GB2076543A

GB2076543A - Force Balance Transducer System

Info

Publication number: GB2076543A
Application number: GB8017121A
Authority: GB
Original assignee: Pitney Bowes Document Messaging Technologies Ltd
Current assignee: Pitney Bowes Document Messaging Technologies Ltd
Priority date: 1980-05-23
Filing date: 1980-05-23
Publication date: 1981-12-02
Also published as: GB2076543B

Abstract

A force balance transducer comprises a magnet 12 within the magnetic field of which is arranged a coil 20. The magnet and coil are relatively movable and means 22, 24 are provided for applying to the movable member (the coil as illustrated), in opposite directions, a reference force and an unknown force respectively. By applying currents to the coil to balance the applied forces and measuring the current in the presence of the reference alone and with the unknown force respectively, the magnitude of the unknown force can be calculated from the value of the reference force. The reference force may be applied by a weight 22 or a spring and the unknown force by a pressure fluid applied to a differential pressure transducer 24. <IMAGE>

Description

SPECIFICATION Improvements in Force Balance Transducers This invention relates to force balance transducers.

According to the invention there is provided a force balance transducer comprising a cylindrical magnet, a coil mounted co-axially with the magnet with an air gap therebetween so that the coil is influenced by the magnetic field of the magnet, one of the coil and magnet being a movable member with respect to the other along the axis of the magnet, means for applying a reference force in one axial direction to the movable member, means for applying an unknown force to be measured in the other axial direction to the movable member, means for applying a first reference current to the coil to cause the coil and magnet to assume a datum position with respect to each other in the presence of the reference force only, means for applying a second, balancing current to the coil to cause the coil and magnet to return to said datum position when the unknown force is applied and means for determining from the magnitudes of said first and second currents the magnitude of said unknown force.

The invention will now be described by way of example with reference to the accompanying drawing which shows one embodiment of a force balance transducer according to the invention arranged for the measurement of fluid pressures.

Referring to the drawing there is shown a force balance transducer 10 comprising a right circular cylindrical permanent magnet 12. Secured to one end of the magnet 12 is a magnetic cap member 14 having extending therefrom a rigid, elongate, magnetic support member 1 6 which is circular in crosssection and co-axial with the magnetic 12. Slideably mounted on the member 1 6 is a coil former 1 8 having a coil 20 wound thereon, such that the coil is in the magnetic field of the magnet 12. Secured to the upper part (in the drawing) of the coil former 18 is a reference mass 22 which biases the coil former 1 8 and coil 20 downwardly.

The ends of the coil 20 are coupled through terminals 20a and 20b to an electronic circuit for providing a current through the coil 20 to thereby maintain it in a datum position with respect to the magnet 12.

Thus only one coil is used and the current through it is first adjusted to balance the weight of the mass 22, coil 20 and former 1 8 to maintain the coil in a datum position with respect to the magnet 12 while compensating for any changes in field strength of the magnet. Having set the coil to its datum position and by adjustmnt of the current through the coil 20 and measured the magnitude of the current, the transducer can now be used to measure the magnitude of an unknown force applied to the underside of the coil former 1 8 in an axially upwardiy direction. This is achieved by measuring the new current required through the coil 20 to restore it to its datum position and then performing a simple mathematical calculation.

If a current is made to pass through the coil 20 which just balances the force exerted by the weight W of the mass 2 plus the weight of the coil assembly 18 and 20 then PR=B91R7rNd (1) Where P=forne due to gravity of the weight W in Newtons N=number of turns of the coil IR=current through the coil B9=flux density in the air gap d=average diameter of the coil If now a force Px is applied upwardly to the coil 20 (as shown in Figure 1) the new current equired to balance the resultant force will be PRPx=lxB97rNd (2) dividing equation 1 by 2

Therefore the unknown force is directly proportional to the reference weight W, the difference and the ratio of two currents 1R and Ix.

Of course the transducer could be implemented in other ways, for example, by replacing the reference weight with a reference force generated by a spring and if a spring is used then in order to retain a very low temperature coefficient the spring should preferably be of a low temperature coefficient material such as Ni-Span C Alloy 902.

