GB2255961A - Mechanical actuator. - Google Patents

Abstract

A mechanical actuator comprises two beams A.B, a traction element M, and a coiled spring S. Beams A,B are pivoted at a point (10) away from their ends. The traction element M is secured to beam A at one end and attaches to beam B at the other end. A strain sensing element SSE is connected in line with either the traction element M or the spring S. It provides an output which may be used in controlling the traction element M. Pref. the sensing element is an optical emitter and receiver e.g. infrared device. Pref. a shutter device is moved away from the emitter and receiver as the strain increases. A number of different examples of sensor are disclosed in the specification. <IMAGE>

Description

MECHANICAL ACTUATOR The present invention relates to a mechanical actuator termed by the inventors a "shadow digit". The system provides a small general purpose actuator which may be controlled by a computer to carry out a function analogous to that of a finger. The invention also encompasses a strain sensing element suitable for use in such an actuator.

According to a first aspect of the present invention, there is provided a mechanical actuator comprising two elongate structural members pivotally interconnected at a point away from their ends; a traction element extending between one member and the other member on one side of the pivotal interconnection; a resilient tensioning element extending between the one member and the other member on the other side of the pivotal interconnection; and a strain sensing element connected with either the resilient element or with the traction element between the one and the other member and arranged to output a signal varying with the configuration of the actuator and suitable for use in controlling the traction element.

The shadow digit is a motive unit based on two beams joined by a pivot. One beam is able to pivot around the other beam, powered by a shadow muscle, and return to its original position by a spring. The spring contains sensory equipment enabling the device to provide feedback on its position. Shadow digits can be easily combined to build a larger device, due to the deliberately modular design. The larger device could be used in robots and other autonomous devices. One use would be the creation of artificial limbs, such as fingers, arms and legs. The overall range of uses is very broad. Examples of use include automatic airbrush, a one-legged semi-walking robot, a wall-climbing robot, a remote gripper, a musical keyboard playing finger and a self-closing vice.Possibilities of use include puppet manipulation, a carrot slicer, a remote shutter release mechanism for photographic work, aids for handicapped persons such as an automatic page turner, or a weighing machine. It is intended that operators of the digits will use them in new ways, which they have discovered, and in the educational environment this is the primary intention. The main benefits of the digits over similar devices is that they are far more flexible in terms of power, speed and fineness of control. Their design inherently lends itself to rough-and-ready installation within a matter of seconds.

Preferably the traction element comprises a radially expansible tube formed of elastomeric material, arranged to be connected to a source of pressurised fluid and covered with a braided sheath which is connected at at least two points to the structural members, the element being arranged so that the introduction of pressurised fluid into the tube causes a radial expansion of the braided sheath and hence a longitudinal contraction of the sheath which in turn causes relative movement of the structural members towards one and other.

Preferably the signal output by the strain sensing element is input to control means arranged to control the traction applied by the traction element.

Typically, in use the shadow digit will be interfaced, for example, to a personal computer. This provides a control loop with control signals going from the computer to a valve regulating the supply of air or other pneumatic fluid to the traction element, with the signal from the strain sensing element providing corresponding feedback on the configuration of the shadow digit.

Preferably the sensing element includes an optical emitter, an optical receiver, and a variable transmission path between the emitter and the receiver arranged such that the amount of light received at the receiver varies with the strain applied to the sensing element.

This preferred aspect of the invention is not limited to use with light in the visible spectrum, and may use infra-red emitting and sensing elements.

According to a second aspect of the present invention, there is provided a strain sensing element comprising an emitter and a receiver arranged generally side-by-side and a shutter member normally overlying the emitter and receiver so as to block or substantially to restrict the transmission of radiation from the emitter to the receiver, and being moveable away from the emitter and receiver in response to strain applied to the sensing element thereby providing increased transmission.

Preferably the emitter and receiver are mounted in a common body, and the shutter member is provided as part of a housing enclosing the common body and resiliently mounted with respect to the body.

While it is much preferred that the actuator of the first aspect of the present invention should use a strain sensing element in accordance with this second aspect, the strain sensing element is not limited in application to such actuators, and can also advantageously be used wherever a simple, compact and robust strain sensing element is required.

