Torque Sensors
Sensing of torque and force is useful in many applications, including the following:
1. In robotic tactile and manufacturing applications—such as gripping, surface gauging, and material forms— where exerting an adequate load on an object is the primary purpose of the task.
2. In the control of fine motions (e.g., fine manipulation and micromanipulation) and in assembly tasks, where a small motion error can cause large damaging forces or performance degradation.
3. In control systems that are not fast enough when motion feedback alone is employed, where force feedback and feedforward force control can be used to improve accuracy and bandwidth.
4. In process testing, monitoring, and diagnostic applications, where torque sensing can detect, predict, and identify abnormal operation, malfunction, component failure, or excessive wear (e.g., in monitoring machine tools such as milling machines and drills).
5. In the measurement of power transmitted through a rotating device, where power is given by the product of torque and angular velocity in the same direction.
6. In controlling complex nonlinear mechanical systems, where measurement of force and acceleration can be used to estimate unknown nonlinear terms and an appropriate nonlinear feedback can linearize or simplify the system (nonlinear feedback control).
In most applications, sensing is done by detecting an effect of torque or the cause of torque. As well, there are methods for measuring torque directly. Common methods of torque sensing include the following:
1. Measuring strain in a sensing member between the drive element and the driven load, using a strain gage
1. bridge.
2. Measuring displacement in a sensing member (as in the first method)—either directly, using a displacement sensor, or indirectly, by measuring a variable, such as magnetic inductance or capacitance, that varies with displacement.
3. Measuring reaction in support structure or housing (by measuring a force) and the associated lever arm length.
4. In electric motors, measuring the field or armature current that produces motor torque; in hydraulic or pneumatic actuators, measuring actuator pressure.
5. Measuring torque directly, using piezoelectric sensors, for example.
6. Employing a servo method—balancing the unknown torque with a feedback torque generated by an active device (say, a servomotor) whose torque characteristics are precisely known.
7. Measuring the angular acceleration caused by the unknown torque in a known inertia element.
The remainder of this section will be devoted to a discussion of torque measurement using some of these methods. Note that force sensing may be accomplished by essentially the same techniques.
