GB2183037A

GB2183037A - Acoustic sensor systems for controlling processes or machines

Info

Publication number: GB2183037A
Application number: GB08528079A
Authority: GB
Inventors: Trevor James Holroyd
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1985-11-14
Filing date: 1985-11-14
Publication date: 1987-05-28
Also published as: DE3638814A1; JPS62156531A; FR2590362B1; FR2590362A1; GB8528079D0; GB2183037B

Abstract

A sensor (10) according to the invention comprises a transducer (14) which is acoustically coupled to a machine (12) or process etc. and which detects acoustic energy and converts it into an electrical signal. The electrical signal is amplified by an amplifier (16) and the level of acoustic energy is detected by a level detector (18). A comparator (20) compares the level of acoustic energy as detected with a preset level of acoustic energy and detects changes about the preset level and sends a signal. The signal can be used to control the machine or process, i.e. switch it on or off, or to indicate a change in the level of acoustic activity. A keyboard may comprise a plurality of such sensors each acoustically coupled to a key pad, adjacent pads being acoustically isolated. <IMAGE>

Description

SPECIFICATION Sensors for controlling processes or machines This invention relates to sensors for controlling processes or machines, particularly sensors which switch a process or machine on or off.

Switches are devices used for effecting the completion/interruption of an electrical circuit.

The prior art comprises many arrangements which are mechanical, for example lever switches, knife switches, turn switches, push-button switches and microswitches, which involve movement. Other switches are electromagnetic and pressure switches.

The prior art devices generally involve moving parts and or are not simple.

The present invention seeks to provide a sensor which has no moving parts and is simple and can be used to control processes or machines.

Accordingly the present invention provides a sensor comprising a transducer adapted to provide an electrical output signal dependent upon the degree of acoustic emission activity, a level detector adapted to measure the level of the electrical signal which corresponds to the level of acoustic emission activitiy, a comparator adapted to compare the level of acoustic emission activity with a preset level of acoustic emission activity, the comparator detecting changes in the level of acoustic emission activity from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity and producing a signal.

The comparator may produce a digital signal level change, or a signal to change the state of a relay contact.

The comparator may produce a signal which is an on or off signal, or a signal which indicates that the acoustic emission activity level has changed.

The transducer may be acoustically coupled to a housing of and apparatus to detect changes in the level of acoustic emission activity in the housing.

The sensor may be mounted in and acoustically coupled to a housing, the sensor detecting changes in the level of acoustic emission activity in the housing an producing a signal to control the apparatus, and the sensor may be hermetically sealed in a solid metal housing or other suitable material.

The level detector may measure the r.m.s.

level or peak level of acoustic emission activity detected by the transducer.

A keyboard comprising a housing adapted to receive a a plurality of key pads, the housing providing an increased attenuation path for acoustic emission activity, each key pad being acoustically coupled to respective sensor, each sensor comprising a transducer adapted to provide an electrical output signal dependent upon the degree of acoustic emission activity in the respective key pad, a level detector adapted to measure the level of acoustic emission activity, a comparator adapted to compare the level of acoustic emission activity with a preset level of acoustic emission activity, the comparator detecting changes in the level of acoustic emission activity from a level less, or greater, then said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity and producing a signal.

A method of controlling a process or machine comprising detecting the level of acoustic emission activity from a position of interest of the process or machine, comparing the level of acoustic emission activity from the position of interest of the process or machine with a preset level of acoustic emission activity, detecting changes in the level of acoustic emission activity from the position of interest of the process or machine from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustice emission activity, said changes producing a signal to control the process or machine.

A method of controlling a process or machine comprising detecting the level of acoustic emissions activity from a position of interest of a first process or or first machine, comparing the level of acoustic emission activity from the position of interest of the first process or first machine with a preset level of acoustic emission activity, detecting changes in the level of acoustic emission activity from the position of interest of the first process or first machine, from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity, said changes producing a signal to con trol a second process or second machine.

The signal to control the process or machine may be an on or an off signal.

