Self-Sensing Heating Method for Shape Memory Alloy

Abstract

When an electric current is used to heat the shape memory alloy (SMA), a strong one can shorten the phase transformation but may overheat the SMA or it's surroundings. Here a method which ensures no overheating and enables the heating process depend on either the heating current or the heating time is proposed. It bases on the detection of the ending of the phase transformation and can be used together with other control processes concerned with heating to prevent overheat. Noticed when martensitic phase begins to transfer to austenitic phase by heating, the variation of resistance increases; after reaches the maximum, it decreases and reaches the minimum again when it completely transfers to austenitic phase, the beginning and the ending of the phase transformation can be detected by monitoring the variation of the SMA's resistance. Based on this principle, a circuit with resistance feed back is proposed here. In the verifying experiment, the SMA is heated for a short period, then it's resistance is determined and the variation of the resistivity is calculated, repeatedly. When the variation of the resistivity increases from the minimum, it means the beginning of the phase transformation; while it falls down to the minimum again, it means the ending of the phase transformation, then the heating current is cut off. The experimental result shows a favorable agreement with the analytical solution. With this method, regardless of however the ambient temperature, the heat convection and heat radiation change, the complete phase transformation of the SMA can be ensured and the overheating can be prevented.