GB2285865A - Strain measurement system - Google Patents

Abstract

A strain measurement system comprises a wheatstone bridge 20 having strain gauge arms 21 - 24 adhesively bonded to a surface of a component e.g. a power drive shaft (51, Figure 3). A potential divider for monitoring temperature comprises low and high stability resistors 34, 35 and is co-mounted with the bridge across supply lines 25, 26. The output of the bridge is connected by lines 36, 37 with an application specific integrated circuit (ASIC) 38. A line 39 connects the ASIC 38 and a junction of the resistors 34, 35. The ASIC includes an analogue switch 40 for switching between a strain related bridge output signal on line 36 and a temperature related signal on line 39. The signals are fed via an operational amplifier 41 to an A/D converter 42. Digital signals are passed from the converter to an algorithmic operator 43 which accesses an external EEPROM 44. The EEPROM 44 holds a matrix look-up table storing data obtained by calibration of the system over desired strain and temperature ranges. The algorithmic operator 43 reads down correction data and uses interpolation logic to calculate a strain value which is corrected for zero offset, zero drift with temperature and span drift with temperature. The corrected strain value is passed to a digital frequency synthesizer 45 which produces a frequency signal on line 46. If desired a visual or audio warning is given if a predetermined strain level is exceeded. Calibration data is input to the EEPROM 44 via lines 89, 90. <IMAGE>

Description

Description of Invention Title: Strain Measurement Systems This invention relates to strain measurement systems which use strain sensing elements attached to a surface of a component and is also concerned with calibration of such a system.

A particular requirement to measure strain is that of monitoring, controlling or limiting torque in components of a mechanical power transmission system. Torque measurement systems have been proposed for this requirement which measure strain by metal foil or semiconductor strain gauges; by means of magnetostriction; by phase displacement of the ends of a shaft; or by means of surface acoustic wave devices. Disadvantages of such systems are that they all suffer from drift with temperature and some produce a non-linear output.

A known strain measurement system 10, as depicted in Figure 1 of the accompanying drawings has four metal foil strain gauges 11, 12, 13, 14 provided one in each arm of a wheatstone bridge circuit. Variations in the resistances of the strain gauges and in their angles with respect to each other, as well as other minor imperfections, generally result in the output of the bridge being other than absolute zero at zero torque. There is also a requirement to adjust the bridge span, that is the range of the bridge output between zero and 100% torque. Furthermore, there is a requirement to compensate for zero drift and span drift with temperature change. In the system 10 compensation elements in the form of metal foil resistors 15, 16, 17, 18 are included in the bridge circuit. Each of these resistors provides a ladder of resistance elements (rungs) 19 connected in parallel. Adjustment of resistance is achieved by cutting out a rung of the ladder judged to be that which will effect the desired adjustment.

Resistor 15 manufactured from a suitable material is located in the bridge between strain gauges 11 and 12, whilst resistor 16 manufactured from a different material is provided between strain gauges 13 and 14. These resistors 15 and 16 are used to trim the bridge output at zero torque and for zero drift with temperature change. Resistors 17 and 18, which are also manufactured from suitable different materials, are provided one in each of the bridge power supply lines and are used to trim the bridge at 100 per cent torque and for zero drift with temperature change at 100 per cent torque.

Calibration of the system 10 when used to monitor torque in a drive shaft of a power transmission system requires that a number of excursions over the full operating temperature range be conducted whilst simultaneously applying torque to the shaft at a number of discrete levels. Each level of torque must be held constant at a constant temperature whilst readings of the bridge output are taken. These readings are then analysed and used to choose which rung (i.e. resistance element 19) of the ladder to cut. This process is iterative, and a number of temperature and torque excursions are required, together with a number of ladder cuts, before temperature stable calibration is achieved.

Thus this calibration process is labour intensive and time consuming, especially if the thermal inertia of the thermal characteristics of the signal conditioning circuits downstream of the wheatstone bridge is large. Further, cutting of the ladder rungs requires considerable skill as particular care must be taken not to cut through the resistor insulating backing material with the attendant risk of the cut edge of the rung contacting base metal of a component to which it is bonded. This cutting operation is made more difficult when the strain gauges and resistors are bonded to the internal surface of a hollow drive shaft.

Another problem arises when it is a requirement to provide a strain measurement system having redundancy, that is to say two or more bridge circuits so that, for example, torque can continue to be monitored in the event of failure of one of the circuits. For example, in the case of one particular drive shaft six wheatstone bridges and their associated conditioning circuitry are required. In calibrating each bridge it is not generally possible to achieve absolute zero bridge output at zero torque. A zero offset of plus or minus 0.2 millivolts is generally considered acceptable. Thus in calibrating six wheatstone bridges there will be variations between the zero offsets and output spans of the bridges.