The present invention will be more fully described by way of reference to the accompanying drawing in which Fig. 1 shows diagrammatically a sensor according to the present invention.

The sensor 10 shown in Fig. 1 comprises a transducer 14, an amplifier 16, a level detector 18 and a comparator 20. The transducer is acoustically coupled to, for example, a machine 12, and the transducer detects stress waves, also known as acoustic emission or secondary acoustic emission, and converts the acoustic emissions into electrical signals. The electrical signals produced by the transducer are then amplified in the amplifier and supplied to the level detector. The level detector analyses the amplified signal to detect or measure the level of acoustic emission activity detected by the transducer, this may be either a r.m.s.

level detector or a peak level detector.

The comparator then compares the level of acoustic emission activity as detected by the level detector with a preset level of acoustic emission activity, the comparator detects changes in the level of acoustic emission activity from a value less, or greater, than the present level to a value greater, or less respectively, then the preset level, and sends a signal to a control 22 for the machine 12.

The acoustic emission activity being detected is not the type generated by crack growth and plastic deformation, but is the type generated due to impacts, rubbing and turbulence, in fact any process which generates broad band stress wave transients.

The invention will be further illustrated by way of reference to the following examples, which are examples of many possible uses for the sensor.

An impact between two objects generates a broadband stress wave transient, the frequency content and amplitude of the stress wave is controlled by the physical properties of the objects. These stress waves are detectable in a wide variety of structures.

EXAMPLE 1 The impact of two items coming into contact can be detected by the generated stress waves. The sensor can be used to produce a signal to confirm that a mechanism has latched or fully retracted, or can be used to establish the coordinates of an object by registering the impact of a probe.

EXAMPLE 2 Blasting processes, such as sand, grit, bead and vapour, which are used for cleaning articles consist of a rapid succession of violent impacts. Since these impacts are intentional the sensor would be useful in detecting the absence of the blasting medium or the absence of a workpiece, and would send a signal accordingly.

EXAMPLE 3 Debris often produces impacts under the influence of gravity, for example attachments falling of machines or structures. The sensor could be used to indicate that an attachment has fallen off, or switch off the machine.

EXAMPLE 4 Debris from engines, machines often becomes entrained in a fluid flow, for example bearing debris in an oil, lubrication, system and impacts of the debris with the oil system housing will generate stress waves. The sensor could be used to indicate bearing wear or to switch off the machine if excessive stress wave levels are being detected.

Perturbations within a fluid can generate broadband stress waves which propagate through the fluid and into the walls of the containing vessel. The broadband stress waves can be detected by using an immersed transducer, or more conveniently using a transducer mounted onto a non-wetted surface of the container.

EXAMPLE 5 Turbulence is generally generated within fluid flows and the turbulence produces stress waves, the level of stress wave activity is dependent upon the particular arrangement.

The sensor may be used to detect excessive turbulence, stress wave activity, due to excessive flow rates because of a downstream leak or failure of a valve to close and to stop the system. Similarly the sensor could be used to detect insufficient flow rates in a coolant system, or a lubricating system, due to loss of coolant, lubricant, and to prevent operation.

This would be useful for an IC engine or turbines etc.

EXAMPLE 6 Transient stress waves are generated by the collapse of bubbles within a fluid whether the bubbles are caused by cavitation, boiling or effervescence, and the level of acoustic emission activity will depend upon the particular situation. A sensor could be used to produce a signal when the bubbles caused by the above processes are formed or when they cease to be formed. This would be applicable in pumps where cavitation occurs and in chemical and food processing.

In addition to the generation of heat during friction, broadband stress waves are also generated as part of the energy loss. Sliding contact between two solid surfaces is characterised by slip-stick processes, macroscopic and microscopic, each of which acts as a source of stress wave transients.

EXAMPLE 7 The unintentional sliding of a clamped, wedged, tied, slung or bolted objected is accompanied by broadband stress wave activity.

A sensor could be used as a slip detector for a robot hand.

