GB2076469A - Fluid actuated ram with position measuring device - Google Patents

Abstract

In a fluid ram device the extension of the piston relative to the cylinder can be measured by means of a nut and leadscrew (13) arrangement. The leadscrew is housed within a sealed chamber (7) defined by the piston and cylinder, and has an extension which extends out through a seal (16) in the cylinder body. The leadscrew and extension are free to rotate relative to the cylinder body but cannot move axially relative thereto. The leadscrew engages with a threaded portion (12) on the piston, and is received in a hollow (11) in the piston when the piston is retracted into the cylinder. In use as the piston moves axially relative to the cylinder the leadscrew and its extension rotate to an extent dependent upon the magnitude of the axial movement. The extension drives a transducer (19) such as a helical potentiometer, which can provide an electrical signal indicative of the magnitude of the movement. <IMAGE>

Description

SPECIFICATION Fluid actuated ram with position measuring device This invention concerns fluid actuated devices of the kind comprising a cylinder body defining a cylinder, and a piston slideable axially therein, the piston and cylinder defining therebetween at least one chamber to or from which a working fluid can be admitted or vented to effect relative retraction or extension of the cylinder body and the piston.

Such devices are referred to herein for convenience as fluid actuated ram devices. The action of the working fluid can be opposed or opposable e.g. by resilient means or by pressurised fluid within a second chamber defined between the piston and the cylinder-body. The working fluid can be a liquid or a gas.

In particular, the invention relates to fluid actuated ram devices which include means for measuring the amount of the relative retraction or extension of the cylinder body and the piston.

In a typical known servohydraulic device, the extension of the piston is measured electrically by means of a linear variable differential transformer (LVDT) housed within the piston, the windings being carried by the piston and the displaceable coil being fixed relative to the cylinder body. An LVDT provides a rapid and accurate response to movement, and is thus particularly useful where the piston is required to execute rapid cyclic movements having a predetermined waveform.

One useful application of this type of system is thus for fatigue testing of mechanical components, where cyclic deformations, closely approximating to those encountered in practice, can be imposed on a component under test in the laboratory.

However, the LVDT is a sensitive device requiring careful handling. Also, the LVDT is of necessity considerably longer than the stroke it is required to measure. This means that its use considerably increases the length of a fluid actuated ram device.

In other applications where the position measuring means is not required to provide such an accurate or rapid response, some form of transducer mounted externally of the piston and cylinder body can be employed, particularly where considerations of space occupied are unimportant.

One example of a suitable transducer is the linear potentiometer, in which a wiper carried by the piston moves over a coil fixed relative to the cylinder body. Another possibility is to mount a diffraction grating on or in fixed relation to the piston, with a sensor carried by the cylinder body.

Whilst such known transducer devices are quite satisfactory in a laboratory environment, they are all unsatisfactory for one reason or another for use in a less favourable situation. For example oil and dirt can prevent proper electrical contact between wiper and coil of a potentiometer thus inducing false readings, or can obscure the surface of a diffraction grating. Such transducers as these just mentioned are relatively sensitive to vibrations and mechanical shocks.

For heavy engineering applications such as earth-moving or mining, or other applications where the environment is hostile to sensitive equipment, there is thus a requirement for a more rugged position measurement device.

In certain applications, e.g. in manufacturing industry, control of fluid actuated ram devices is effected by means of an array of limit switches which can be used to effect a form of step-wise control in stages dependent upon the piston reaching a certain extension at which a limit switch is actuated. Such systems are rather clumsy, and the control is difficult to alter once set up. Moreover, the step-wise output provided by such systems is not really ideal for the actuation of a micro-processor control unit.

Finally it should be noted that for many applications, particularly in mobile machinery such as earth moving machines, mining machines, mobile cranes, etc. space is at a considerable premium, so that any position measurement device which substantially increases the overall length of a fluid actuated ram device will be unacceptable.

There thus exists, particularly for use in manufacturing industry and/or heavy engineering applications, the need for a fluid actuated ram device provided with a position measurement transducer which is rugged, reliable, requires little extra space, and which can provide a continuous output indicative of the relative position of the piston and cylinder body at all times. For the particular applications envisaged, a very high degree of accuracy is often not required.

According to the present invention there is provided a fluid actuated ram device as herein defined, comprising a lead screw receivable in a hollow within the piston, a screw threaded portion integral with or fixed to the piston and engaging with the lead screw, the lead screw being mounted for rotation about its longitudinal axis relative to the cylinder body but restrained from movement relative thereto along its longitudinal axis.

Preferably the lead screw is contained within a sealed chamber defined between the piston and the cylinder body, and has an extension which extends through a seal in the cylinder body to the exterior of the sealed chamber.

Conveniently the device includes a transducer which can provide an electrical signal representative of the rotation of the lead screw relative to the cylinder body.

The transducer is conveniently driven by the extension of the lead screw.

The transducer may conveniently be a helical potentiometer.

The invention will now be described by way of example only with reference to the accompanying drawing, which shows in diagrammatic form, part in section a fluid actuated ram device in accordance with the invention.

As shown in the drawing a fluid actuated ram device comprises a cylinder body 1 defining a cylinder 2 in which a stepped piston 3 is slideable axially. The cylinder body 1 and the piston 3 are provided with respective integral lugs 4, 5 by means of which they can be secured to associated equipment. The piston 3 has a head portion 6 which forms a sliding seal with the wall of the cylinder 2 so that there are defined between the cylinder body 1 and the piston 3 a first chamber 7 and a second chamber 8 to which pressurised hydraulic fluid can be admitted via respective ports 20, 21 in controlled manner so as to effect extension and retraction of the piston 1 relative to the cylinder body 3 as desired.As viewed in the figure, the first chamber 7 is closed at its left hand end by an end portion 9 of the cylinder body, and the second chamber 8 is sealed to the piston 3 by means of a seal 10 through which the piston can slide.

