CONTACT PRESSURE MEASUREMENT

Abstract

The knowledge of the correct clamping force in a bolted joint is important in the prevention of leakage and in extending the life span of the joint. One of the methods to tighten the joints to the correct clamping force would be to use torque wrenches but this and other techniques do not indicate the actual gasket pressure in the interfaces of the joint. An extensive literature survey was carried out to identify suitable sensing material that could be configured into a miniature sensor to be inserted between the two interfaces of bolted joints for the purpose of contact pressure measurement. The ternary semiconductor material A1xGa1-xAs has been known to exhibit piezoresistive behaviour. An investigation was carried out to find out the behaviour of the material and the possibility of configuring it into a contact pressure sensor for the measurement of contact pressure at the interfaces of bolted joints. Some of the results from the investigation were submitted to a journal called "Sensors and Actuators". A paper, entitled "A1xGa1-xAs Semiconductor Sensor for Contact Pressure Measurement" (by Siew-Lok Toh, Cho-Jui Tay, Mustafizur Rahman and Sum-Huan Ng), was accepted by the journal; but due to plans to patent the findings, it was withheld from publication till further notice. Miniature pressure sensors based on thin films of A1xGa1-xAs were designed and fabricated using cleanroom techniques for testing. A series of tests was conducted on the sensor to study its behaviour under contact pressure, hydrostatic pressure, light and temperature changes. Creep tests were also conducted on the sensors to study its stability under constant contact pressure for long periods of time. The sensors were found to display highly linear dependence on the both contact and hydrostatic pressure. An increase in pressure will lead to an increase in the resistivity of the A1xGa1-xAs material. There is no noticeable hysterisis but in terms of repeatability the sensors responded better to hydrostatic pressure. The pressure limit which the sensor fractures is also higher for hydrostatic pressure. In fact, the sensors show no sign of deterioration when brought to over 40 MPa of hydrostatic pressure. These are due to the more uniform pressure distribution over the sensor in the hydrostatic environment. The sensors were cooled to a minimum temperature of 6.5°C and heated to a maximum temperature of 200°C. The relationship between its resistance and the temperature is quadratic with an increase in temperature resulting in a drop in resistance. The sensors were also found to be sensitive to electromagnetic radiation like light and other stronger forms of radiation; but an opaque coating is sufficient to cut down drastically the effect of light on the sensor. The effect of radiation falling on the sensor material would be a lowering of the sensor's resistance. Even under constant contact pressure for over 740 hours, the sensors do not show any drift in their resistance. In addition to the A1xGa1-xAs sensor, an investigation was also carried out on electrically conducting polymers. Some of these sensors called Force Sensing Resistors (FSRs) were tested. The resistance of the sensors were found to drop with an threshold in the applied pressure when the resistance of the sensor suddenly drops to a much lower value resulting in a "switch like" response. The saturation pressure of the FSR is low- about 3 MPa. Under the application of a constant contact pressure, the resistance of the sensor tends to drift to lower values. Due to all these inherent characteristics, the FSR is more suitable for use as contact switches rather than for pressure or force measurement. From the results of the tests, the A1xGa1-xAs sensor displayed good potential in being the sensor for contact pressure measurement in bolted joints. In addition, the sensor can also be used widely for applications in measuring hydrostatic pressure due to its small size. However, before it can be put into use, further developments would be required to increase the contact pressure limit of the sensor, and compensate for temperature effects. One way to increase the contact pressure limit might be to remove the GaAs substrate completely which is causing the problem. The A1xGa1-xAs film would then sit on another hard and strong backing. This will also decrease the thickness of the sensor. It is to be noted that the present 1 mm x 1 mm x 0.6mm size of the smallest sensor can still be lowered further should the need arises. The temperature effect problem can be solved by pairing the A1xGa1-xAs sensor with another temperature sensor and subsequently compensate for the temperature effect, or changing the wafer structure of the A1xGa1-xAs sensor to reduce or eliminate the sensitivity of the sensor to temperature changes.