Robotics

All hobby servos have the standard mounting holes, but there are times when the standard mounting holes do not fit the way you want to mount them. It seems, especially when it comes to robotic applications. This document proposes an alternate method of mounting servos. This document deals with mounting one servo to the shaft of another, although the method can be adopted for many other mounting requirements.

In a nutshell we will (1) dismantle the servo, (2) drill a hole to place a screw from the inside of the servo, (3) drill a hole to allow the screwdriver access to the screw's head, (3) mount the servo to the desired mount while dismantled, and finally (4) reassemble the servo while it is mounted.

Getting Started

The servos used in this case were Hitec HS-300 a standard and very popular servo. Servos may vary in design but I have used this method of mounting on other servos such as Airtronics and Apollo 5. The process of mounting each servo will take no more than ten or fifteen minutes when you become comfortable with the process. You will need a small Phillips screwdriver, Pliers, and a drill. Optionally, you may use super-glue for an extra strength bond.

Dismantling the Servo

Fig 1. The servo is apart with new mounting holes drilled.

The first step is to take the servo apart. We must remove the circuit board and potentiometer from the servo casing. Note this does not involve cutting or otherwise disconnecting any wires. Remove the four screws at the back of the servo and the back plate. Inside you will see a small circuit board and the back of the motor. Pull the circuit board out of the casing to reveal the potentiometer.

Now we must remove the front casing to the servo and all of the cogs. Pull on the front of the servo where the standard mounting holes are, the front casing should easily pull off. You should now see about 4 cogs, remove all cogs necessary to allow you to unscrew the potentiometer from the servo casing. Once the potentiometer is removed you are ready to drill the mounting hole through the servo casing.

Drilling the Mounting Holes

In this example I have drilled a hole through both sides of servo near the center of the servo casing (fig. 1). One of the holes is where the screw will be placed with the head of the screw inside the casing. The other hole is to allow the screwdriver access to the screws head inside the servo. The most ideal location for the mounting holes is in between the potentiometer and the motor. There is not much clearance anywhere else in the servo once it is put back together. You can add an extra set of holes for another mounting screw if desired. Choose the best size screw and mounting location for your project, but in any case, make sure the head of the screw will not interfere with the motor, potentiometer or circuit board.

Fig 2. The servo is dismantled. The other servos radial lever is glued to the side of the dismantled servo.

Mounting the Servo

We are almost complete, it is time to mount the servo. The servo should still be dismantled. In this example I super-glued the other servo's radial lever to the side of my dismantled servo. The hole of the radial lever and my mounting hole are lined up. The super-glue is optional but I find that for a single screw mount it prevents the servo from twisting, while the screw prevents the servo from coming off. If you used two screws this will not be necessary. I can now mount the dismantled servo onto its paired servo, reinforced with the screw from inside the dismantled servo. We are now ready to reassemble the dismantled servo while it is mounted.

Fig 3. The servo is partially reassembled with only the front and back plate of the servo missing.

Reassembling the Servo

Insert the potentiometer into the servo casing. It is important that we place the potentiometer and its cog properly. Any misalignment here will cause your servo to malfunction. There is a small metal tab on one side of the potentiometer; this tab lines up with a small slot inside the servo casing. When placed properly the base of the potentiometer should not be able to rotate in the servo casing. Use the pliers to screw the potentiometer's nut back on. Attach the cogs as follows: lower middle, top motor, potentiometer and top middle. To finish, replace the circuit board into the casing, attach the front and rear plate and four screws. You have now finished mounting the servo.

If you do mind the screwdriver access hole you can cover the hole with small rubber inserts. Be sure to test the servo to make sure that it has been reassembled properly.

Help! My Servo's Malfunctioning! (Troubleshooting)

If your servo no longer works it is surely a problem in the reassembling process. For any other problems you can contact me at the email address below.

• Servo does not function at all.

  I will assume that there is power and a proper signal going to the servo and that another servo works in its place. Thus it is definitely a problem with the servo. The most obvious answer here is that one of the wires inside the servo is being short-circuited or has been cut. Open the servo and look for cut wires leading to the circuit board, motor and potentiometer. I most often find that in this case when replacing the back plate of the servo one of the wires was pushed up against the motor terminals, make sure that the back plate fits easily into place.

• Servo does not go to the right position. Servo Buzzes constantly.

  You have most likely placed the potentiometers cog incorrectly. The design of the cog is varies between various servos so it is difficult to describe. You can try re-placing the cog rotated 180�. You can also activate the servo while the front place is off and command the servo to the center position. The motor should be rotating very fast. Rotate the potentiometer shaft until the motor slows and then stops, the potentiometer is now at the proper center position, now replace the cog.