876A

GB2230876A - Hydraulic control or testing apparatus

Info

Publication number: GB2230876A
Application number: GB8909210A
Authority: GB
Inventors: Christopher Howden
Original assignee: HOWDEN MACHINERY
Current assignee: HOWDEN MACHINERY
Priority date: 1989-04-22
Filing date: 1989-04-22
Publication date: 1990-10-31
Anticipated expiration: 2009-04-22
Also published as: GB8909210D0; GB2230876B

Abstract

A hydraulic control or testing apparatus for applying a selected high pressure or selected force, for example to a test piece, comprises a fluid pressure actuator (11) to be supplied, in use, with an hydraulic pressure fluid, a fluid reservoir (14), and a pump (16) interconnecting the reservoir and the actuator. The pump is driven by a variable speed motor (18) the speed of which is determined by an electrical signal generated as an error signal between a demand value which can be set to represent the required force and a feedback signal from a sensor (33) responsive to the force exerted by the actuator. The sensor (33) may consist of a strain gauge or a transducer responsive to pressure in the delivery line to the actuator. The demand value may represent position rather than force or pressure requirements of the actuator. <IMAGE>

Description

HYDRAULIC CONTROL OR TESTING APPARATUS The present invention relates generally hydraulic control or testing apparatus, and particularly to such apparatus designed for applying selected high pressure or selected force. The present invention may find application, for example, in hydraulic test equipment for applying a test pressure or force to a test piece.

In such application it is necessary to be able to predetermine accurately the force or pressure applied by the apparatus in order accurately to determine the forces resisted by the test piece or at which the test piece fails. Conventional pressure control apparatus involves the use of servo controlled valves for regulating the pressure from a pressure source to a delivery line to an actuator. At very high pressures, however, namely those in excess of five to six thousand p.s.i distortion in servo valves results in leakage and inaccuracy and such pressures are usually considered an absolute maximum for the use of such servo valves. It is desirable, however, in some circumstances to apply test pressures considerably in excess of those at which servo valves start to distort.

For example, in compression testing concrete samples it may be necessary to apply pressures up to ten or eleven thousand p.s.i.

The present invention seeks, therefore, to provide hydraulic control or testing apparatus capable of applying very high pressures under accurate control without the use of conventional servo valves.

According to one aspect of the present invention, therefore, hydraulic control or testing apparatus for applying a selected high pressure or selected force (for example to a test piece) comprises a fluid pressure actuator adapted to be supplied, in use, with an hydraulic pressure fluid, a fluid reservoir, and a pump interconnecting the reservoir and the actuator, in which the pump is driven by a variable speed motor the speed of which is determined by an electrical signal generated as an error signal between a demand value which can be set to represent the required force and a feedback signal from a sensor responsive to the force exerted by the actuator.

Preferably there is provided a pressure compensated flow control valve connected to the fluid delivery line from the pump to the actuator, allowing a leakage flow of fluid to a subsidiary reservoir at a fraction of the maximum delivery rate of the pump. It has been found to be of critical importance to the accurate operation of such hydraulic control apparatus that the leakage flow through the pressure compensated flow control valve be sufficiently great to allow for rapid pressure variation in a decreasing sense as well as in an increasing sense. It will be appreciated that when the pressure variation is controlled by varying the speed of operation of a pump delivering the fluid a reduction in the speed of the pump will only result in a reduction in the pressure applied at the actuator providing the fluid previously delivered to the actuator has a route to escape.Such escape route is provided by the pressure compensated flow control valve and it is preferred that this valve has a range of settings allowing it to provide a leakage path up to one half of the maximum delivery rate of the pump. This gives a potential fall in the pressure applied to the actuator equivalent to the maximum pressure increase so that the feedback error signal control circuit can cause a rapid increase or rapid decrease in the pressure at the actuator simply by controlling the speed of the motor. This makes it possible to control very high pressures accurately since case distortion of servo valves which previously limited the maximum pressure which could be applied with accuracy is no longer a determining factor.

In one embodiment of the invention the sensor responsive to the force exerted by the actuator is a strain gauge positioned to detect the force exerted on the mechanical system comprising the actuator and the test piece or other component stressed thereby. In such a system the control circuit is entirely separated from the hydraulic circuit so that any leakages or inaccuracies in the components of the fluid pressure circuit are automatically compensated.

