Velocity Control
Some input devices only provide on/off control signals. With these, it is not possible to control the speed; it is only possible to activate/deactivate
Figure 3.6 Menu structure for controlling a 6-DOF robot and gripper with a joystick with two control signals in Cartesian space.
Figure 3.7 Example of an assistive keypad with five single switches.
the movement in a given direction. This can also be addressed using a menu structure. Consider, for example, the problem of controlling a 6-DOF ARM with a gripper in Cartesian space using a small keypad with five single switches (Figure 3.7). The menu structure in Figure 3.8 provides a solution to the problem.
Position Feedback
To implement the control methods described, it is necessary for the ARM electronics to know the joint angles exactly. One way is to measure the angle of each joint with respect to a reference, such as by counting the number of the joint motor rotations. However, with this method, it is necessary to have a way of measuring the initial position of the robot. For
Figure 3.8 Menu structure to control a 6-DOF robot and gripper with five single switches.
industrial robots, it is common that they perform a calibration sequence when the robot is turned on. Each joint is moved slowly in a certain direction until it hits an end switch. At that moment, the position is known for that joint. This is typically a process that takes some time. Adopting such a method for assistive robotic arms would therefore not be practical.
Another solution is to perform a one-time factory calibration and then put a brake on all joints such that the joints cannot be rotated when the robot is turned off; thus, the previous joint angle measurement will still correspond to the joint position after turning it on. One can also measure the absolute angle of the joint using a potentiometer mounted to each joint or other absolute encoders.
