PROCEDURE AND APPARATUS IN THE CONTROL OF A PNEUMATIC LOAD APPLICATION DEVICE

Abstract: Procedure and apparatus in the control of a pneumatic load application device, said load application device comprising a pneumatic power means (3) connected to a pressure source (2) and applying a load to a test specimen (1) and a sensor (4) for measuring the effect of the load on the test specimen. In the procedure, a continuous and constant gas flow is supplied into the pressure circuit (6) of the pneumatic power means (3) and the pressure in the pressure circuit is regulated by regulating a gas flow passed out of the pressure circuit. The apparatus comprises a constant flow element (5) for maintaining a constant flow of a gas flowing from the pressure source (2) into the pressure circuit (6) of the pneumatic power means (3) and a regulator valve (7) for regulating a flow of gas flowing out of the pressure circuit of the power means on the basis of information supplied by the sensor (4).

Description: PROCEDURE AND APPARATUS IN THE CONTROL OF A PNEUMATIC LOAD APPLICATION DEVICE The present invention relates to a procedure as defined in the preamble of claim 1 and to an apparatus as defined in the preamble of claim 5 in the control of pneumatic load application device. In various load application and testing equipment for the testing of materials, traditionally servo- hydraulic systems are used in which the force applied to the test specimen is regulated by regulating the flow and pressure of hydraulic oil. However, hydraulic systems have certain drawbacks. Even the slightest errors that may appear in the control signals have an immediate effect on the specimen being tested, so a hy- draulic system requires an extremely accurate and precise control system. In addition, in many cases, especially when materials are to be tested in actual operating conditions, e.g. in process industry or in the reactor core of a nuclear power station, the material to be tested is used in circumstances in which the use of a hydraulic system is out of the question because of the risk of oil leaks. In prior art there are also various pneumatic systems in which the force applied to the test specimen is regulated by adjusting the flow and pressure of compressed air, such as patents US 3 548 646 and US 3 628 378. In these, two valves are used, and the pressure is increased in a controlled manner by letting compressed air flow via a valve into a pressure cir- cuit. The pressure is kept constant by keeping both valves closed. The pressure is decreased by opening an outlet valve. A drawback with pneumatic systems like this is inaccuracy and irregularity of control. Opening and closing the valves, however accurately and cau- tiously, generates impulses in the oil flow and produces pressure shocks, which substantially impair the accuracy of the measurement results. The object of the present invention is to eliminate the drawbacks mentioned above. A specific object of the present invention is to disclose a new type of procedure and a corresponding apparatus that make it possible to implement research of materials or application of load to test specimens in a highly accurate and safe manner in all possible environments in different pressure and temperature conditions. As for the features characteristic of the in- vention, reference is made to the claims. In the procedure of the invention, a pneumatic load application device is used, said device comprising a pneumatic power means connected to a pressure source for applying a load on a test specimen and a sensor for measuring the effect of the load on the specimen. According to the invention, a continuous, uninterrupted and constant gas flow is supplied into the pressure circuit of the pneumatic power means during the measurement, and the pressure in the pressure circuit, i.e. the force applied by the power means to the test specimen, is regulated by only adjusting a gas flow let out from the pressure circuit . In a preferred embodiment, the constant gas flow supplied into the pressure circuit can be adjusted or adapted to a desired magnitude as required by the testing arrangement used in each case. The constant gas flow is preferably supplied into the pressure circuit from a constant pressure. Thus, in the procedure of the invention, the pressure in the pressure circuit of the power means remains constant when the incoming gas flow is equal to the outgoing gas flow. When the pressure in the pressure circuit is to be increased, the outgoing gas flow is throttled, and when the pressure in the pressure circuit is to be decreased, the throttling effect on the outgoing gas flow is reduced. The apparatus of the invention is used to control a pneumatic load application device, which comprises a pneumatic power means connected to a pressure source for applying a load on a test specimen and a sensor for measuring the effect of the load on the test specimen. The apparatus of the invention for controlling a load application device comprises a constant flow element for maintaining a constant gas flow from the pressure source into the pressure circuit of the power means and a regulator valve for regulating the gas flow flowing out of the pressure circuit of the power means on the basis of information obtained from a sensor. The constant flow element used may be e.g. a throttle or some other suitable flow resistance valve, such as a needle valve, that can be used to restrict the gas flow so as to render it a smooth constant flow. The constant flow element, such as a throttle, preferably comprises an adjustment or setting that permits a desired level of constant flow through the valve to be set . The regulator valve used is preferably a magnet-controlled or motor-controlled servo