An analogue to digital converter (ADC) may be used to measure the two currents 1R and l, and if the same resistor is used to convert the currents to a voltage input for the A to D Converter, then any change in the value of that resistance will be cancelled out as will the effects of the reference voltage of the A to D converter.For example: VR=I RR (4) Where R is the I to V converting resistance and Vx=IxR (5) Now Vx Dα (6) Vref Where: D=the digital output from an A-D converter V -the reference voltage of the converter Vthe measured voltage

No substitute in equation (3)

Hence the answer is obtained by solving a fairly simple equation, in a computer for example, and the effects of R and Vref of the A to D converter are all cancelled out.

One problem is that if W is not made nearly equal to the full-scale force required, then when PR < PX, it is obvious that the current through the coil 20 must be reversed. Hence a bipolar A to D Converter must be used. Also if PR is much smaller than Px max the A to D converter must change range otherwise the resolution when measuring the field strength of the magnet will not be high enough.

Range changing of the A to D converter will, of course, result in errors due to the range changing components.

One solution to this problem is to provide a tap on the coil, therefore instead of range changing the A to D converter a tap could be changed on the coil. Provided the full coil and the tapped part of the coil are subject to the same magnetic field density from the magnet 12 then the force developed will be directly proportional to the turns ratio. The turns ratio of a coil is, of course, stable and does not suffer from a temperature coefficient. To facilitate this a moving magnet assembly could be used with a fixed coil, this has the advantage of simplifying the lead outs from the coil 20 to the terminals 20a and 20b. The weight of the magnet could then be used as, or as part of, the reference weight.

Referring again to Figure 1, this shows an embodiment of the transducer arranged for pressure measurement. In this case the force balance transducer 10 is arranged so that it acts directly against the diaphragm 24 of a differential pressure transducer 26. Pressure to the transducer 10 is applied to the reverse side of the diaphragm 24 by way of a two way valve 28 having first and second inlet ports 28a and 28b to which a reference pressure and an unknown pressure may be coupled.

The main control of the circuit is by means of a three way shift register 30 which is driven by the pulse generator 32. With the shift register set with a logic "one" on output "1" a logic "one" is applied to an amplifier driver 34 via "or" gate 36 which operates the two way valve 28 to apply reference pressure from port 28a to the pressure transducer 24. Also in this state of the shift register 30 a logic "one" is applied to the amplifiers 38 and 40 which apply a voltage to the switching electrodes of gates 42 and 44 respectively which in turn sets both switches to their low resistance condition.

Switch 42 applies a voltage V, to the inverting input of an amplifier 45 by way of a resistor 46 of value R,.

The resultant output voltage from amplifier 45 drives a current through the coil 20 of the force balance transducer 10 in such a direction and magnitude to lift the force transferring member 48 clear of the pressure transducer diaphragm 24. In this state if the reference pressure Pr is atmospheric pressure then the electrical output of the transducer 10 will be representative of its zero drift. Because switch 44 is in its low resistance state this drift voltage is stored on a capacitor 51 of value C. The next clock pulse from clock 32 causes the shift register 30 to provide a logic "one" on output 2.This leaves the valve 28 in the same position but, via "or" gate 50 and amplifier 52, switch 54 is set to its low resistance condition and switches 42 and 44 are driven to their high resistance conditions therefore a voltage V2 is applied by way of a resistor 55 to the amplifier 45, which voltage is an off-set voltage of some small fraction of the full range output voltage of the pressure transducer 26. In this condition the feed back loop from the force balance transducer 10 via the pressure transducer 26 is closed. The current through the force balance transducer drive coil 20 will be, after the feed back loop has settled, sufficient to generate a force which just balances the force due to the reference weight (W) plus a small force from the diagraphm 24 of the pressure transducer 26.The current generated in this mode of operation is calibrating the force balance transducer 10 as shown in equation (1).