EXAMPLE 8 A lubrication system is intended to minimise damage and frictional losses. The effectiveness of lubrication will control the generated stress wave activity. A sensor could be used in all types of machines and engines to detect excessive stress wave activity, due to loss of lubrication, and to shut down the machine or engine.

EXAMPLE 9 Many static objects have low levels of stress wave activity except when physically disturbed, for example articles in unoccupied office buildings and houses are likely to have very low background stress wave activity. A sensor could be used to detect tampering with doors, desks, cabinets, vaults etc, and operate a warning.

A sensor according to the invention could be used as a switch, the teansducer could be attached to the housing of a domestic or industrial item or apparatus. The entire housing of the item would then be active as a switch, touching the housing would create a stress wave which could be detected by the sensor, and if the level of the stress wave activity changes about the preset level, and used to switch the domestic or industrial item on or off, or be used to detect tampering with the item.

A manually operated sensor could be mounted within a very robust housing which is vandal or tamper proof. The sensor could if necessary be hermetically sealed in solid metal for example. Such a switch would have no moving parts, and could be simply rubbed or impacted to switch on an apparatus. It would be possible to form a keyboard arrangement comprising a plurality of key pads, each of which is connected to a respective sensor.

The key pads could be mounted in a robust housing, in which encreased attenuation is provided between the key pads, i.e. using an impedance mismatch or damping material to reduce the transmission of stress waves between the key pads. When a key pad is rubbed or impacted a stress wave is transmitted to the respective sensor, which may be of a sufficient level to give a switching action, but the stress wave transmitted through the housing to the adjacent key pads and sensors is insufficient to cause a switching action. Such a device could be used as a typewriter or for example as a keyboard for a Bank automatic cash dispenser.

The sensors could be mounted in and acoustically coupled to the respective key pad, and the sensors could be hermetically sealed in a metal or other suitable material key pad.

All mechanical means of metal removal, e.g.

milling, drilling, turning, grinding, broaching and polishing, generate broadband stress wave activity. The sensor can be mounted on the tool or the workpiece, or on the fixture that holds the workpiece or tool so that the workpiece or tool can be changed without affecting the sensor.

EXAMPLE 10 When the metal removal process commences there is an abrupt increase in stress wave activity. The sensor can be used to recognise this change and to produce a signal to confirm that the workpiece is within starting tolerances.

EXAMPLE ii The stress wave activity level is related to the rate of metal removal, an excessive cutting demand would result in an abnormally high stress wave activity level. The sensor could be used to recognise this and produce a signal to stop the processes and so prevent damage to the machine, tool and workpiece.

EXAMPLE 12 Excessive tool wear such as that which produces binding and a clear audible signal also generates increased broadband stress wave activity. The sensor could be used to recognise this condition and stop the machine and prevent more damage to the workpiece.

EXAMPLE 13 The breaking of a cutting edge is characterised by a very brittle fracture which is associated with a high amplitude stress wave transient. The sensor could be used to detect this event or the transition from prebreak to postbreak levels of stress wave activity and stop the machine.

A wide variety of industrial processes, industrial plant, machinery and equipment generate acousticemission activity in carrying out their function. A signal form a sensor can be used to switch on or switch off a machine apparatus, process to ensure that the machine or process is operating within the desired operating range.

Also a sensor can be used to switch on a second process or machine when the acoustic emission activity of a first process or machine changes from a level less, or greater, than a preset level to a level greater, or less respectively, than the preset acoustic emission activity.

The invention need not be limited to the examples given, but could be applied to any situation where there is a change in the level of acoustic emission activity, the sensor can be used to produce a signal when the comparator detects a change in the level of acoustic emission activity from a level less, or greater, than a preset level to a level greater, or less respectively, than the present level.

The signal from the sensor can be used to switch on or switch off a machine, apparatus, process, or otherwise control a machine or process or indicate that a particular point of a process or machining process has been reached.

Broadband stress waves generated by processes are ever present and in many cases are very sensitive indicators of process variations.

A single sensor according to the invention provides real time monitoring of an appreciable area or volume. The positioning of the sensor is not critical since the stess waves propogate from their source to the sensor.