The piston 3 is hollow in that it has formed therein a blind bore 11 which opens towards the first chamber 7. At the entrance to the blind bore 11 there is fixed to the piston 3 a nut 12 having an internal screw thread. Received within the blind bore 11 is a lead screw 1 3 having a single or multiple start screw thread which is engaged within the internal screw thread of the nut 12. The screw threaded portion of the lead screw 13 extends through the first chamber 7, and terminates at a shoulder 14 which lies against the end portion 9 of the cylinder body.Extending through the end portion 9 is a reference shaft 1 5 forming an extension of and integral with the lead screw 1 3. The reference shaft 1 5 is rotatably supported in a ball or roller bearing 1 6 housed in the end portion 9, and passes through a seal 1 7 which seals the first chamber 7 against the escape of pressure fluid around the shaft 1 5. A thrust member 1 8 is secured to the shaft 1 5 on the exterior of the end portion 9, so that between the shoulder 14 and the thrust member 18, the lead screw 13 is restrained against axial movement, whilst it is able to rotate about its longitudinal axis in the nut 12 and the bearing 1 6.

Housed within the lug 4 is a transducer in the form of a helical potentiometer 19, having a wiper (not shown) secured to and hence rotatable with the reference shaft 1 5, and a resistor with which the wiper cooperates fixed in relation to the lug 4.

In use of the device pressure fluid is admitted to the first chamber 7 to cause extension of the piston or ram 3, and to the second chamber 8 to cause retraction thereof, as is known. As the piston is retracted, the screw 13 passes through the nut 12 and is received in the blind bore 11, and as the piston is extended the screw 1 3 is withdrawn through the nut 12. A rotation of the screw 1 3 and hence of the shaft 1 5 is hence induced, provided that the piston 3 and cylinder body 1 are anchored to external equipment which prevents their relative rotation, the rotation of the shaft 15 being proportional to the extension of the piston relative to the cylinder body.

Obviously the rotation of the shaft per unit extension of the piston depends upon the pitch of the screw thread. Subject to not exceeding the angle of friction, this pitch can be as small as desired, and the smaller the pitch the more accurate the measurement of extension is attainable. However, where the transducer employed is a helical potentiometer, the range thereof will be limited to a certain number of turns, for example ten. The pitch of the screw thread would then be chosen such that the full stroke of the piston 3 results in ten turns of the screw 13, and this consideration will be the determining factor in the choice of pitch.

The helical potentiometer 19 will be supplied with input electrical power from a constant voltage source, and its output will be in the form of a d.c. voltage whose magnitude is indicative of the degree of extension of the piston. It is thought that piston extension can readily be measured to an accuracy of + 1% with a system of the kind described in accordance with the invention, and such a degree of accuracy is quite satisfactory for many applications. This is especially so in view of the relatively low cost of production of devices according to the present invention.

The invention is thought to have application for example in many industrial production processes, e.g. control of injection moulding, conveyor systems, devices for automatic clamping and manoeuvering of articles during manufacture; in heavy engineering applications; and in earth working machines. Devices according to the invention are particularly useful where microprocessor control is to be used because they can provide a reliable indication of piston extension which is continuous over the entire possible stroke. The working parts are simple and capable of rugged construction, and moreover are largely protected within the cylinder body/piston assembly whilst the electrical components are readily accessible for replacement or maintenance. A most important feature of the invention is its compactness.It will readily be appreciated that a fluid actuated device incorporating a position transducer in accordance with the invention is hardly longer than the corresponding prior art device without any position sensing means. Or put another way, within a given space, the addition of a position transducer in accordance with the invention need result in hardly any appreciable loss of stroke.

As will be apparent to those skilled in the art, many variations on the specific embodiment described are possible without departing from the scope of the invention. Thus, for example, other transducers than the helical potentiometer can be used to convert the rotary movement of the lead screw to a useable (e.g. eiectric) signai -- for example various other resistive (potentiometer) types of transducer, electronic digitisers, selsyns or the like. In some circumstances the transducer may include electronic circuitry to convert the signals it provides into any appropriate form of coded electrical information -- possible alternatives including modulation of voltage, frequency, current or a modulated a.c. waveform.

The precise form and method of actuation of the piston and cylinder body can of course be varied. For example, a coil spring could be provided instead of fluid pressure in the first chamber 7 or the second chamber 8 for retraction of the piston, or gravity could be employed.

Claims (7)

1. A fluid actuated ram device as herein defined, comprising a lead screw receivable in a hollow within the piston, a screw threaded portion integral with or fixed to the piston and engaging with the lead screw, the lead screw being mounted for rotation about its longitudinal axis relative to the cylinder body, but restrained from movement relative thereto along its longitudinal axis.

2. A fluid actuated ram device according to claim 1 wherein the lead screw is contained within a sealed chamber defined between the piston and the cylinder body, and has an extension which extends through the seal to the exterior of the sealed chamber.

3. A fluid actuated ram device according to claim 1 or claim 2 which includes a transducer which can provide an electrical signal representative of the rotation of the lead screw relative to the cylinder body.

4. A fluid actuated ram device according to claim 3 wherein the transducer is driven by the extension of the lead screw.

5. A fluid actuated ram device according to claim 3 or claim 4 wherein the transducer is a helical potentiometer.

6. A fluid actuated ram device according to claim 1 and substantially as hereinbefore described.

7. A fluid actuated ram device substantially as hereinbefore described with reference to the accompanying drawing.