Examples of actuators and strain sensing elements in accordance with the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a side elevation of an actuator system; Figure 2 shows a detail of a first example of a strain sensing element; Figure 3 shows a detail of a second example of a strain sensing element; Figures 4a and 4b are sectional views of a further preferred example of the strain sensing element; and Figure 5 is a partially sectional side elevation of a shadow muscle.

A shadow digit is shown diagrammatically in Figure 1.

It consists of two beams, marked Beam A and Beam B, a shadow muscle, marked M, and a coiled spring, marked S.

Beam A and Beam B are pivoted at point 10, a few centimetres from their ends, as shown.

The muscle N is secured to Beam A at the point marked as 11 at one end, and attaches to Beam B at the point marked as 12 at the other end.

The coiled spring S is secured to Beam A at the point marked as 13 at one end, and attaches to Beam B at the point marked as 14 at the other end.

When air is forced into the muscle M through the tube marked T the muscle M contracts, causing Beam B to rotate about point 10 in an anti-clockwise direction.

When air is released from the muscle M it relaxes.

When this happens the spring S applies a restoring force to point 14 causing Beam B to rotate around point 10 in a clockwise direction so that it ends up occupying its initial position.

Situated within the spring S is a sensory mechanism which communicates to a computer or other controlling device via the electrical cable marked C.

Figure 2 shows the spring S in greater detail.

Secured to the inside of one end of the spring S is a photo-reflective infra-red combined transmitter and receiver, or more simply the infra-red sensor, marked I.

Positioned about half way along the spring S is a reflector, marked R.

Whilst in operation, the infra-red sensor I continuously transmits light of infra-red wavelength along the inside of the spring towards the reflector R, and at the same time measures the intensity of the light reflected back.

In between the infra-red sensor I and the reflector R, and on inside of the spring S, is a length of braiding material, marked D, formed of the same material used for the shadow muscle M which is described in further detail below. The purpose of the braiding material D is to prevent reflections from the coils of the spring S interfering with the readings taken by the infra-red sensor I.

As the digit moves through its aforementioned actions, the spring S stretches and relaxes. As it does this the distance between the infra-red sensor I and the reflector R varies, thus causing the intensity of light reflecting back onto the infra-red sensor I to vary also. This causes a change in the signal sent through the electrical cable marked C, enabling a calibrated computer or other controlling device to continuously monitor the position of Beam A relative to Beam B. In summary, each digit has both powered movement and feedback.

Figure 3 shows an alternative design for the strain sensing element. In this example, rather than a combined emitter/detector, a separate infra-red emitter I is provided at one end of the spring and a receiver J at the other end of the spring. The spring is surrounded by a dark shroud M and at the end of each spring a wire loop allows attachment to the beam or fitting.

Figure 4 shows a further and particularly preferred example of a strain sensing element. This element may be used either in series with spring or in series with the muscle itself. The element comprises two separate units which fit together. Unit A comprises a body Al and a mechanical/electrical termination A2. Unit B comprises a housing which is fitted over unit A. The housing is able to move slightly: when it is pulled, two rubber pads located between the housing and the body Al are compressed.

A flat black surface B1 is provided on the inner end wall of the housing.

An emitter diode A3D and receiver diode A2D are located in recesses in the body and facing the flat black surface. The output levels of the receiver are such that further amplification is not required. Normally the black surface blocks transmission from the emitter to the receiver. However as strain is supplied to the element, compressing the rubber mountings, the black surface moves away from the receiver and emitter and allows infra-red light to pass between them. The common body Al is mechanically linked to the termination A2 by the legs of the emitter and receiver diodes. Within the termination A2, the legs are fixed to a PCB-like structure A4 having a copper layer A5 on its inner side. A soldered connection is made between the legs of the diodes and cables which carry the output signal from the sensing element.Figure 4b is a view of the sensing element with the two units separated. In use, the element is secured by one of the loops AL, BL to either the traction element or the spring, and by the other loop to the line extending to the structural member from the element. As an alternative to a black surface a white or other reflective surface may be used. In that case, the emitter and sensor are not recessed but lie substantially in the surface plane of the body, with the shutter normally abutting them. With such an arrangement only a very small displacement of the shutter is sufficient to cause a marked increase in transmission. A resilient mounting is not then needed, the play between the housing and body, and the flexibility of the housing being sufficient to allow the required travel when stress is applied to the element.

