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This paper investigates and realizes the self-sensing capability of shape memory alloy (SMA) actuators. SMA exhibits large stroke, high energy density, and requires low driving voltage. To make SMA more applicable to small scale robotic manipulations, its motion control using accurate self-sensing is necessary. The presented technique builds a self-sensing model by measuring the SMA electrical resistance. Effects of pretension force on strain and force self-sensing are investigated. The model is polyfitted to replace sensor electronics for strain or force feedback. A pretensioner is specifically designed to provide sufficient pretension force without affecting the subject to be actuated. The advantages gained from using polyfitted self-sensing models are demonstrated through several step response control experiments. With the merits shown, we expect this technique can be utilized for SMA actuators in meso to micro scale applications.