GB2387650A

GB2387650A - Displacement sensors

Info

Publication number: GB2387650A
Application number: GB0209099A
Authority: GB
Inventors: Mark Paul Norman
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-04-20
Filing date: 2002-04-20
Publication date: 2003-10-22
Also published as: GB0209099D0

Abstract

A displacement sensor includes an outer tubular member (10) formed of a light impervious material having a reflective inner surface, and an inner member (11) movable relative to the outer member (10). A light emitter is mounted on one of the members and a light detector is mounted on the other. The light detector (12) generates an output signal, the value of which is dependent on the intensity of the light incident on it and this signal can be used to provide an indication of the displacement of one member relative to the other. In an alternative embodiment, emitter and detector may be disposed upon the same member. Rotation may be measured and force or torque may be measured with the use of a spring.

Description

-.L Lab DISPLACEMENT SENSORS

Field of the Invention

This invention relates to linear and/or rotary displacement sensors and is particularly concerned with the provision of an improved form of displacement sensor which includes a light emitter and a light detector.

Backaround to the Invention

Analogue light emitter/detector pairs are not normally used to sense displacement, whether linear or angular. This is due to a number of factors, for example, the interference which can be caused by ambient light and the highly non-linear response of received light incident on the detector in a physically unconstrained environment. It is also necessary to take into account changes of output resulting from small changes in relative cross-axial orientation between the emitter and the detector.

It is accordingly a specific object of the present invention to provide a displacement sensor the design of which is such that the

limitations referred to above can be overcome without resorting to expensive parallel light sources such as lasers and laser detectors.

Summarv of the Invention According to a first aspect of the present invention there is provided a displacement sensor which includes an outer tubular member formed of a material which is impervious to light and an inner member which is movable relative to the outer member, a light emitter being mounted on one of the members and a light detector being mounted on the other member, the outer tubular member having a reflective inner surface and said light detector being arranged to generate an output the value of which is dependent on the intensity of the light incident thereon.

The inner and outer tubular members may be arranged either for relative linear movement or for relative rotary movement.

The light emitter and the light detector may thus be a fixed distance from each other, with one of the two members arranged for movement crossaxially relative to the other member. An aspherical optical system may be employed, or a cover having a specifically shaped aperture may be attached to the emitter. The emitter or detector may alternatively be designed to produce a signal which varies in dependence on the relative rotary positions of __ _

According to a second aspect of the present invention there is provided a displacement sensor which includes an outer tubular member formed of a material which is impervious to light and an inner member which is movable relative to the outer member, a light emitter and a light detector mounted on one of the members and a reflector mounted on the other member, the outer tubular member having a reflective inner surface and said light detector being arranged to generate an output the value of which is dependent on the intensity of the light incident thereon.

The light emitter and the light detector may again be a fixed distance from each other, with one of the two members arranged for movement crossaxially relative to the other member. An aspherical optical system may be employed, or a cover having a specifically shaped aperture may be attached to the emitter. The emitter or detector may alternatively be designed to produce a signal which varies in dependence on the relative rotary positions of the emitter and detector.

One or both of the members may be made of a flexible material which allows the sensor to operate over a curved surface.

In addition to the outer member being of tubular form, the inner member may also be of tubular form. The two tubular members will have crosssectional configurations determined by the required relative movements between the two members. For example, they may be of round or square cross-section.

According to a third aspect of the present invention there is provided a force sensor which comprises a displacement sensor as defined above and spring means acting between the inner and outer members.

If the two members of the sensor are arranged for relative rotary movement, the spring deflection may be arranged to give a measure of torque.

Although reference has been made above to "light", other forms of electromagnet waves (such as microwaves) may be employed. Ultrasound may also be used. The term "light" as used above is thus to be interpreted as including other forms of electromagnetic waves and ultrasound.

The output from the detector is normally very small and is, therefore, preferably passed through a pre-amplifier.

