GB2039057A - Digital strain measurement technique - Google Patents

Abstract

An electrical circuit provides a digital representation of strain and uses electrical resistance strain gauges 2, 4 arranged in a Wheatstone bridge 1. The bridge unbalance output is integrated at 16 and compared with a reference at 19. When the reference is exceeded, flip-flop 23 changes state and closes analogue CMOS switches 10, 11 to connect resistors 8, 9 in parallel with bridge resistors 3, 5. The length of time the resistors are switched in parallel with the bridge is proportional to the magnitude of the applied strain and this time is determined by counting clock pulses. The result is displayed as a digital representation of the strain applied. <IMAGE>

Description

SPECIFICATION Digital strain measurement technique The present invention relates to apparatus for the measurement of a physical parameter of a system.

The invention has particular application in strain measuring apparatus, such apparatus using electrical resistance strain gauges which are arranged so as to form an electrical bridge circuit.

The electrical output of such an electrical bridge circuit is a relatively small analogue voltage which is substantially proportional to the applied load and, if external currents are supplied to the output terminals such that the bridge is balanced, the amount of strain can also be assessed from the amount of external current required to achieve balance.

Measurement of either current or voltage requires an instrumentation amplifier and filter circuits prior to any analogue to digital conversion and additional complex circuitry to suppress power supply variation and enable operation of a remote sensing mode. These requirements may be considered disadvantages of the prior art since they increase the complexity and reduce the reliability of the electrical circuitry.

An object of the present invention is to obviate or mitigate the above said disadvantages.

According to the present invention there is provided apparatus for measurement of a physical parameter of a system, comprising transducing means for producing an analogue electrical signal representative of a change in the physical parameter being measured, said transducing means forming first elements of an electrical bridge network, and means for monitoring and balancing said bridge network by connecting second elements into said bridge network for a period of time, said period of time being representative of the magnitude of the system parameter being measured.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of one embodiment of the apparatus of the present invention; Fig. 2 shows part of the apparatus of Fig. 1 in a remote sensing mode; Fig. 3 shows waveforms which are produced by a part of the apparatus; Fig. 4 is a modification of a detail of the apparatus of Fig. 1; and Figs 5a, 5b show additional modifications to the apparatus of Fig. 1.

Referring now to Fig. 1, apparatus for measuring strain includes an electrical bridge 1 having four resistors 2, 3, 4, 5 of which 2 and 4 are strain gauges although all four resistors on the bridge may be strain gauges. Current is supplied to the bridge 1 via positive and negative bridge supply terminals 6, 7 respectively. External resistors 8, 9 are connected in series with complementary metal oxide semi-conductor (CMOS) analogue switches 10, 11 respectively; resistor 8 and switch 10 being connected between a first bridge output terminal 12 and the negative supply rail, and resistor 9 and switch 11 being connected between a second bridge output terminal 13 and the positive supply rail. When said analogue switches 10, 11 are closed external resistors 8, 9 are connected in parallel with bridge resistors 5, 3 respectively.External resistors 8, 9 have the same resistance value, which is related to the sensitivity and impedance of the bridge 1.

The output signal of the bridge 1 is taken between terminals 12, 13 and is fed to an integrator 1 6 via inverting and non-inverting inputs 14, 1 5 respectively. The output 1 7 of said integrator 1 6 is connected to the inverting input 1 8 of a comparator 1 9, the non-inverting input 20 of the comparator being connected to the supply rails by a potential divider such that reference signal level equal to half the supply terminal potential is fed to said input 20. The comparator output 21 is connected to a data input 22 of "D type" flip-flop 23 and a waveform generator (not shown) is connected to a clock input 26 and sends a fixed frequency clock pulse signal 25 thereto.The flip-flop 23 has a Q output 27 which is connected to each of analogue switches 10, 11.