This makes the installation of the sensor easy and in most cases non-invasive, e.g. the sensor may be externally mounted monitoring the interior of a machine. The orientation of the sensor is unimportant. The sensor is generally simple and requires no expensive components.

It may be necessary to position the sensor as acoustically close to the source of the stress waves of interest in the case where stress waves of interest are masked by another more dominant source of stress waves, because the use of acoustic emission as a sensor will only succeed if the source of interest is dominant. Another method would be to analyse the output of the sensor differentially with respect to that of a second sensor positioned close to the competing source of stress waves.

Claims

1. A sensor comprising a transducer adapted to provide an electrical output signal dependent upon the degree of acoustic emission activity, a level detector adapted to measure the level of the electrical signal which corresponds to the level of acoustic emission activity, a comparator adapted to compare the level of acoustic emission activity with a preset level of acoustic emission activity, the comparator detecting changes in the level of acoustic emission activity from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity and producing a signal.

2. A sensor a claimed in claim 1 in which the comparator produces a digital signal level change.

3. A sensor as claimed in claim 1 in which the comparator produces a signal to change the state of a relay contact.

4. A sensor as claimed in any of claims 1 to 3 in which the comparator produces a signal which is an on or an off signal.

5. A sensor as claimed in any of claims 1 to 4 in which the transducer is acoustically coupled to a housing of an apparatus to detect changes in the level of acoustic emission activity in the housing.

6. A sensor as claimed in any of claims 1 to 4 in which the sensor is mounted in and acoustically coupled to a housing, the sensor detecting changes in the level of acoustic emission activity in the housing, and producing a signal to control the apparatus.

7. A sensor as claimed in claim 6 in which the sensor is hermetically sealed in a solid metal housing, or other suitable material.

8. A sensor as claimed in any of claims 1 to 3 in which the comparator produces a sig nal which indicates that the acoustic emission activity level has changed.

9. A sensor as claimed in any of claims 1 to 7 in which the level detector measures the r.m.s. level of acoustic emission activity detected by the transducer.

10. A sensor as claimed in any of claims 1 to 7 in which the level detector measures the peak level of acoustic emission activity detected by the transducer.

11. A keyboard comprising a housing adapted to receive a plurality of key pads, the housing providing an increased attenuation path for acoustic emission activity, each key pad being acoustically coupled to a respective sensor, each sensor comprising a transducer adapted to provide an electrical output signal dependent upon the degree of acoustic emission activity in the respective key pad, a level detector adapted to measure the level of acoustic emission activity, a comparator adapted to compare the level of acoustic emission activity with a preset level of acoustic emission activity, the comparator detecting changes in the level of acoustic emission activity from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity and producing a signal.

12. A keyboard as claimed in claim 11 in which each sensor is mounted in and acoustically coupled to a respective key pad.

13. A keyboard as claimed in claim 12 in which each sensor is hermetically sealed in a solid metal key pad, or other suitable material.

14. A method of contolling a process or machine comprising detecting the level of acoustic emission activity from a position of interest of the process or machine, comparing the level of acoustic emission activity from the position of interest of the process or machine with a preset level of acoustic emission activity, detecting changes in the level of acoustic emission activity from the position of interest of the process or machine from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity, said changes producing a signal to control the process or machine.

15. A method of controlling a process or machine comprising detecting the level of acoustic emission activity from a position of interest of a first process or first machine, comparing the level of acoustic emission activity from the position of interest of the first process or first machine with a preset level of acoustic emission activity, detecting changes in the level of acoustic emission activity from the position of interest of the first process or first machine from a level less, or greater, than said preset level of acoustic emission activity to a level greater, or less respectively, than said preset level of acoustic emission activity, said changes producing a signal to control a second process or second machine.

16. A method of controlling a process or machine as claimed in claim 14 in which the signal to control the process or machine is an or an off signal.

17. A sensor substantially as hereinbefore described and with reference to the accom panying drawings.

18. A method of controlling a process or machine substantially a hereinbefore described.