It is an aim of the present invention to provide an improved strain measurement system.

It is a further aim of the present invention to de-skill the calibration of a wheatstone bridge in a strain measurement system.

Other aims of the invention are to reduce the man-hours required to perform the calibration; reduce the elapsed time required to perform the calibration; normalise zero offset and output span to the same levels in a multi-bridge system; and to take into account, during calibration, the thermal characteristics of the signal conditioning circuits downstream of the wheatstone bridge.

Accordingly, in one aspect the invention provides a method of calibrating a strain measurement system comprising strain sensing elements bonded to a surface of a component in which strain is to be measured and connected in a wheatstone bridge, including the steps of: a. energising the bridge; b. cycling the component through a desired operating load range at a number of discrete temperatures in a desired operating temperature range; c. inputting to a computer strain related signals output by the bridge at a plurality of loads at each discrete temperature, signals representative of sensed temperature of the bridge at each discrete temperature, values of the true load applied to the component at each discrete temperature, and values of the true discrete temperature at each load; d. processing the data in the computer to build a look-up table of data for correcting strain related signals output by the bridge for temperature variation in operation of the system; and e. downloading the look-up table to an electrically-erasable-programmable-read-only memory (EEPROM) provided with signal processing circuit means of the strain sensing system.

Preferably the bridge output signals at zero and at maximum operating load are input to the computer at each discrete temperature so that the strain values measured in operation can be corrected for zero offset, zero drift with temperature and span drift with temperature.

In an embodiment of the invention temperature signals are acquired from a potential divider co-mounted with the wheatstone bridge, the divider comprising one stable resistive element and one unstable resistive element connected in series, the unstable element providing temperature sensing.

In a modification of this embodiment the stable element of the potential divider is provided by the wheatstone bridge.

In another aspect the invention provides a system for measuring strain in a component comprising a wheatstone bridge of strain sensing elements bonded to a surface of the component, means for energising the bridge, means for monitoring the temperature of the bridge, an EEPROM for storing in the form of a look-up table values of bridge output and temperature signals output in calibration of the bridge over a range of strain and temperature, signal processing electrical circuit means for receiving signals output by the bridge and signals from the temperature monitoring means, the signal processing electrical circuit means including means for converting bridge output signals and temperature signals to digital signals, means for addressing locations in the EEPROM look-up table, means for processing data obtained from the look-up table to obtain a strain value corrected for temperature variation, and means for transmitting a signal representative of the corrected strain value.

The signal processing electrical circuit means may be mounted with the bridge circuit on the component or, alternatively, may be provided remote from the component.

Preferably the signal processing electrical circuit means comprises an application specific integrated circuit (ASIC).

The ASIC includes an analogue switch for switching the ASIC between acquiring a bridge output signal and a bridge temperature signal. These signals are converted from analogue to digital format by an analogue to digital converter incorporated in the ASIC.

The ASIC further comprises an algorithmic operator including averaging logic means which is fed by the analogue to digital converter to output EEPROM addresses for strain and temperature. The algorithmic operator further includes interpolation logic means which processes data acquired from the EEPROM to calculate a corrected strain value.

A signal representative of the corrected strain value is passed to digital frequency synthesizer means incorporated in the ASIC which produces a frequency signal whereby the corrected strain value is transmitted to a display station.

Whilst the corrected strain value may be displayed it may be further processed, either before or after transmission, to provide a strain related parameter such as stress, load or torque.

In a particular embodiment a strain measurement system in accordance with the invention is utilised to monitor torque in a drive shaft transmitting power in a power transmission system.

In this embodiment the system may have redundancy by the provision of two or more bridges and associated signal processing circuits whereby torque monitoring continues in the event of failure of one of the bridges or its associated circuits.

Any suitable strain sensing elements may be used, such as metal foil strain gauges or semiconductor strain gauges.

The invention will now be further described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagram of a wheatstone bridge circuit used in a strain measurement system in accordance with the prior art; Figure 2 is a circuit diagram of a strain measurement system in accordance with an embodiment of the present invention; and Figure 3 is a view in cross-section of a rig for calibrating a power drive shaft having a strain measurement system in accordance with the present invention.

A strain measurement system in accordance with one embodiment of the invention, as shown in Figure 2, comprises a wheatstone bridge circuit 20 having four strain gauge arms 21, 22, 23, 24. The strain gauges may be of metal foil type or semi-conductor type and are adhesively bonded to a surface of a component in which it is required to measure strain, for example a power drive shaft 51 (reference Figure 3).

