GB2310495A - Displacement sensor imposing zero force - Google Patents

Abstract

A sensing arrangement able to measure in an accurate and linear manner a very small positional change in the "X" direction, without applying any back forces to the system to be measured, comprises a pair of oppositely oriented linear Hall effect sensors 1,2 mounted on non-magnetic supports (3,4 Fig. 1) so as to face each other, and a magnet 8 attached to the movable part of the system so as to be displaced parallel to the faces of the two Hall sensors in the gap therebetween. The Hall sensors 1,2 are connected to a differential amplifier, providing the arrangement with a doubling of its output sensitivity at C, and with a method of cancelling out temperature drift of the sensors. The arrangement is stated to be insensitive to displacement of the magnet in the "Y" direction and of use in systems measuring movement, position, force, pressure, torque etc.

Description

This invention relates to a zero force measurement sensor.

When measuring small amounts of movement, position, force, pressure, torque, angular position etc. the sensors that are available on the market at present tend to apply a back force on the system that is to be measured. This back force will in itself produce a positional error, that will then add to any inaccuracy that may exist in the sensor itself.

Sensors that measure small changes in position, are * Relatively expensive.

* Have a low milli-volt to volt output signal.

* Require expensive instrumentation amplifiers before a useful signal can be obtained from them.

* Dedicated to the particular range it is required to measure, and therefore can be easily overloaded.

* Susceptible to small changes in temperature, and would normally require some form of temperature compensation circuit to be applied to the output signal before it can be related back to the true measurement.

According to the present invention there is provided two Ratiometric, Linear Hall Effect Sensors, (Type UGN3503U) and an operating rectangular bar magnet.

The two Hall Effect Sensors are mounted on non-magnetic material used as supporting frames. The sensors are separated by an air gap and the bar magnet is positioned in this gap, with air space left between the

of each sensor and the side faces of the magnets.

The magnet would be firmly attached to a non-magnetic material which would then be firmly coupled to the system under measurement. This arrangement of the new sensor provides no back forces to the system to be measured.

The Essential Technical Features are as follows 1. The new sensor system exerts no back force to the system to be measured, and therefor will not interfere with the true measured displacement.

2. The new sensor system has a linear output that can be related back to the applied force, etc.

3. The new sensor system, can measure both positive or negative forces applied to it, as the design feature allows accurate linear measurement in both directions.

4. The design of the new sensor, self compensates for changes in temperature/expansion. This will be described in a later passage.

5. The new sensors design would allow it to be constructed to measure any loads from micro-grams to tons of weight, by simply building the correct back-force load to the device.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which : Figure 1 shows in perspective the sensor system with the movement arm unattached to the system under measurement.

Figure 2 shows in elevation, front and side view.

Figure 3 shows connection details and principles of operation.

(This will form the main invention to be applied to the patent) Referring to the figure 1 drawing, the sensor comprises of Hall Effect sensors (1) and (2). They are mounted permanently into the two adjustable sensor frames (3) and (4). These frames a secured to the two bottom adjustable mounting brackets (5) and (6).

On the movement arm (7) is mounted a permanent rectangular magnet (8). (North pole to the rear and South pole to the front).

The movement arm (7) and hence the magnet (8) shall be made to move only in the direction of the arrows. i.e. parallel to the two sensor mounting brackets (3) and (4). in the "X" direction.

The movement arm (7) shall be suitably attached to the system to be measured.

To set up the new sensor system to measure very minute positional change in movement, is carried out as follows : a) The two Hall effect Sensors (1) and (2) would be specially chosen to match both there null voltage output (no magnetic field present) to the same value, i.e. 2.500 Volts and also equally matched temperature drift characteristics.

b) Hall effect sensor (1) is installed with its Socr+L\ active area facing towards the magnet (8), and the Hall effect sensor (2) is mounted so its

active area is facing towards the magnet (8).

c) The sensors are electrically cabled as in figure three.

d) With no magnetic field present, sensor (1) is measured for null, and the voltage noted.(say 2.500 volts) The sensor (1) via sensor frame (3) is then moved in towards the magnet (8) in the "Y" direction.

Note :- The sensors (1) and (2) and hence the frames (3) and (4) being positioned closer to the magnet (8) in the "Y" axis will increase the sensitivity of the sensors and moving them away will reduce the sensitivity. Each sensor can be independently have its sensitivity set, therefore taking into account any slight differences at manufacture of each sensor.

The null setting value that was noted above (2.500) is now used to position the active area of the sensor (1) to the null position of the magnet (8) by moving mounting bracket (5) in the "X" axis. i.e This will position the active area of the sensor to approximately half the magnets (8) length.

