PNEUMATIC AMPLIFIER

Abstract: A pneumatic actuator which allows precise control as regard position, speed and acceleration. Piston (4) and cylinder (1) define a chamber (7) and inlet and exhaust valves (13 and 18) control the flow of gas into and out of the chamber under the control of a stepper motor (12) which rotates a screw-threaded spindle (10) in one direction for extending the actuator and in the opposite direction for retracting the actuator. Any movement of the piston due to a change of loading on the piston rod opens up either the inlet (13) or exhaust valve (8) for returning the piston automatically to the correct position. Pressure chamber may be defined on both sides of the piston to provide pneumatically powered strokes in both directions.

Description: Pneumatic Amplifier This invention relates to a fluid actuator, and more particularly concerns a pneumatic actuator which is capable of providing precise control over position, speed and acceleration. Pneumatically driven actuators are well known and have the advantages of being able to generate large forces and being comparatively inexpensive. The pneumatic devices currently available, however,are not suitable for applic¬ ations, e.g. in robotics, where very accurate control is required, the main reason for which is the compressiblity of air. Also known in the art are motors, and especially electric stepper motors enegrised by voltage pulses, which enable precise speed and position control. Such motors are particularly suitable for low power applications, but where power demands are higher the cost and weight of the motors and their associated circuitry constitute severe drawbacks and often makes them impractical to use. The aim of the present invention is to overcome the disadvantages of the devices mentioned above by providing a pneumatically driven actuator which is adapted to permit precision control. In accordance with this aim the invention provides a pneumatic actuator comprising a cylinder, a piston slidable in the cylinder and confining a chamber therewith, a piston rod attached to the piston and projecting through a n end of the cylinder, and valve means operable to admit pressurised gas into and release gas from the chamber, characterised in that a screw threaded spindle is rotat- able in the cyliner and is coupled to the piston and the valve means whereby rotation of the spindle in one direction causes gas to be admitted into the chamber to displace the piston in one direction along the cylinder, and rotation of the spindle in the opposite direction causes gas to be released from the chamber for the piston to be displaced in the other direction relative to the cylinder, the distance through which the piston is displaced being determined by the angle through which the spindle is rotated, and a rotary motor is connected to the spindle for rotating the spindle relative to the piston. With such an actuator the motor is required only to rotate the spindle for operating the valve means and it can be of comparatively low power compared with the output power of the actuator. As a result a relatively small motor can be employed. In a preferred construction an electric stepper motor is used to enable accurate control over the rotation of the spindle. The output force of the actuator is produced by the gas pressure, but as the piston displacement, and hence its position, are determined by rotation of the spindle the actuator is readily capable of accurate control. In one preferred construction of actuator embodying the invention the piston separates two chambers in the cylinder and the valve means is operable to admit gas to and release gas from both chambers with gas being released from each chamber as gas is admitted into the other chamber. In this way the piston is driven pneumatically in both directions for providing power strokes whether the actuator is being extended or retracted. A more complete understanding of the invention and its features will be had from the following detailed description given with reference to the accompanying ■ drawings, in which:- Figure 1 is an axial section through a pneumatic amplifier embodying the invention: Figure 2 shows the device of Figure 1 during extension thereof; Figure 3 shows the device of Figure 1 during retract¬ ion thereof; Figure 4 is an axial section through an actuator similar to that of Figure 1 but modified to provide power strokes in both directions; Figure 5 is a schematic representation in axial section of an alternative linear pneumatic actuator constructed in accordance with the invention; Figure 6 is a similar axial view of a modified form of the actuator shown in Figure 5 ,' and Figure 7 is an exploded perspective view of one end cover of the cylinder of the actuator shown in Figure 6. In the drawings illustrating the different embodi¬ ments the same reference numerals have been used to designate corresponding features for convenience and clarity. The pneumatic amplifier or actuator shown in Figure 1 has a cylinder 1 fitted with front and rear end covers 2 and 3, respectively. A piston 4 is slidable in the cylinder and carries a tubular piston rod 5 which extends freely through a central opening in the front end cover 