The current is converted to a voltage by a resistor 56 and is measured by an A to D converter 58 when the trailing edge of a pulse from a mono-stable circuit 60 commands the converter 58 to take a measurement which measurement is coupled to a store in a micro computer 62. A third clock pulse from clock 32 causes the shift register 30 to provide a logic "one" at output 3. The valve 28 is operated via an amplifier 64 to couple the unknown pressure Px to the pressure transducer 26. The switch conditions of switches 42, 54 and 44 remain unchanged from the previous shift register condition. The output from the amplifier 45 will now drive a current through the coil 20 which will generate a force in the force balance transducer 10 to just balance the difference of the force generated by the pressure on the diaphragm 24 and the force caused by the weight (W).The resultant voltage across resistor 56 is again measured by the A to D converter 58 and the output coupled to the computer 62. This current is equivalent to ix in equation 2. As can be seen by equation 3 and following through to equation (10) the resultant unknown pressure can be calculated from the two digital outputs. As stated earlier the output for the transducer 10 is not summed at the reference pressure output voltages from the transducer but at a small offset voltage. This is to make the feed back control easier but, of course, the small force applied to the force balance transducer 10 by the pressure sensing diaphragm 24 constitutes an error. If this small error force is constant then it can be allowed for, therefore the only real concern is the variation of this error with time and temperature.

As the zero drift is corrected then it is only the variation of span that is of concern. Therefore if the small offset voltage is set to correspond to a 1% of full range and if the time and temperature stability of span is better than 1% then the overall error due to this would be less than 0.01%. Also it is not necessary for the transducer 10 to work over the full range of the pressure being measured, provided the pressure transducer has sufficient overload capability. For example on a 20 psi (14,000 kg/sq meter) system if a 2 psi (1,400 kg/sq meter) transducer was used, in theory the error would be reduced to 0.001%.

The idea of making the force balance transducer 10 act directly onto a diaphragm of a transducer without linkages to give a mechanical advantage means that the coil and magnet assembly has to generate a relatively large force. While there is no reason why this concept could not be made to work with some form of mechanical linkage system to give a mechanical advantage it is simpler and probably more accurate if it can be made to work without linkages. Fortunately the recent advances made in rare earth magnets has made this concept more feasible.

Taking an example of a 20 psi (14,000 kg/sq meter) device and using a pressure transducer with an effective diaphragm diameter of 0.8 inches (2.03 cm) then the summing force at full scale will be P=20xnx 0.42 Ibs force .-. Pl O Ibs force (4.54 Kg) Taking a practical example of an assembly to generate this force.

Using SI units

Where Flux density in the air gap of the magnetic circuit Vg=Volume of the air gap Hm=Magnetising force of the magnet Bm=Flux density of the magnet Vm=Volume of the magnet o=4&num;x1 0-7 (constant BJHg) Also the force generated in a coil with current flowing through it in a magnetic field is as shown in equation I.

Where P=Bg Il Newtons (12) I=Current in amps l=length of wire in the magnetic field in meters Bg=FIux density in Webers/sq. meter P=Force in Newtons Taking an example where the average coil diameter is 2 cm and let the coil be wound with 800 turns then the length wire will be approximately 50 meters.Now the reasonable diameter for the wire carrying 100 mA would be 0.09 mm therefore the total volume of the wire would be 7rx0.0452x 1 0-6x50=0.3 1 8x 1 0-6m Assume the volume in the air gap is twice the volume require by the wire Vg=0.64x 1 O-6m3 Now from the equation 12 P Bg=-xWebers/sq. meter II Force required in this example is 10 Ibs force=50 Newtons 50 Bg=- =10 Webers/sq. meter 50x0.1 Now from equation 11 the volume of the magnet can be calculated. For a typical sammarium coblat magnet the By may factor in SI unit is: BHmax=135x1O

Vm=3.79x10~4m3=379 cm3 Therefore the dimensions of the ring magnet could be inner dia. 4 cm outer dia. 9 cm height 11 cm The above calculation does not take into account the effects of flux leakage and the reluctance of the soft iron pole pieces which would increase the volume of the magnet by some small amount.

While the invention has been described usiny a diaphragm of a pressure transducer as the means for detecting the datum position of the force balance coil and for applying the unknown force to member 48, any other suitable means could, of course, be used such as a linear variable differential transformer (LVDT) for determining the datum position.

