A system was developed to maintain an approximate distance between an end-effector and a surface. The effector is part of a robotic arm system with two servo actuated joints in the arm, though only one is used to adjust position. A sonar distance sensor is mounted on the end-effector, which maintains a constant planar angle with respect to the ground through mechanical links. The code was designed to be simple, made for exposition rather than performance. A 10 mm (+/- 5 mm) hysteresis window and measurement averaging (5 point) was used to get rid of noisy measurements and unneeded actuation.

It performed better than I expected after some simple tweaks to angle adjustment values, delays, and hysteresis width, maintaining distances to surfaces and some objects as they moved. I expect to be able to use a similar algorithm to control end-effector height and distance to relatively large objects instead of having to use kinematics (reverse kinematics).

Many improvements could be made, including (but not limited to) the following:

• incorporate fundamentals of control theory to improve performance;
• use both the boom arm and turret arm servos to manipulate end-effector height;
• include the use of kinematic equations to better respond to the error signal, for example: to translate the distance error signal into an angular error signal;
• improve mechanical rigidity and actuator quality (strength, rigidity, accuracy); and
• improve distance sensing with more effort put into filtering measurements.

Code is here: https://github.com/tyblu/comp444-random/blob/master/robot_arm_assign2_extra/robot_arm_assign2_extra.ino

YouTube video link: https://goo.gl/hxeEBs 

Note: The study guide states an assignment (#2) requirement not covered by units 3 and 4: "Working examples will include employing actuators to interact with the environment as well as using sensors to complete an assigned task." The preceeding example fulfills this requirement.