2. A return spring 6 is interposed between the piston and the front end cover and acts on the piston 4 to urge it rearwardly. The piston and cylinder confine a pressure chamber 7 having an outlet or exhaust port defined by a central orifice in the piston which opens into the bore of the piston rod 5« On the side of the piston facing the chamber 7 is a conical valve seat surrounding the outlet port. A poppet valve member 8 cooperates with this seat assembly to close the outlet port, the valve member being slidable axially relative to the piston for opening and closing the port OMPI but being held against rotation relative to the piston by a radial pin or screw 9 which enters a longitudinal groove or slot in the valve member 8. The valve member is internally screw-threaded and is carried by an externally screw threaded spindle 10 which extends axially through the cylinder. The rear end cover 3 s provided with a front plate 11 and defines therewith a motor compartment in which an electric stepper motor 12 is accommodated. The stator of the motor is fixed to the cover 3 and the rotor is rotationally fast with the spindle 10. The end cover 3 includes also an inlet port for pressurised gas which opens axially into the motor compartment and communicates with the pressure chamber 7 through this compartment. The inlet port is surrounded by a conical valve seat with which a poppet valve member 13 cooperates, the member 13 being urged towards the seat by a coil spring 14 acting between the valve member and the inner end of a gas connector 15 screwed into the cover 3. The spindle 10 includes a projection 16 at its rear end which enters a blind hole in the inlet valve member 13 for opening the valve in a manner which will become clear from the following explanation of the actuator operation. In a normal static condition of the actuator the parts will occupy the positions as illustrated in Figure 1 with the gas inlet and outlet valves being closed against the respective seats and the force on the piston 4 due to the gas pressure in chamber 7 being balanced by the spring 6 and the external load on the piston rod 5- If the actuator is to be extended, e.g. from the retracted condition illustrated in Figure 2, the appropriate voltage pulses are fed to the stator of the motor to cause the rotor to turn in a given direction, indicated by the arrow in the drawing, thereby causing ■ ^SZ XΪ OMPI the spindle 10 to rotate and to be driven rearwardly as a result of its threaded engagement with the valve member 8 which held against forward movement by the piston 4. The rearward movement causes the spindle extension 16 to push the inlet valve member 13 off its seat against the force of the spring 14 so that pressurised gas enters the chamber J via the compartment housing the motor. The rise in pressure in the chamber 7 acts on the piston pushing it, and hence the rod 5, forwardly thus extending the actuator. Due to the spring 14, the spindle 10 also moves forward with the piston 4 whereby the inlet valve 13 closes against its seat again and the actuator stops in the adjusted position. The exact length of the forward extension is dependent upon the angle through which the spindle 10 is rotated by the motor, i.e. the distance which valve member 13 is displaced along the spindle, and can be precisely controlled by the pulses fed to stepper motor 12, If the actuator is to be retracted, e.g. from the extended position of Figure 3, the stepper motor is energised to rotate the spindle 10 in the opposite direction to that in which it is turned to extend the actuator. Due to abutment of a shoulder on the spindle with the plate 11 the spindle 12 is unable to move forwardly and instead the valve member 8 is driven back off its seat to open the outlet port. Gas is thereby allowed to escape from the chamber 7 reducing the pressure in this chamber so that the piston 4 is dis¬ placed rearwardly under the combined forces of the return spring 6 and any external load on the piston rod 5, until the valve member 8 once more contacts its seat. The new adjusted position of the piston 4 is determined by the stepper motor 12 and can be controlled accurately. It is to be noted that the actuator will also compensate automatically for any unintentional movements of the piston 4, e.g. to leakage of gas from the cylinder or changes in the loading on the piston rod since any unwanted piston movement will open up either the inlet valve 13 or the exhaust valve 8 so that gas is either admitted to or released from the chamber 7 to re-establish the correct position of the piston. From the foregoing it will be appreciated that the pneumatic actuator provided in accordance with the invention can be accurately controlled electronically as regards position speed and acceleration. The stepper motor need be of low power only since it is only required to operate the valves, and the actuator as a whole may have a high power to weight ratio, making it particularly suitable for robotic systems. In Figure 4 there is illustrated a modified actuator of basically similar construction to that of Figure 1, and only the main differences will be described in detail. The actuator of Figure 4 is adapted to pro- vide power