valve for which an electric control signal is provided via elec- trie feedback from the sensor being used. Thus, the apparatus preferably comprises a control unit, to which the sensor and the regulator valve are connected, for controlling the regulator valve in accordance with the measurement results obtained from the sensor. The apparatus preferably comprises before the constant flow element a pressure reducer for regulating the constant pressure acting on the constant flow element . This ensures that the gas flow through the constant flow element is as smooth and unchangeable as possible. The procedure and apparatus of the invention have been tested in research on nuclear power plant ma- terials in the control of bellows-loaded fracture strength testing apparatus under a maximum pressure of 200 bar. The same technique of the invention can also be used in the control of other pneumatic load applica- tion devices. The advantages of the invention as compared with commonly used servo-hydraulic systems include a better ability to compensate for error signals that may occur in the control and the fact that any leaks occur- ring in the system will not contaminate the environment. Furthermore, the control system is remarkably fast and sensitive, allowing very low transition speeds and small loading forces. Another advantage of the invention is a very low energy consumption. When combined with a bellows-loaded fracture strength measuring instrument, the invention makes it possible to test materials without frictional losses in pressurised conditions, e.g. in process industry or in the reactor core of a nuclear power station. Moreover, as the correction flow through the apparatus is continuous, regulation is also continuous and uninterrupted, which means that the apparatus and procedure of the invention can also be used to compensate variations in external pressure, which is more difficult when prior-art means are used in which valves are periodically opened and closed. In the following, the invention will be described in detail by referring to the attached drawing, which presents a diagram illustrating an apparatus ac- cording to the invention. A test specimen 1 is tested in a loading frame 11, which comprises a power means 3, i.e. a bellows, which exerts a pressure on the test specimen 1, which rests on supports 12. The movement of the centre of the test specimen is measured using a sensor 4, which may be e.g. an LVDT sensor. The pressure source 2 feeding the power means 3 consists of a compressor and a pressure accumulator 13 connected to it. From the pressure source, compressed air flows via a pressure reducer 10 under con- stant pressure through a constant flow element 5, i.e. a throttle, into the pressure circuit 6 of the power means 3. The amount of gas 9 allowed to flow out of the pressure circuit 6 is controlled by means of a regulator valve 7, which is e.g. a servo-controlled magnetic valve. The entire system is controlled by a control unit 8, which comprises amplifiers 14, a function generator 15, main controls 16 and measuring cards 17. The sensor 4 provides electric feedback 18 to the control unit 8, and the electric control input 19 controlling the servo valve is likewise connected to the control unit . The apparatus works as follows. The pneumatic servo-controlled pressure regulation system is based on cooperation between the throttle 5 and only one valve, the servo valve 7. Pressure regulation in the pressure circuit 6 of the pneumatic power means 3 is effected by decreasing or increasing the throttling of the gas flow passed through the servo valve 7 while the gas flow through the throttle 5 remains constant . When the amount of gas flowing through the servo valve is throttled, the pressure in the bellows 3 connected to the pressure circuit 6 rises, producing a growing load force on the test bar 1. The actual inventive idea of the pressure regulation system will become manifest when the bellows pressure is to be reduced. To do so, the throttling action of the servo valve is reduced, which means that the amount of gas passed through the servo valve increases and pressure difference produced tends to be equalised. When the system tries to equal- ise the pressure difference, this results in a practically insignificant increase in the gas flow through the throttle 5; in other words, the flow through the throttle 5 is practically constant, so the servo valve will be able to drop the pressure in the pressure circuit 6 to the desired value. Thus, the control loop with feedback via the sensor 4 can accurately regulate the force acting on the test specimen 1 without any variations in the pressure and load applied to the test specimen. The apparatus makes it easy to implement material testing e.g. in the reactor pressure vessels in nuclear power stations because the loading frame, suspended from only a single pneumatic hose and a single electric conductor, can be lowered into practically any sort of circumstances, free of mechanical and hydraulic power transmission arrangements. The apparatus can be used to implement constant displacement measurements, in which the test specimen is held in a constant position during the measurement, linear displacement measurements, in which the displacement is changed, i.e. increased or de- creased linearly as a function of time with an accuracy of change as fine as 0,05 μm per minute, and constant force measurements, in which the test specimen is subjected to a constant pressure during a desired period of time. In the foregoing, the invention has been described by way of example by referring to the attached drawing while different embodiments of the invention are possible within the framework of the inventive idea defined by the claims.