Claims

Claims

1. A force balance transducer comprising a magnet providing a magnetic circuit with an annular gap, a coil mounted co-axially with said gap so that the coil is influenced by the magnetic field of the magnet, one of the coil and magnet being a movable member with respect to the other along the axis of the said gap, means for applying a reference force in one axial direction to the movable member, means for applying an unknown force to be measured in the other axial direction to the movable member, means for applying a first reference current to the coil to cause the coil and magnet to assume a datum position with respect to each other in the presence of the reference force only, means for applying a second, balancing current to the coil to cause the coil and magnet to return to said datum position when the unknown force is applied and means for determining from the magnitudes of said first and second currents the magnitude of said unknown force.

2. A transducer as claimed in Claim 1, in which the axis of said magnet is arranged to be vertical, in use, and said means for applying the reference force to the movable member comprises a reference mass for biasing said member downwards under gravity.

3. A transducer as claimed in Claim 1, in which the said means for applying the reference force comprises a biasing spring.

4. A transducer as claimed in any one of the Claims 1-3, in which the said means for applying the balancing currents to the coil comprises amplifying means to an input of which there is connected a transducing means for sensing the position of said coil, the said amplifying and transducing means being connected in a feed back loop such that the output of said amplifying means reaches a stable state when the coil is in said datum position.

5. A transducer as claimed in any one of Claims 1-4, wherein the said means for determining the magnitude of the unknown force include an analogue to digital converter to the input of which is connected a shunt resistor in series with the said coil, and a processor for calculating the said magnitude from the outputs of the analogue to digital converter in the presence of said first and second currents respectively.

6. A transducer as claimed in Claim 5, in which said analogue to digital converter is a bipolar analogue to digital converter.

7. A transducer as claimed in any one of Claims 1-6, in which the said coil is a tapped coil and means are provided for enabling said first and second currents to be applied via different tappings of the coil to enable the turns ratio of the coil to be selected in accordance with the ratio of the reference force and the unknown force.

8. A transducer as claimed in any one of Claims 1-7, in which the said coil is fixed and the said magnet comprises the movable member.

9. A transducer as claimed in Claim 8, as appended to Claim 2 in which the mass of the magnet forms at least part of said reference mass.

10. A transducer as claimed in Claim 4 or any one of Claims 5-9 as appended thereto, in which the said position sensing means comprises a differential pressure transducer incorporating a pressure sensing diaphragm one side of which is arranged to be coupled to said movable member and the other side of which is exposed to fluid within a fluid pressure chamber, the said means for applying an unknown force to said movable member comprising valve means for introducing a pressure fluid into said chamber.

1 A transducer as claimed in Claim 10, in which the arrangement of said feedback loop is such that the datum position of said coil is reached when the force generated by said coil is just insufficient to balance said reference force, whereby a residual force is applied to said diaphragm which is small in relation to the range of the differential pressure transducer.

1 2. A transducer as claimed in Claim 10 or 1 wherein said amplifying means is coupled to the output of said differential pressure transducer by means for storing an offset voltage to compensate for the zero drift in a voltage output of said differential pressure transducer.

1 3. A transducer as claimed in Claim 11 as appended to Claim 5, including a logic curcuit for operating the transducer in accordance with a control program including the following steps:- (a) coupling of the output of said differential pressure transducer to said storage means with said valve means venting the pressure chamber to atmosphere and the diaphragm decoupled from said movable member whereby the storage means is caused to receive said offset voltage, (b) closing of the feedback loop of said amplifying means with said diaphragm coupled to said movable member, whereby said first balancing current is generated with the said pressure chamber vented to atmosphere, (c) storage by said processor of the output of the analogue to digital converter as a first value.

(d) actuation of said valve means to introduce a pressure fluid into said chamber whereby said second balancing current is generated in the presence of said pressure fluid.

(e) storage by said processor of the output of the analogue to digital converter as a second value.

(f) computation of the said unknown force by said processor from said first and second values.

14. A force balance transducer substantially as described herein with reference to the accompanying drawings.