The bridge is energised by a 5 volts dc voltage on lines 25 and 26 connected to one side of the bridge. This voltage is supplied by a power signal conditioning circuit 27 which is preferably provided as a printed circuit board. The circuit 27 receives a 1870 Hz ac power signal input on a pin 28 of a pair of pins 28, 29. The power signal is converted to dc by a voltage rectifier 30 and regulated by a voltage regulator 31 to provide the 5 volts dc signal for bridge energisation. Capacitor 32 is a reservoir capacitor for rectifier 30 and capacitor 33 is a reservoir capacitor for regulator 31.

A potential divider for monitoring temperature comprises low stability resistor 34 and a high stability resistor 35 connected in series with each other, and co-mounted in parallel with the input side of the bridge across lines 25 and 26.

The output side of the bridge is connected by lines 36 and 37 with an application specific integrated circuit (ASIC) 38. A line 39 connects between the ASIC 38 and a junction of the resistors 34 and 35. The ASIC includes an analogue switch 40 for switching between lines 36, 37 to obtain a strain related bridge output voltage signal and line 39 to obtain a bridge temperature related voltage signal. The signals are amplified by an operational amplifier 41 before being converted to digital form by an analogue to digital (A/D) converter 42. Digital signals are passed from the A/D converter to an algorithmic operator 43 which accesses an electrically-erasable-programmable-read-only memory (EEPROM) 44 provided external of the ASIC 38. The EEPROM 44 holds a matrix look-up table storing data obtained by calibration of the system over desired strain and temperature ranges as will hereinafter be described. The algorithmic operator 43 reads down correction data and uses interpolation logic to calculate a strain value which is corrected for zero offset, zero drift with temperature and span drift with temperature. The corrected strain value is passed to a digital frequency synthesizer 45 which produces a frequency signal on line 46. The frequency signal is transmitted to an appropriate station (not shown) where a strain value or other strain related value is displayed. If desired a visual or audio warning is given if a predetermined strain level is exceeded.

Calibration data is input to the EEPROM 44 on a pair of lines 47, 48, as well hereinafter be further described.

Referring now to Figure 2, there is shown a calibration rig 50 for calibrating a strain measurement system in accordance with the present invention when used to measure torsional shear strain in a power drive shaft 51 and, hence, to monitor torque transmitted by the shaft.

The drive shaft 51 is hollow and has the wheatstone bridge circuit 20, power signal conditioning circuit 27 and ASIC 30 bonded to the internal surface at one end of the shaft.

For calibration purposes the opposite end of the shaft is supported in a thrust bearing 52 which is fixed to a base plate 52 of the rig. An intermediate plate 54 is spaced from the base plate 53 by legs 55 and has a central hole 56 through which the drive shaft projects from its support in the thrust bearing. A tubular member 57 is fixed at one end to the intermediate plate 54 and extends upwardly to encompass a substantial part of the length of the drive shaft. At its opposite end the tubular member 57 is provided with a flange plate 58. An environmental chamber 59 encloses the upper end length of the drive shaft and has a flange 60 intermediate its ends which is supported on a base plate 61 that is located on the flange plate 58 by dowels (not shown) and secured with quick release clamps (not shown). Two load cells 62 and 63 are supported on the base plate 61 and are connected to outer ends of arms 64 and 65, respectively, which project radially outwardly from connection with an internally splined ring member 66 that engages with a splined length of the drive shaft near its upper end. A similar internally splined ring member 67 engages with a splined length of the drive shaft near its lower end and radially projects two arms 68, 69 which are connected at their outer ends with hydraulic jacks 70, 71, respectively. The jacks 70, 71 are supported by legs 72, 73, respectively, which extend between base plate 53 and intermediate plate 54.

In calibration of the strain measurement system a power signal from a power supply (not shown) is input to the power signal conditioning circuit 27 over a line 80. As previously explained, the conditioning circuit 27 provides a 5 volts dc signal for energisation of the bridge 20. Voltage signals output by the bridge due to out of balance are passed over a line 81 to a computer 82 which is also connected by a line 83 for receiving a signal from a sensor 84 sensing temperature of the drive shaft internally of the chamber 59. The computer is further connected by lines 85, 86 for receiving signals output by the load cells 62, 63 when the shaft is subject to torque loading.