Alternatively, however, the sensor responsive to the force exerted by the actuator may be a pressure transducer in the hydraulic delivery line from the pump to the actuator.

In the preferred embodiment of the invention there is provided a unidirectional valve in the fluid line between the pressure compensated flow control valve and the subsidiary reservoir which may be a separate reservoir from the main reservoir from which the pump draws fluid.

In an alternative embodiment the demand signal from the transducer may be a position demand signal rather than a force or pressure demand signal.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic hydraulic circuit diagram illustrating the principal components of the embodiments.

Referring now to the drawing, the hydraulic circuit shown is adapted for a compression testing machine typically for compression testing concrete. The compression is applied by a fluid pressure actuator 11 which may be a conventional single acting hydraulic ram biased by a spring schematically indicated 12 and supplied from a delivery line 13. A reservoir 14 connected to the low pressure side of the ram receives surplus fluid ejected upon extension of the ram via a line 15.

A delivery line 13 is supplied with hydraulic fluid from a pump 16 via a unidirectional valve 17. The pump 16 is driven by a variable speed electric motor 18 and is immersed within the hydraulic fluid in a main reservoir 19 equipped, in a conventional way, with a filler/breather 20 and a level gauge 21.

Connected to the delivery line 13 is an adjustable pressure compensated flow control valve 22 the adjustment of which determines the rate of leakage flow of fluid from the delivery line 13 via a unidirectional valve 23 to a reservoir 24.

The delivery line 13 is also provided with a pressure gauge 25, a safety pressure relief valve 26 and a solenoid valve 27 for allowing discharge of fluid pressure from the actuator 11 at the end of the test. The discharge valve 27 is connected by a discharge line 28 to a subsidiary reservoir 29 and the pressure relief valve 26 is connected by a discharge line 30 to a further subsidiary reservoir 31 which, although shown as a separate reservoir from the reservoir 29 may in fact be made as a common reservoir therewith. Indeed all the separate reservoirs shown in the fluid circuit may be one single reservoir.

In this embodiment the actuator 11 is provided with a fluid pressure transducer 33 which acts to generate an electrical signal representative of the instantaneous pressure within the delivery line 13 and thus the force exerted by the actuator. The electrical signal from the transducer 33 is fed to one input of a differential amplifier (not shown) the other input of which is fed with a demand signal generated, by means not shown, for example a variable resistor suitably calibrated with an indication of the force exerted by the actuator 11 in correspondence with the settings of the resistor. The output from the differential amplifier 35 is used to generate a ramp waveform for controlling a triac by which the speed of the motor 18 is determined.

This electrical control circuit acts to control the speed of the motor 18 in dependence on the setting of the potentiometer to a required or "demand" value modified by the feedback signal from the pressure transducer 33 which acts to generate an error signal varying the speed of the motor up or down in dependence on the difference between the demand value and measured value of the pressure.

Claims

1. A hydraulic control or testing apparatus for applying a selected high pressure or selected force, for example to a test piece, comprises a fluid pressure actuator to be supplied, in use, with an hydraulic pressure fluid, a fluid reservoir, and a pump interconnecting the reservoir and the actuator, in which the pump is driven by a variable speed motor the speed of which is determined by an electrical signal generated as an error signal between a demand value which can be set to represent the required force and a feedback signal from a sensor responsive to the force exerted by the actuator.

2. Apparatus according to claim 1 wherein a pressure compensated flow control valve is connected to the fluid delivery line from the pump to the actuator, allowing a leakage flow of fluid to a subsidiary reservoir at a fraction of the maximum delivery rate of the pump.

3. Apparatus according to claim 1 or claim 2, wherein the sensor responsive to the force exerted by the actuator is a strain gauge positioned to detect the force exerted on the mechanical system comprising the actuator and the test piece or other component stressed thereby.

4. Apparatus according to claim 1 or claim 2, wherein the sensor responsive to the force exerted by the actuator may be a pressure transducer in the hydraulic delivery line from the pump to the actuator.

5. Apparatus according to claim 2, wherein a unidirectional valve is provided in the fluid line between the pressure compensated flow control valve and the subsidiary reservoir which is a separate reservoir from the main reservoir from which the pump draws fluid.

6. Apparatus according to claim 4, wherein the demand signal from the transducer is a position demand signal rather than a force or pressure demand signal.

7. A hydraulic control or testing apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.