Position control of shape memory alloy actuators (cont'd...)
Resistance temperature detector
The resistance thermometers also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature. They are having higher accuracy, Low drift ,Wide operating range and repeatability. The relationship between an RTD resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD resistance can be calculated, and the temperature can be determined. RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms , increases. RTD elements are commonly specied according to their resistance in ohms at zero degrees Celsius. In our setup RTD is used to monitor the temperature of the SMA so that excessive heating could be avoided.
On-Off control is the simplest form of feedback control. An on-off controller simply drives the manipulated variable from fully closed to fully open depending on the position of the controlled variable relative to the setpoint. Simple on-off feedback control systems are cheap and effective.
On-off controller chattering problem
In practical on-off control systems are designed to include hysteresis: there is a deadband, a region around the setpoint value in which no control action occurs. The width of deadband may be adjustable or programmable. When the measured value lies within this dead-band the controller does nothing. If the measured value crosses the upper limit of the band, the switch is off, on the other switch turns on when the process variable goes beyond lower set point. The interesting point is that if the designer does not provide the band, the system would chatter - repeatedly switch on and off at very high frequency. It will reduce the performance and life span of the on-off controller. To avoid chattering, practical on-off controllers usually have a dead band around the set point.