5.2: Proprioception of Robot Kinematics and Internal forces

Proprioception refers to the perception of internal states of a robot. This is different from exteroception, which describes sensing of anything outside of the robot. Proprioception includes awareness of the robot’s joint angles, its speeds, as well torques and forces.

The main internal sensor is therefore the encoder. Encoders can be used for sensing joint position and speed, as well as — when used together with a spring — a simple force sensor. There are both incremental and absolute encoders. The latter are mostly used in industrial applications, but are not common in mobile robotics. There are magnetical and optical encoders, which both rely on a magnetic or optical beacon turning together with the motor and being sensed by an appropriate sensor that counts every pass-through. The most common encoder in robotics is the optical quadrature encoder. It relies on a pattern rotating with the motor and an optical sensor that can register black/white transitions. Whereas those patterns can be precision manufactured, simple encoders can be made by simply laser-cutting a pattern such as shown in Figure 5.1.1 and reading it with a light sensor.

While a single sensor would be sufficient to detect rotation and its speed, it does not allow for determining the direction of motion. Quadrature encoders therefore have two sensors, A and B, that register an interleaving pattern with distance of a quarter phase. If A leads B, for example, the disk is rotating in a clockwise direction. If B leads A, then the disk is rotating in a counter-clockwise direction. It is also possible to create absolute encoders, an example of which is shown in Figure 5.1, right. This pattern encodes 3-bits, encoding 8 different segments on a disc. Notice that the pattern is arranged in such a way that there is only one bit changing from one segment to the other. This is known as “Gray code”. The function of linear encoders is analogous, both for incremental and absolute encoders.

If combined with a spring, such as in a series elastic actuator, rotary and linear encoders can be used as simple force/torque sensors using Hooke’s law (F = kx, force equals distance times spring constant). Whereas the series elastic actuator is the most illustrative examples, most load cells operate on the premise of measuring displacements within materials of known properties. Here, measuring changes in resistance or capacitance might be easier choices.

Other means of measuring the actual force at the end-effector or joint torques is measuring the actual current consumed at each joint. Knowing a mechanism’s pose allows to calculate the resulting forces and torques across the mechanism as well as the currents required for empty loading conditions. Derivations of these then correspond to additional forces that can hence be calculated.

Finally, there are other means of proprioception, such as simple sensors that can detect when a robot gets picked up, e.g.