As compared with currently available displacement sensor techniques, such as LVDT's and linear potentiometers, the sensor of the present invention provides the following benefits: a) intrinsic EM immunity, b) a complete sold state, non-contacting sensing system giving very high reliability, c) excellent linearity, d) can operate over a wide range of displacements,

e) is well suited to mass-production techniques thereby providing economic advantages as compared to existing sensors, f) it is small in size, and 9) light in weight.

Brief Description of the Drawings

Figure 1 is a sectional view of a first form of displacement sensor which includes sliding square-section tubes, Figure 2 is a sectional view of a second form of displacement sensor which includes sliding flexible tubes, Figure 3 is a sectional view of a third form of displacement sensor which includes three relatively movable tubes, and Figure 4 is a sectional view of a fourth form of displacement sensor arranged to act as a force sensor.

Descrintion of the Preferred Embodiments The displacement sensor shown in Figure 1 comprises an outer, square-section tubular member 10 and an inner, square-

section tubular member 11 which is a close sliding fit within the outer member 10. The tubular members 10 and 11 are of similar lengths with the lengths of the two tubular members 10 and 11

depending on the particular application of the measurement system. l An infra red detector 12 is fitted at one end of the outer tubular member 10 such that the detecting element of the detector 12 faces inwardly of the outer tubular member 10. An infra red transmitter 13 is fitted to the adjacent end of the inner tubular; member 11 such that it is contained within the inner tubular member 11 facing outwardly thereof.

The inner tubular member 11 is arranged to slide inwardly of the outer tubular member 10 until it reaches a limiting position l determined by contact of the emitter 13 with the infra red detector I 12. The intensity of light incident on the detector 12 will vary in dependence on the distance between the emitter 13 and the detector 12. The outer tubular member will shield the detector 12 from unwanted ambient light and, at the same time, will act as a light guide to ensure that the majority of the light emitted by the emitter 13 is received by the detector 12. During sliding movement of the inner tubular member 11 relative to the outer tubular member 10, relative cross-axial movements are prevented by the interengagement of the two square-section formations of the tubular members 10 and 11. Brackets (not shown) can be provided at either end of each of the tubular members 10 and 11 to provide stable mountings.

The light received by the infra red detector 12 is converted using a suitable electronic circuit (not shown), with amplification as

- required. The output signal in this form is non-linear but can be converted to linear by a calibration process against a known linear sensor. The optical detecting element, i.e. the infra red detector 12, is of a commonly available type and provides an output response in proportion to the intensity of the incident light. The output response may be a current, voltage, resistance or frequency, and the emitter 13 is chosen to match the peak wavelength response of the detector 12. The maximum detection length is determined by the sensitivity of the detecting electronic circuit, and can be selected to suit. The inner and outer tubular members 11 and 10 can be of very small cross-section, for example, less than 1 mm. square, and the emitter 13 and the detector 12 will be of corresponding size.

In a modification to the arrangement described above, both the emitter 13 and the detector 12 are mounted on the same tubular member 10 and 11 and a mirror or like reflective surface is provided on the other tubular member 10 or 11. The intensity of the light from the emitter 13 which is reflected on to the detector 12 will vary in dependence on the distance between the emitter/detector and the reflector and the output from the detector will be used, via a suitable calibration procedure, to provide an indication of the displacement of the one tubular member relative to the other tubular member.

o c) Turning next to Figure 2, this shows an arrangement in which at least one of the tubular members 10, 11 is flexible such that linear displacements around curved surfaces can be measured.

In addition to measuring relative linear movement, it is also possible to measure angular displacement at the same time as linear displacement. Figure 3 shows an arrangement for enabling I this to be carried out. It includes an outer tubular member 20 formed of a light-impermeable material and an inner tubular member 21, also formed of a light- impermeable material. A detector 22 is mounted on the outer tubular member 20, facing inwardly thereof, and an emitter 23 is mounted on the adjacent end of the inner tubular member 21 so as to emit light towards the detector 22 as well as emitting light in the opposite direction, i.e. towards the opposite end of the inner tubular member 21.

