Motors: expansible chamber type – Working member position feedback to motive fluid control – Electrical input and feedback signal means
Patent
1988-02-11
1989-11-07
Garrett, Robert E.
Motors: expansible chamber type
Working member position feedback to motive fluid control
Electrical input and feedback signal means
91459, 73517R, F15B 903, F15B 909
Patent
active
048784176
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method and an apparatus for servo-controlling the position of a pneumatic actuator used for displacing a load, for example in robotics.
Up to now, pneumatic actuators have been used on a large scale in industry as on/off actuators because of their robustness, their low cost price, and the convenience of feeding them with compressed air.
PRIOR ART
Attempts have been made to servo-control their position like hydraulic actuators by means of a position sensor connected to a corrector circuit controlling a feed valve of the actuator and taking account of the difference between a reference position and a measured position, and possibly taking account of the first derivative of said difference. However difficulties have always been encountered due to the compressibility of air or of the gas contained in the actuator, and these difficulties give rise to instability and lack of stiffness in the pneumatic actuator, or else to oscillations on either side of the reference position.
SUMMARY OF THE INVENTION
The invention provides a method and an apparatus for servo-controlling the position of a pneumatic actuator, while avoiding these drawbacks.
To this end, the invention provides a method of servo-controlling the position of a pneumatic actuator, consisting in determining the position of the moving member of the actuator, in comparing it with a reference position, and in taking account of the difference between said positions in order to generate a control signal for controlling a valve feeding the actuator with air or gas under pressure, the method being characterized in that it also consists in determining the speed, the acceleration, and the inertia of the moving member, in determining correction factors for the speed and for the acceleration from said inertia, and in generating a valve-control signal which takes account of the above-mentioned position difference and also of the speed and the acceleration of the moving member as corrected by applying the above-mentioned factors in order to cause the moving member to stop in a predetermined position while avoiding oscillations.
By measuring the inertia of the moving member, which varies as a function of the displaced load, and by taking account of it for correcting the speed and acceleration terms in the servo-control loop it is possible to avoid oscillations of the moving member of the actuator and to obtain good servo-control stiffness and stability. Accuracy is improved by the fact that the control signal to the valve can be determined or corrected more quickly from a measurement of acceleration than from a measurement of speed. In addition, the servo-control is automatically adaptable to the size of the load displaced by the actuator.
In practice, the invention will most often be used in cases where it is necessary to displace a load from one point to another as fast as possible without shock and without oscillation on either side of the point of arrival, to return empty or with a different load to the starting point, and to move back to the other point, etc.
To do this, the method in accordance with the invention is characterized in that it comprises an initial stage during which the actuator is fed at a constant predetermined flow rate, e.g. the maximum feed rate of air or gas under pressure, in order to displace the moving member from a completely stopped position, and during which the inertia of the moving member and the corresponding speed and acceleration correction factors are determined, an intermediate stage during which the moving member continues its stroke towards its reference position and the actuator is fed at said flow rate of air or gas under pressure, with the servo-control having no effect on the valve, and a final stage during which said speed and acceleration correction factors are taken into account while generating the valve control signal in such a manner as to cause the moving member to reach the reference position and to stop there.
Thus, from the beginning of each new stroke of the movi
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patent: 4279192 (1981-07-01), Kleinwaks
patent: 4362978 (1982-12-01), Pollard et al.
patent: 4481451 (1984-11-01), Kautz et al.
patent: 4510428 (1985-04-01), Dunne
patent: 4537077 (1985-08-01), Clark et al.
Bertin & Cie
Garrett Robert E.
Williamson Mark A.
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