Claims: CLAIMS 1. Procedure in the control of a pneumatic load application device, said load application device comprising a pneumatic power means (3) connected to a pressure source (2) and applying a load to a test specimen (1) and a sensor (4) for measuring the effect of the load on the test specimen, charac t er i sed in that a continuous and constant gas flow is supplied into the pressure circuit (6) of the pneumatic power means (3) and the pressure in the pressure circuit is regulated by regulating a gas flow allowed to escape from the pressure circui . 2. Procedure as defined in claim 1, char act eri sed in that said constant gas flow is ad- justed to the level required in each individual measurement . 3. Procedure as defined in claim 1, char acteri sed in that the continuous gas flow is supplied from a constant pressure. 4. Procedure as defined in any one of claims 1 - 3, charac t eri s ed in that the procedure is applied in measurements carried out in closed spaces, such as those in power plant and process environments. 5. Apparatus in the control of a pneumatic load application device, said load application device comprising a pneumatic power means (3) connected to a pressure source (2) and applying a load to a test specimen (1) and a sensor (4) for measuring the effect of the load on the test specimen, charac t er - i s e d in that the apparatus comprises a constant flow element (5) for maintaining a constant flow of a gas flowing from the pressure source (2) into the pressure circuit (6) of the pneumatic power means (3) and a regulator valve (7) for regulating a flow of gas flow- ing out of the pressure circuit of the power means on the basis of information supplied by the sensor (4) . 6. Apparatus as defined in claim 5, characterised in that the constant flow element (5) is a throttle. 7. Apparatus as defined in claim 6, char- acterised in that the throttle comprises an adjustment for setting a desired level of constant flow. 8. Apparatus as defined in any one of claims 5 8, characterised in that the regulator valve (7) is a magnet-controlled or motor-controlled servo valve. 9. Apparatus as defined in any one of claims 5 - 8, characterised in that the apparatus comprises a control unit (8) , to which the sensor (4) and the regulator valve (7) are connected, for controlling the regulator valve in accordance with the information supplied by the sensor. 10. Apparatus as defined in any one of claims 5 - 9, characterised in that the apparatus comprises a pressure reducer ( 10), placed before the constant flow element (5) , for applying a desired constant pressure to the constant flow element .

Inventors: ARILAHTI ESKO, MOILANEN PEKKA, VARIS ESA, ARILAHTI, ESKO, MOILANEN, PEKKA, VARIS, ESA
Applicants: ARILAHTI ESKO, MOILANEN PEKKA, VALTION TEKNILLINEN, VARIS ESA, ARILAHTI, ESKO, MOILANEN, PEKKA, VALTION TEKNILLINEN TUTKIMUSKESKUS, VARIS, ESA
Application year: 1998
Main classification: G01N3
IPC codes: G01N3