A reaction torque transducer for mounting a motor of an instrument drive unit includes a motor plate a mounting plate and a flex ring. The motor plate is fixed to the motor and the mounting plate is fixed to a fixed structure. The flex ring is positioned between the motor plate and the mounting plate. The flex ring includes a body having a first end and a second end that are moveable relative to one another. The first end of the body is fixed to the motor plate and the second end of the body is fixed to the mounting plate. The body is configured to flex in response to the first and second ends moving with respect to one another. A strain gauge is positioned on the body of the flex ring and is configured to measure flexation of the body.
