Optical torque sensors

The distinction between a force sensor and a torque sensor is not clear, because in all cases a mechanical movement resulting from a force is converted into optical, electrical, or magnetic signals. This conversion defines the characteristics of the sensor. All sensors are made for limited ranges. 

One of the latest developments in the area of steering torque sensors is an optical concept created by Bishop Innovation with Bosch. As in conventional hydraulic steering systems, a torsion bar is a part of the steering column. A disc with a barcode on its surface is attached to each end of the torsion bar. A light source within the sensor module illuminates the surface of the coded discs, partially reflecting the light through a lens onto an optoelectronic ASIC. 

The light intensity on the sensitive surface of the optical ASIC, an array of photodiodes, depends on the code of the discs. The intensity distribution over the photodiodes allows the optoelectronic ASIC to calculate the absolute angular position and the angular displacement of the steering shaft Fig. 7.12.6 shows the sensor concept. This concept allows measurement of the absolute angular position of the steering wheel and the torque applied to the steering system. 

FIGURE 35.3 The ARM Guide. The patient s arm is attached to a hand splint (S) that is attached to an orientable linear track. A dc servo motor (M) can assist in movement of the subject s arm in the reaching direction (R) along the linear track. Optical encoders record position in the reach (R), pitch (P), and yaw (Y) axes. A six-axis force sensor (F) records the forces and torques at the interface between the device and the subject. The device is statically counterbalanced with two counterbalance weights (C). 

Interpolators compute a series of intermediate points, in space and time, compatible with the maximum torque or force available at the joint motors. For dynamic control, a precise mechanical model of fhe robof is needed in order to compute the torques or forces to be applied to actuators in order to follow a given trajectory. Sensors (typically optical encoders) at the joints allow closing the loop of the controller. Position control loop, usually provided by the manufacturer in a rather optimized way, can be exploited as the kernel for the synthesis of more complicated control schemes. 

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