strokes in both directions and not just during extension as in the actuator of Figure 1. The piston rod 5 is sealed to the forward end of the cylinder so that the piston 4 separates two chambers 7 and 20. The stepper motor 12 is mounted externally of the cylinder, which has the advantage that the compressed gas does not flow through the motor and possibly contaminate it with any material entrained with the gas. The motor rotor is coupled to the spindle 10 through a sliding coupling 21 which holds the spindle rotationally fast with the rotor but allows it to move rearwardly for opening the inlet valve 13 which is carried on the spindle. A gas inlet duct 22 has one branch leading to the cavity inlet accomodating valve 13 for supplying gas to the chamber 7 , and a second branch leading to chamber 20 via a spool valve 23 and a passageway 24- The spool valve also controls communication of the passage 24, and hence chamber 20, with an outlet port 25, the spool being biased in one direction by the pressure of gas in chamber 7 acting through a port 26, and in the opposite direction by the force of a spring 27. The operation of the actuator is essentially the same as that described above for Figure 1. It differs in that when the motor 12 is operated to open the inlet valve 13 for extending the actuator, the pressure of the gas in chamber 7 pushes the spool of valve 23 against the force of spring 27 to allow gas from chamber 20 to escape through passage 24 and port 25, so that the piston 4 moves forward. On the other hand, when the motor is driven in the opposite direction to open the outlet valve 8 (as depicted in t e drawing) for retracting the actuator, the force of spring 27 over¬ comes the pressure of gas in chamber 7 so that the spool of valve 23 is adjusted to connect the passage 24 to the gas inlet duct 22. The gas under pressure thereby enters the chamber 20 and acts on the piston 4 to push it back to re-seat the valve 8. Thus, the piston is driven pneumatically in both directions. It will be realised that in this embodiment the return spring 6 of Figure 1 is not required. Another form of double acting or bi-directional amplifier is shown in Figure 5« In this case the s_pindle 10 has a screw threaded connection with the piston 4 and in place of the inlet and outlet valves 8,13,23 a spool valve 30 is provided. The valve housing includes an inlet port 31 and two exhaust ports 32, 33 - The valve spool is fixed to the spindle 10 and includes a port 34 which is connected to an axial passage 35 through the spindle 10, this passage being in constant communication with the chamber 20 via a hole 36 in the spindle, the interior of piston rod 5 and a hole 37 in the piston rod. The end of the spindle 10 is sealed to the rod 5 by a seal 38. As an alternative to the gas flow path 34-37 an external connection could be provided between the spool valve and the chamber 20, as indicated in broken line in the drawing. A duct 39 connects the chamber 7 with the spool valve. Due to the threaded connection between the piston 4 and spindle 10, rotation of the spindle by the motor 12 in one direction displaces the spool of valve 30 to connect the duct 39 with the inlet port 31 and the port 34, and hence chamber 20, with the exhaust port 32. The piston 4 is driven forwardly and the actuator is extended. Reverse rotation of the spindle brought about by the motor displacing the spindle and spool in the other direction so that the chamber 20 is connected to the inlet port 1 and the chamber 7 is connected to the exhaust port 32. The piston 4 is then driven rear¬ wardly until the spool valve is returned to the closed position. The actuator of Figures 6 and 7 is very similar to that of Figure $ , except the stepper motor 12 is mounted externally of the cylinder and is coupled to the spindle through the valve spool and by means of a coupling 21 which allows axial movement of the spool and spindle. The cylinder is of simple construction with the end covers being held together on the cylinder body 1 wall by bolts 40. The end cover 3 is integrally moulded with the valve housing which, as may be seen in Figure 7 consists of a single injection moulding 41 to which a cover plate 42 is fixed, e.g. by welding. In the three pneumatic amplifiers of Figures 4 to 7 the pressures in the two chambers 7 and 20 are dependent upon the axial force exerted on the piston rod 5« The valves in these embodiments regulate the pressure and maintain a stable position of the piston rod and piston irrespective of the load fluctuations. In other words the pneumatic amplifier compensates atuomatically for load changes without loss of positional reference. The speed of actuation is determined by the operating speed of the stopper motor and the pitch of the screw thread on the spindle, so that the speed and acceleration can be precisely controlled by the pulses fed to the stepper motor. Although the embodiments of the invention have been described as having electric stepper motors it will be appreciated that other types of motor, e.g. a servo motor, could be used provided it allows the spindle to be rotated in a closely controlled manner. Other modifications are also possible and will occur to skilled readers.