In use, when nq strain is applied, the potential difference between the output terminals 1 2, 13 is small so that the integrator output 1 7 is small and thus the reference signal at the non-inverting input 20 of the comparator 1 9 is greater than the signal at the nverting input 18. In this case the comparator output applied to the data input 22 of flip-flop 23 is such that the a ouput 27 is a 'lo' data line signal and the analogue switches 10, 11 remain open.When strain is applied, strain gauge resistance values change whereby the bridge 1 is unbalanced and a potential difference appears between output terminals 1 2, 1 3. The integrator output 17 is large and is greater than the signal at the comparator reference input 20 which changes the value of comparator output 21 which in turn causes a 'hi' data-line output signal to be produced from the 5 output 27 which starts a counter (not shown). The counter counts the number of clock pulses received during the period when the data line output is 'hi'. A measure of strain in digital form is obtained by counting the number of clock pulses received when the data line output was high and comparing this value to the fixed number of pulses received in the same time.The 'hi' output signal also closes analogue switches 1 0, 11 thereby connecting said external resistors 8, 9 in parallel to bridge resistors 5, 3 respectively in order to balance the bridge. Said external resistors 8, 9 are retained in parallel with bridge resistors 5, 3 for a period of time which is a function of the magnitude of the ambient strain.

The operation of the apparatus according to the present invention under various conditions is described as follows: (a) at no load, the bridge 1 is balanced and thus the output from the bridge is very small, the data line signal from the Q output 27 is 'lo' and therefore no pulses are counted by the counter 31 and the corresponding strain recorded is zero; (b) at small loads the strain is small, however a large error is created at the output of the integrator 16 and is sensed by the comparator 19 which in turn permits a clock pulse to switch the flip-flop 23 to produce a 'hi' data line output signal at output 27. This 'hi' data line output signal closes the analogue switches 1 0, 11 and extemal resistors 8, 9 are connected in parallel with the bridge resistors 5, 3 respectively thereby balancing the bridge 1.The bridge output signal then drives the integrator 1 6 so that the integrator output 1 7 is reduced until the comparator 1 9 senses the need for the external resistors 8, 9 to be switched in parallel with the bridge resistors 5, 3 again. As the bridge ouput is small the external resistors 8,9 are only occasionally switched in for one complete clock period 32 as shown in Fig. 3.

Thus few pulses are counted and the ratio of counted pulses to the number of fixed frequency pulses is small; consequently, the output corresponding to the measured strain is small.

(c) at half load (which is equivalent to half full starin for linear strain gauges) the bridge output signal when external resistors 8, 9 are switched in parallel with bridge resistors 5, 3 is equal in magnitude but opposite in polarity to the bridge output signal when the external resistors 5, 3 respectively. The integrator output 17 therefore ramps up and down at equal rates hence the external resistors 8, 9 are switched in and out of the bridge circuit on alternate clock periods. Thus the counted pulses are half the number of fixed frequency pulses so that the ouput displays a value of strain equivalent to half the maximum of strain value.

(d) when the bridge 1 is near full load (or when the strain is near maximum), the external resistors 8, 9 are switched into the bridge circuit 1 almost continuously by data line pulse 33 as shown in Fig. 3 in order to maintain the bridge in balance, and theoretically the bridge output could be balanced at full load if the external resistors 8, 9 were permanently switched in parallel with resistors 5, 3 respectively.

In the described embodiment, without departing from the scope of the invention, linearity of the bridge output signal is improved by the following circuit modifications.

Additional analogue switches 29, 30 may be connected in series between said analogue switches 10, 11 as shown in Fig. 4. Switches 29, 30 are controlled by a signal from the complementary Q output 28 of the 'D' type flipflop, so that when switches 1 0, 11 are closed during measurement switches 29, 30 are open, and when switches 8, 9 are open circuit, i.e. when no measurement is being taken, switches 29, 30 are open, and when switches 8, 9 are open circuit, i.e. when no measurement is being taken, switches 29, 30 are closed. Thus external resistors 8,9 are connected so that capacitive charge, which is created when analogue switches 1 0, 11 are switched off, is cancelled, thus any distortion of the bridge Output signal due to this effect is eliminated.The same effect can also be achieved if only one of switches 29, 30 is used instead of both in series.

A differential propagation time delay of the 'D' type flip-flop which is present when switching such analogue switches 10, 11, 29 and 30 on and off, may be used to provide a control over the linearity of the bridge output signal. The shape of the clock pulse is affected by various propagation delays in such 'D' type flip-flops. The Q output 27 of the 'D' type flip-flop 23 has a greater propagation delay when tuming on to tuming off, the result is a positive non-linearity wherein half scale readings in excess of the true value are produced.

A propagation delay, in the form of an electrical circuit comprising a diode 39 and a variable resistor 40 connected in parallel as shown in Figs.

