one of my friends has gone through a similar problem with the usage of the rotary shaft to shaft torque sensor. He used strain gauge instead of the rotatory shaft. With strain gauges bonded to the shaft, the shaft becomes the transducer. Secondly, the shaft to be calibrated, a process that usually involves loading the shaft statically and tabulating the results. This is relatively easy to do in small systems, but as loads and shaft size increase, it becomes an onerous task. Selecting a location for the strain gauges, mounting them carefully and protecting them become problematic for users inexperienced in such techniques. Outside contractors are usually available through the torque sensor suppliers for most applications and locations.
Thank You.