Claims: C LAIMS 1. A pneumatically driven actuator comprising a cylinder (1), a piston (4) slidable in the cylinder and confining a chamber (7) therewith, a piston rod (5) attached to the piston and projecting through an end of the cylinder, and valve means (8, 13, 23; 30) operable to admit pressurised gas into and release gas from the chamber, characterised in that a screw threaded spindle (10) is rotatable in the cylinder and is coupled to the piston and valve means whereby rotation of the spindle in one direction causes gas to be admitted into the chamber (7) to displace the piston (4) in one direction along the cylinder, and rotation of the spindle in the opposite direction causes gas to be released from the chamber (7) for the piston (4) to be displaced in the other direction relative to the cylinder, the distance through which the piston (4) is displaced being deter¬ mined by the angle through which the spindle is rotated, and a rotary motor is connected to the spindle for ro¬ tating the spindle (10) relative to the piston (4) to control the position of the piston. 2. An actuator according to claim 1, wherein the valve means is responsive to displacement of the piston (4) from an adjusted position due to changes of loading on the piston rod (5) to admit gas into or release gas from the chamber (7) thereby to return the piston to said adjusted position. 3. An actuator according to claim 1 or 2, wherein the piston (4) separates two chambers (7,20) in the cylinder, and the valve means (8, 13, 23; 30) controls flow of gas into and out of both chambers and is arranged to release gas from each chamber (7 or 20) when admitting gas to the other chamber. 4- An actuator according to claim 1,2 or 3, wherein the valve means includes separate inlet and exhaust valves (13 and 8) controlling flow of gas into and out ■ ϊi- of the chamber (7)> 5. An actuator according to claim 4, wherein the piston has an opening defining an exhaust port and de¬ fines a valve seat surrounding said port, and an exhaust valve member (8) is screw threaded onto the spindle (10) for cooperation with the piston valve seat. 6. An actuator according to claim 4 or 5, wherein the spindle (10) is movable longitudinally for operating the inlet valve (13), rotation of the spindle in said one direction causing the spindle to move axially re- lative to the piston (4) and the exhaust valve (8) to open the inlet valve. 7. An actuator according to any one of claims 1 to 3, wherein the spindle (10) has a screw threaded conn¬ ection with the piston (4), and the valve means is coupled to the spindle (10) to be operated in response to longitudinal movement thereof produced by rotation of the spindle relative to the piston. 8. An actuator according to claim 7, wherein the valve means is a spool valve (30) connected to the spindle (10) for longitudinal movement therewith. 9. An actuator according to claim 4,5 or 6 and claim 3, wherein said separate inlet and exhaust valves (13 and 8) control flow of gas into and out of a first chamber (7), and the valve means comprises a further valve device (23) responsive to the pressure in said first chamber to control flow of gas to and from the second chamber (20). 10. An actuator according to any one of the pre¬ ceding claims, wherein the motor is an electric stepper motor (12) or a servo motor. 11. An actuator according to any one of the pre¬ ceding claims, wherein the motor (12) is mounted ex¬ ternally on the cylinder.

Inventors: JONES TIMOTHY JOHN, JONES, TIMOTHY, JOHN
Applicants: ORIGINAL ANDROID CO LTD, THE ORIGINAL ANDROID COMPANY LIMITED
Application year: 1983
Main classification: F15B9
IPC codes: F15B9