To calibrate the shaft over a desired temperature range, say for example -55 to 1600C, it is first cooled to the lower end of the temperature range by passing a refrigerant through refrigerant coils (not shown) disposed internally of the chamber. The bridge 20 is energised and bridge output voltage at zero torque is acquired by the computer as a datum. Torque load is then applied to the shaft by the hydraulic jacks 70, 71 and is gradually increased to the top end (100 per cent) of the desired shaft operational torque range. At the same time strain representative bridge output voltages at a number of discrete load levels are input to the computer together with true load values read out from the load cells. Bridge temperature representative voltage signals acquired between resistors 34 and 35 are also input to the computer together with true temperature values as measured by the temperature sensor 84.

If desired calibration may be extended beyond 100 per cent torque, such as for example to 180 per cent of the operational torque range, in order to obtain a calibration for over torquing of the shaft during operation. At the outset of calibration the EEPROM 44 is empty so that the strain and temperature representative signals are passed to the computer without correction.

The temperature of the shaft is then increased to a new value and another calibration through the torque range is carried out with signal values again being acquired by the computer. This is repeated at a number of temperatures throughout the temperature range. The computer processes the data to build a look-up table of true torque levels applied to the shaft as measured by the load cells 62, 63 and measured torque levels as derived from the strain representative voltage signals output by the bridge circuit 20 at a number of temperatures over the temperature range as measured by the temperature sensor 81 and by acquiring the voltage between resistors 34 and 35.

When calibration of the torque measurement system has been completed through the temperature range the contents of the look-up table built in the computer are input to the EEPROM 44 over an I2C data input line 87 and a clock signal is input over an I2C clock line 88. The lines 87, 88 are connected to input pins 89, 90, respectively, and after input of the data the lines are disconnected and the pins are sealed.

In operation of the shaft, when it is transmitting power in a power transmission system, an electrical power signal is transmitted to the torque measurement system by any one of a number of known techniques, such as a rotary transformer. The wheatstone bridge circuit is energised and voltage signals are output by the bridge due to out of balance caused by torsional shear strain in the shaft. The analogue switch 40 switches the ASIC between acquiring bridge output voltage signals representative of strain and bridge temperature representative voltage signals as measured between resistors 34 and 35. The inputs are toggled by the switch to the amplifier 41 at a clock frequency and the gain of the amplifier is adjusted to suit the sensitivities of the bridge and the temperature sensing potential divider provided by resistors 34 and 35. Each time a pair of readings are taken the polarity of the amplifier and the A/D converter are reversed to cancel any errors caused by offset voltages.

The A/D converter provides two digital words, one describing bridge temperature and the other the strain to which the bridge is subjected. These words are passed to the algorithmic operator 43 and are processed by averaging logic which passes the most significant bits of the two words to the EEPROM 44 as addresses to enable stored correction data to be retrieved. Each individual correction data word is stored in the EEPROM as an eight bit byte and its inverse to allow data integrity checks to be carried out on the data obtained from the EEPROM. The data obtained from the EEPROM consists of four eight bit bytes and their inverse. These are checked and passed to interpolation logic in the operator 43, together with the least significant bits of the two digital words derived from the A/D converter. These latter are used with the data from the EEPROM to calculate the true corrected strain in the drive shaft. The corrected strain value is passed to the digital frequency synthesizer which generates a strain related frequency signal for transmitting the corrected strain to a display station where it may be displayed as a torque value. Also. the signal may be used to trigger audible and/or visual warning in the event that the drive shaft is overtorqued.

In a non-illustrated embodiment of a strain measurement system in accordance with the present invention six wheatstone bridges and their associated electrical circuits are provided.

Three of the bridges together with their associated electrical circuits are wired up so as to be operational in providing three torque related outputs to logic circuitry which votes on whether the torque transmitted by the shaft is within a permissible range or not. The other three bridges and their associated electrical circuits are provided as spares which may be hard wired into the system in the event of failure of one of the operational bridges. In this embodiment the bridges may be calibrated for clockwise and anti-clockwise torque, and calibration values stored in dual matrices in the EEPROM.

This provides a shaft having a strain sensing system which can be used to monitor torque in either direction of rotation of the shaft. This is particularly advantageous in an application where two counterrotating shafts are required and which would otherwise necessitate shafts having handed strain sensing systems to be provided and correctly installed.

The present invention substantially reduces the time and skill required for calibration of a strain measurement system, particularly in a case where a plurality of bridges and associated electrical circuits are required.

It will be appreciated, of course, that the present invention is not limited to the measurement of strain for the purpose of monitoring torque and that modifications to the described embodiments are possible.