Note :- The mounting brackets (5) and (6) can be moved along the "X" axis independently of each other, allowing individually nulling of each sensor.

e) The set-up of the second sensor (2) is repeated as in (d) using frame (4) and mounting bracket (6).

REFER to figure 3 After the set-up procedures have been followed the following results should be obtained.

The voltage output from sensor (1) should now be 2.500 volts which connects to input A of a differential amplifier.

The voltage output from sensor (2) should also be 2.500 volts, which connects to input B of the differential amplifier.

The difference signal between the two sensors will appear on the output (C) of the differential amplifier and should now be 0.000 volts.

A very small movement of the mounting arm (7) in the forward direction of the "X" axis, will move the magnet (8) and the north pole end closer to the active area of the Soc\th facing sensor (1), which will result in an increase from the null voltage of 2.500 volts to say 2.520 volts. An increase of +20 milli-volts.

The same small movement will move the north pole of the magnet (8) closer to the active area of the not facing side of sensor (2), which will result in a decrease from the null voltage of 2.500 volts to say 2.480 volts. A decrease of 20 milli-volts.

The nett effect of this will produce a difference signal of 40 millivolts at the input of the differential amplifier. With a small gain setting on the amplifier of 50 the resulting output would 2.000 volts.

If the movement arm (7) is moved in the opposite direction with the same force then the inverse of the above will apply and the result at the output of the differential amplifier (C) will be -2.000 volts.

Changes in temperature and metal expansion is minimised due to cleaver design of this system.

Please refer to figure 3.

If the temperature rises on the sensors (1 and 2) then both output voltages from these two sensors will fail together, cancelling out the effects of thermal drift.

If the movement arm (7) and magnet (8) move in the Y direction due to thermal expansion, then one sensor will become closer to the magnet and thereby increasing its sensitivity while the other move further away from the magnet and thereby reducing its sensitivity. The resultant measurement will theri be the same as before the expansion took place.

These two effects will therefor ensure accuracy through self compensation.

This is the main design feature of this zero force measurement sensor.

SYSTEM sb:4t < O ssy RONALD AYRE SHAW.

Claims (6)

1. The Zero Force Measurement Sensor, comprises of two temperature matched linear hall effect sensors, positioned with one sensor with its active area facing forward, and the second hall effect sensor with its active area facing away from the first. Both sensors are mounted on a non-magnetic material, with a suitable air gap set between them. A magnet placed in the air gap, can then be moved into a position where the voltage output from both Linear hall effect sensors are equal. When the magnet is then moved in parallel with the two faces of the Linear hall effect sensors, the resultant difference voltage between the two signals, will be directly related to the amount of forward or reverse movement of the magnet and the signal will be Linear.The gain of each sensor can be independently adjusted by moving each of the sensor faces closer or further from the magnet, and with the sensors mounted in the above manner will double the sensitivity of the system.

2. The Zero Force Measurement Sensor, as claimed in claim 1 wherein means are provided to self compensate for changes in temperature, this is achieved by utilising the output from each sensor in a differential mode.. If the temperature rises or falls on one sensor then it will also rise or fall on the other, the resultant output voltage change with a matched pair of sensors will be the same and therefore the difference signal remain the same.

3. The Zero Force Measurement Sensor, as claimed in claim 1 or claim 2 wherein means are provided to self compensate for small changes in the magnet moving at right angles to the desired parallel movement. This is achieved again by using the two linear hall effect sensors in a differential mode. The effect of the magnet moving away from the face of one sensor, will move it the same amount towards the face of the other, the overall effect to the differential output signal will be a zero change, as it will increase the gain on one sensor but reduce the gain on the other by the same amount.

4. The Zero Force Measurement Sensor, as claimed in claim 1 or claim 2 or claim 3 wherein means are provided to suitably mount the magnet on the system to be measured, the measurement can take the form of movement, position, force, pressure, torque or angular position. When the magnet is suitably mounted, and the two sensors positioned correctly in the air gap, then the measurement system will not apply any back force to the system to be measured.

5. The Zero Force Measurement Sensor, as claimed in claim 1 or claim 2 or claim 3 wherein means are provided to apply a fixed amount of back force to the sensor systems magnet assembly, will allow the same zero force measurement sensor to measure any load from micrograms to hundreds of kilograms. The amount of back force applied say by the use of springs, will allow a working range to be fixed.

6. The Zero Force Measurement Sensor, as claimed in claim 1 or claim 2 or claim 3 wherein means are provided to ensure the system cannot be over-loaded, the system is very sensitive to small changes in parallel movement of the magnet and therefore if the magnet is moved > 1000 times the initial movement, it will not be restricted in any way by the two sensors as the air gap is open at both ends. This ensures that the system cannot be damaged through overload.