Intelligent actuators controlling their own stroke have contributed to robot system and factory automation. Usually, stroke is detected by using a sensor located on one end of the actuator and its construction is very complex. The space and the conditions for using the sensor are restricted because of its location. To solve this problem, a new kind of actuator was developed. This actuator's piston rod has magnetic scales made by irradiation with a laser beam. The scaling rod is robust and more accurate. However, irradiating with the laser incurs high cost because of the use of the expensive laser beam machine. Therefore, a new manufacturing process with plastic work is developed to improve productivity. In order to produce a one-body-type magnetic scale, we attempted martensitic transformation of steel by plastic deformation. Using a special austenite steel rod, a part of the surface was transformed into martensite by rotating a roll-die over the rod. The effect of crystal grain size on martensitic transformation was investigated. As a result, martensitic transformation was found to be more stable for finer grain. As magnetic scale became small through the grain refinement, the accuracy of stroke sensing became higher. We also discussed the possibility of fabricating a magnetic scale rod of a carbon steel.