For example, whilst in the case of a system measuring strain for the purpose of monitoring torque in a power drive shaft it is preferred to provide the signal processing circuit on the drive shaft with the wheatstone bridge, it could in this and other applications be provided at a receiving and display station. In the latter case means of installing within the receiving and display station calibration data relating to a particular strain measurement bridge could comprise data stored in non-volatile memory integrated circuits, or data stored in magnetic or optical discs.

The frequency modulated signal for transmitting data to display unit could take the form of a frequency switched key signal.

Claims (1)

1. A method of calibrating a strain measurement system comprising strain sensing elements bonded to a surface of a component in which strain is to be measured and connected in a wheatstone bridge, including the steps of: a. energising the bridge; b. cycling the component through a desired operating load range at a number of discrete temperatures in a desired operating temperature range; c. inputting to a computer strain related signals output by the bridge at a plurality of loads at each discrete temperature, signals representative of sensed temperature of the bridge at each discrete temperature, values of the true load applied to the component at each discrete temperature, and values of the true discrete temperature at each load; d. processing the data in the computer to build a look-up table of data for correcting strain related signals output by the bridge for temperature variation in operation of the system; and e. downloading the look-up table to an electrically-erasable programmable-read-only memory CEEPROM) provided with signal processing circuit means of the strain sensing system.

2. The method claimed in claim 1, wherein the bridge output signals at zero and at maximum operating load are input to the computer at each discrete temperature so that the strain values measured in operation can be corrected for zero offset, zero drift with temperature and span drift with temperature.

3. The method claimed in claim 1 or 2, wherein temperature signals are acquired from a potential divider co-mounted with the wheatstone bridge, the divider comprising one stable resistive element and one unstable resistive element connected in series, the unstable element providing temperature sensing.

4. The method claimed in claim 3, wherein the stable element of the potential divider is provided by the wheatstone bridge.

5. A system for measuring strain in a component comprising a wheatstone bridge of strain sensing elements bonded to a surface of the component, means for energising the bridge, means for monitoring the temperature of the bridge, an electrically-erasableprogrammable-read-only memory (EEPROM) for storing in the form of a look-up table values of bridge output and temperature signals output in calibration of the bridge over a range of strain and temperature, signal processing electrical circuit means for receiving signals output by the bridge and signals from the temperature monitoring means, the signal processing electrical circuit means including means for converting bridge output signals and temperature signals to digital signals, means for addressing locations in the EEPROM look-up table, means for processing data obtained from the look-up table to obtain a strain value corrected for temperature variation, and means for transmitting a signal representative of the corrected strain value.

6. A system as claimed in claim 5, wherein the signal processing electrical circuit means is mounted with the bridge circuit on the component.

7. A system as claimed in claim 5, wherein the signal processing electrical circuit means is disposed remote from the component.

8. A system as claimed in claim 5, 6 or 7, wherein the signal processing electrical circuit means comprise an application specific integrated circuit (ASIC).

9. A system as claimed in claim 8, wherein the ASIC includes and analogue switch for switching the ASIC between acquiring a bridge output signal and a bridge temperature signal.

10. A system as claimed in any one of claims 5 to 9, wherein the bridge output signal and the bridge temperature signal are of analogue format, and the system is provided with an analogue to digital converter whereby the signals of analogue format are converted to signals of digital format.

11. A system as claimed in claim 10, wherein the digital converter is incorporated in the ASIC.

12. A system as claimed in claim 11, wherein the ASIC comprises an algorithmic operator including averaging logic means which is fed by the analogue to digital converter to output EEPROM addresses for strain and temperature.

13. A system as claimed in claim 12, wherein the algorithmic operator further includes interpolation logic means which processes data acquired from the EEPROM to calculate a corrected strain value.

14. A system as claimed in any one of claims 8 to 13, wherein digital frequency synthesizer means are incorporated in the ASIC, and operable to produce a frequency signal whereby the corrected strain value is transmitted to a display station.

15. A system as claimed in claim 14, wherein the corrected strain value is further processed, either before or after transmission, to provide a strain related parameter such as stress, load or torque.

17. A power transmission system comprising a drive shaft operable to transmit power, provided with a strain measurement system as acclaimed in any one of claims 5 to 16, and operable to monitor torque applied to said drive shaft.

18. A method of calibrating a strain measurement system, substantially as hereinbefore described, with reference to the accompiyi ng drawings.

19. A system for measuring strain in a component, substantially as hereinbefore described, with reference to the accompanying drawings.

20. A power transmission system, substantially as hereinbefore described, with reference to the accompanying drawings.