5a and 5b is connected between the 5 output 27 and analogue switches 10, 11. Thus the time for the 'D' type flip-flop to switch off the analogue switches 10, 11 is increased to the same value as the time to switch on thus the non-linearity effect is corrected.

In another modification to the described embodiment the effect of temperature drift on the integrator 1 6 input parameters, in particular the offset voltage, can be reduced incorporating the following circuit modifications: reversing the inputs 14, 1 5 to the integrator 16, which can be carried out at regular intervals either during or at the end of measurement.The inputs 18, 20 to the comparator 19 must be reversed to compensate, alternatively the comparator output 21 may be reversed to compensate for reversal of the integrator inputs 1 4, 1 5 or; disconnecting the integrator inputs 1 4, 1 5 from the bridge 1 and connecting said inputs 14, 1 5 to a half supply voltage (not shown) during an 'autozero' period and applying the integrator output 1 7 to a sample and hold circuit (not shown). The offsets are then measured and applied as a correction to the integrator 1 6. This method is widely used in conventional systems (dual slope etc).

In a further modification of the embodiment the susceptibility to mains induced noise is reduced by driving the clock from a phase locked loop (not shown) driven from supply or a quartz crystal oscillator (not shown) running at a multiple of the mains frequency. A high level of mains induced noise can therefore be tolerated by arranging that the measurement period is always a multiple of one mains cycle, whereby a positively induced error during one half cycle is compensated by negatively induced error in the second half cycle.

The apparatus according to the present invention as shown in Fig. roan be operated in a remote sensing mode which is advantageous in overcoming the effects of resistance change, in the sensing wires is small compared with the current in the supply wires, thus although there may be a voltage change at the bridge due to wire resistance change with temperature, a similar change in the sensing wires will not significantly affect the readings.

Advantages of the described embodiment include; only a single clock input is required to drive the system, the output of the system is in the form of a data line output and the desired digital result is obtained by counting the number of lock pulses during which the data line is high over a fixed number of lock pulses, a single power supply is required, the effect of CMOS analogue switch non-linearity, and the effect of integrator temperature drift is compensated for, the susceptibility to power supply variation is minimal, the system can operate in a remote sensing mode without adding circuit complexity, and the resolution of the system is variable, chosen to suit the application, in particular depending on the sample period chosen as the fixed number of pulses, for example if the fixed number of pulses is one thousand, the resolution isto one part in one thousand.

In a further embodiment of the present invention, only one of external resistors 8, 9 is used to balance the bridge. The digital result is non-linearbutcan be linearisgd as herinbefore described on page 5 line 28 to page 6 line 9 or by energising the bridge from a separate floating power supply. However, when a separate floating power supply is used the result is no longer ratiometric; bridge power supply variations affect the reading.

Claims (13)

1. Apparatus for measurement of a physical parameter of a system, comprising transducing means for producing an analogue signal representative of a change in the physical parameter being measured, said transducing means forming a first element of an electrical bridge network, and means for monitoring and balancing said bridge network by connecting a second element into the bridge network for a period of time, said period of time being representative of the magnitude of the system parameter being measured.

2. Apparatus as claimed in claim 1 wherein said physical parameter is a strain.

3. Apparatus as claimed in either claim 1 or claim 2 wherein each first element is an electrical resistance strain gauge.

4. Apparatus as claimed in any preceding claim wherein said second element is an analogue switch.

5. Apparatus as claimed in claim 4 wherein said analogue switch is of the complementary metal oxide semiconductor type (CMOS).

6. Apparatus as claimed in claim 5 wherein said CMOS switch is connected in series with a fixed resistor, forming a series arrangement.

7. Apparatus as claimed in any preceding claim wherein the electrical bridge circuit is a full bridge circuit having four resistance arms, at least one of which is a strain gauge.

8. Apparatus as claimed in claim 7 wherein two arms are strain gauges.

9. Apparatus as claimed in claim 6 and claim 7 wherein each said series arrangement is electrically connectable in parallel with each said strain gauge when said CMOS switch is closed.

10. Apparatus as claimed in claim 9 wherein one said series arrangement is connected to the other series arrangement via an additional switching element.

11. Apparatus as claimed in claim 10 wherein said additional switching element is a complementary metal oxide semiconductor (CMOS) switch.

1 2. Apparatus as claimed in claim 11 wherein the electrical bridge circuit is a half bridge.

13. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.