-Vipul

=Positional Feedback=

A hobby servo works by sending pulses to the servo from a controller. There is a small potentiometer inside the servo that is attached to the servo arm. The electronics inside the servo compares the position of the potentiometer with the desired position from the pulses and moves the arm as required until they match. The servo does not give any feedback to the controller; therefore the controller has to assume that the servo is always at the desired position. Therefore, the servo has to be modified so that the potentiometer inside the servo provides us with the information about the location of the servo arm. We can ‘close the loop’ with this data and always know the location of the arm. The steps to modify the servo are as follows: 1) Firstly, remove the four screws on the back of the servo casing. The back of the casing should just slide off, exposing the circuit board, as shown in Figure 1

2) The board is only held in place by compression so it should lift free of the casing. Gently pry it up, taking care not to damage the board or the wires holding the board. The potentiometer should now be exposed. The wire in the middle of the potentiometer will be the wiper. This is the wire that we will want to connect to in the future. Shown in Figure 2. 3) Connect the red and black wires from the servo cable to a power supply or battery that will supply the voltage for the servo. Move the servo arm all the way to one side and measure the voltage from the wiper of the potentiometer (yellow wire in Figure 2) to ground (black wire of servo cable). Write down the voltage, move the servo arm all the way around to the opposite position, and record that voltage as well. These values will be used in a moment.

4) Follow the wire that is attached to the wiper on the potentiometer to where it connects to the circuit board. This is shown as the yellow wire in the upper right corner of the circuit board in Figure 3. 5) Gently heat the solder on the wire and remove it from the circuit board. Strip one piece of wire that will be used to make the feedback cable and solder it to the yellow wire. Re solder both wires back to where the wire was just removed. Take extreme care not to damage the board or apply too much heat. Place the board back into the casing and fold the new wire so that it exits next to the motor, as shown in Figure 4. 6) Locate a good ground by checking resistance from various points on the top of the board to the back wire on the servo cable. The anode of the surface mount diode, shown in Figure 5, makes a nice place to attach the ground wire. Strip the second wire of the feedback cable and solder it to the ground, as shown in Figure 5. 7) Move the feedback cable so that it is parallel to the servo cable and secure it with a very small amount of hot glue. This will protect the cables by giving them a small amount of strain relief. Place the cover back on the servo and tighten down all of the screws, as shown in Figure .7