Whether the shutter is black or white, it functions by reflecting light from the transmitter to the receiver as soon as the shutter is pulled away from the emitter and receiver. The use of a black surface has the advantage of reducing the range of the variation in the amount of light transmitted from the state in which the shutter is immediately over the sensors, and the state in which the shutter is pulled away from the sensors. The white shutter, by contrast, provides a more marked transitions between these states.

Figure 5 shows a detail of the shadow muscle or "traction element".

The traction element 1' comprises a braided sheath 2' surrounding a tube 3'. The braided sheath 2' in this example is normally used for holding a number of electric cables together as part of a wiring loom. It is available commercially as EXPANDO EXPT from RayChem. The braids of the braided sheath 2 form two interwoven helices with opposite senses of rotation. As such a sheath 2' is expanded in the circumferential direction its length decreases since the braids themselves are inextensible. In use, a pressurised fluid 4', usually a compressed gas, is introduced into the tube 3' and this is then inflated so circumferentially expanding the sheath 2'. This causes a longitudinal contraction of the sheath 2'. The degree of inflation of the tube 3' determines the degree of contraction of the sheath 2'.

The muscle may be controlled via two valves, one in an inlet line connecting a source of air under pressure to the muscle and the other in an exhaust line from the muscle.

Then the length of time for which the inlet valve is switched on, determines the degree of inflation of the muscle and hence the amount of traction applied by it.

Conversely the muscle can be let down by switching on the exhaust valve for an appropriate period of time. As an alternative arrangement, the muscle may be provided with a "leaky" outlet so that there is a slow continual exhaust of air from the muscle. The inlet valve is then switched on at intervals appropriate to maintain the required overall pressure in the muscle. In either case, the muscle can be controlled over a wide substantially continuous range of states, using valves which are simple on-off devices.

Claims (13)

1. An actuator system comprising: two elongate structural members pivotally interconnected at a point away from their ends; a traction element extending between one member and the other member on one side of the pivotal interconnection; a resilient tensioning element extending between the one member and the other member on the other side of the pivotal interconnection; and a strain sensing element connected with either the resilient element or with the traction element between the one and the other member and arranged to output a signal varying with the configuration of the actuator and suitable for use in controlling the traction element.

2. A system according to claim 1, in which the traction element comprises a radially expansible tube formed of elastomeric material, arranged to be connected to a source of pressurised fluid and covered with a braided sheath which is connected at at least two points to the structural members, the element being arranged so that the introduction of pressurised fluid into the tube causes a radial expansion of the braided sheath and hence a longitudinal contraction of the sheath which in turn causes relative movement of the structural members towards one another.

3. A system according to claim 2, in which the braiding includes two interwoven sets of braids formed as helices with opposite senses of rotation.

4. A system according to any preceding claim, in which the signal output by the strain sensing element is input to control means arranged to control the traction applied by the traction element.

5. A system according to any preceding claim, in which the sensing element includes an optical emitter, an optical receiver and a variable transmission path between the emitter and receiver arranged such that the amount of light received at the receiver varies with the strain applied to the element.

6. A strain sensing element comprising an emitter and a receiver arranged generally side-by-side, and a shutter member normally overlying the emitter and receiver so as to block or substantially to restrict the transmission of radiation from the emitter to the receiver, and being moveable away from the emitter and receiver in response to strain applied to the sensing element, thereby providing increased transmission.

7. A strain sensing element according to claim 6, in which the emitter and receiver are mounted in a common body, and the shutter member is provided as part of a housing enclosing the common body and resiliently mounted with respect to the body.

8. A strain sensing element according to claim 7, in which the emitter and receiver are located in recesses in one end face of the body, and the shutter member is an opposing inner face of the housing generally parallel to the end face.

9. A strain sensing element according to claim 6, 7 or 8, in which the resilient mounting is provided by an elastomeric material positioned between the housing and the body and arranged to be compressed as the shutter is pulled away from the body.

10. A strain sensing element according to claim 9, in which the elastomeric material is a rubber O-ring.

11. An actuator system according to any one of claims 1 to 5, including a strain sensing element according to any one of claims 6 to 10.

12. An actuator system substantially as described with respect to the accompanying drawings.

13. A strain sensing element substantially as described with respect to Figures 4a and 4b of the accompanying drawings.