A second detector 24 is mounted at one end of a central tubular member 25 and the arrangement is such that the inner tubular member 21 is free to move linearly relative to the outer tubular member 20 and is free to move angularly relative to the central tubular member 25. The light emitted by the emitter 23 towards the second detector 24 is angularly oriented and the second detector 24 is arranged to sense the orientation of the received light of maximum intensity so as to generate an output which, by means of a calibration process, can provide an indication of the angular position of the inner tubular member 21 relative to the central tubular member 25. At the same time, the output from the first detector 22 can be used to provide an indication of the

linear position of the inner tubular member 21 relative to the outer tubular member 20.

Figure 4 shows a modification of the arrangement shown in Figure 1 in which a spring 14 (which may be either a compression spring or an extension spring) is placed between the detector 12 and the emitter 13. An applied force will produce a change in the relative positions of the emitter 13 and the detector 12 and measurement of the relative displacement can be used to measure the applied force. The device can thus be used as a force sensor as well as a displacement sensor.

Derivatives of the output signals obtained from the or each detector can, if required, be processed further to provide additional information, for example, velocity and acceleration.

The outer tubular member 10 or 20 acts as a light guide in all versions of the sensor. It has a dual role, the first is to prevent stray radiation from outside the tube entering into the tube and the second is to reduce light losses due to absorption (and to some extent scatter) from light hitting the inner surface. By altering the amount of scatter along the inner tube wall and the efficiency of specular reflection (by changing the tube wall to be made of a more mirror-like material for example) the rate of loss of signal with distance can be altered. For many applications, materials can be used which minimise the rate of loss along the tube but, for short distances, for example, the most mirror- like materials may not be

used. Thus, a short displacement may not give sufficient signal change if the material is too good a reflector.

The intensity on the detector surface has an inverse non-

linear relationship to the distance parameter being measured (rotation or displacement). This can be corrected in the analogue domain or in the digital domain (after passing the signal though an a->d amplifier). This is an important part of the technology, effectively the sensor only works to its optimum with this linearization process. There are a number of methods for performing linearization. For example, one can compare against a known sensor, define the error as a mathematical model (e.g. a fourth order polynomial) and then use this to linearise subsequent inout. The optional use of a post-processing element (such as a microprocessor) enables additional features to be added to the sensor such as: a)digital output formats (conceptually this is a digital sensor), b) improved linearization, c) signal processing to give derivatives, such as velocity & acceleration), d) data logging, e) application specific processing, and f) multiple units e.g. imperial or metric readings.

It is possible to produce a version of the sensor described above with an embedded processor. The user is then able to connect to the sensor (which acts like a terminal) and set various options (such as changing the output units from metric to Imperial) and perform a calibration process on the sensor.

Claims

Claims:

1. A displacement sensor which includes an outer tubular member formed of a material which is impervious to light and an inner member which is movable relative to the outer member, a light emitter being mounted on one of the members and a light detector being mounted on the other member, the outer tubular member having a reflective inner surface and said light detector being arranged to generate an output the value of which is dependent on the intensity of the light incident thereon.

2. A displacement sensor which includes an outer tubular member formed of a material which is impervious to light and an inner member which is movable relative to the outer member, a light emitter and a light detector mounted on one of the members, and a reflector mounted on the other member, the outer tubular member having a reflective inner surface and said light detector being arranged to generate an output the value of which is dependent on the intensity of the light incident thereon.

3. A displacement sensor as claimed in either of the preceding claims, which is arranged to measure relative linear displacements of the inner and outer members

4. A displacement sensor as claimed In Claim 1 or Claim 2, which is arranged to measure relative angular displacements of the inner and outer members.

5. A displacement sensor as claimed in Claim 1 or Claim 2, in which at least one of said inner and outer members is flexible such that linear displacements around curved surfaces may be measured.

6. A force sensor which comprises a displacement sensor as claimed in any one of the preceding claims and spring means acting between the inner and outer members.

7. A displacement sensor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

8. A method of measuring which includes the use of a displacement sensor as claimed in Claim 7.