Servo motors used in classroom projects are typically in the ‘hobby’ category. They are given hobby status because their original design intent was to operate levers attached to control surfaces on model airplanes. Hobby servos are inexpensive, costing under $20 each and are ideal for use in STEM classroom projects that involve programming devices to move. They can be used right out of the box for any purpose that requires a limited range of motion, such as a robot arm or sensor turret. If properly cared for, hobby servos can last a long time in a classroom environment
The other great benefit of using servo motors in that they can be driven directly with an arduino microcontroller without the need for a separate motor driver board, greatly lowering the overall expense of class room projects. However, most hobby servos are limited in their use for experiments that require them to rotate 360 degrees, such as for a drive motor on a robot. Instead, these motors rotate from anywhere between 90 and 270 degrees as shipped from the factory. The good news is that they are easily modified to rotate in a continuous fashion.
While it is possible to purchase pre-modified servo motors, the modification itself is a teachable moment not to be missed. In addition to the cost savings, students will develop a far more complete understanding of this device by doing the modification themselves.To that end, this tutorial will serve to help your students modify a specific type of servo motor that we like to use from Hobby King. This servo is one of our favorites because it is very inexpensive (well under four dollars) and of equal quality in our experience to those costing considerably more. While these specific instructions pertain to this specific servo, the principles involved are common among most brands and styles.
Digital Volt Ohm Meter
Tools and Materials
Flat Sided Cutters
Digital Volt Ohm Meter
Step One: Remove Back of Servo
The screws on the back of the servo tend to be somewhat soft, so the use of a quality screwdriver with a good fit is essential to avoid stripping them. Remove all four screws and set them aside along with the back plate.
With the backing plate removed, you will find the circuit board shown here. It is delicate and should not be touched at this point.
Step Two: Remove the top cover
Pull the top cover off to reveal the gear train. Have students practice removing and replacing the gears in the correct order. In addition, have them calculate the gear ratio to gain an appreciation for why servo motors have so much mechanical advantage over DC motors.
Step Three: Remove Main Gear Stop
Note the protrusion on the main gear below. This will be need to be removed without damaging the gear. Double check students are wearing safety glasses since these parts tend to fly off at high speed.
After removing the stopper from gear, it will probably need to be cleaned up a bit with sandpaper or a dremel for smooth movement.
Step Four: Remove Stop From Top Cover
To ensure smoothest movement, we remove any and all bits that work in conjunction with the main gear stop. This includes the two walls inside the top cover as shown below. Remove with pliers and file any rough edges.
Step Five: Remove the Potentiometer
Now that most of the physical barriers to turning 360 degrees are removed, its time to turn our attention to those contained in the case of the servo. Start by gently removing the circuit board along with the attached motor and control wires. You will not be separating these completely from the case, just exposing the potentiometer. Do not pull on the black wires.
Careful inspection will reveal the potentiometer in the case. There is a philips screw holding it in place. Remove the screw but do not pull on the wires. Instead, push on the potentiometer from above and pop it out.
We will need to get some data from the potentiometer for the next step, so clip the wires attaching it to the control board.
Step Six: Determine the values of the potentiometer
The digital volt-ohm meter will be used to determine the maximum and minimum values of the potentiometer. This part of the project should be used to introduce (or review) the reasons that variable resistors are used in different electronic devices. Make sure the DVOM leads are connected to the correct ports on the meter (generally COM and Volt/Ohm). Rotate the knob to the lowest resistance range. Connect the leads to the center pin and one of the outer pins on the potentiometer. If you get a value of ‘1’ or an error, move up the scale. Rotate knob to the opposite end of the scale. In this case, you can see our maximum value is 5260 ohms. We used the 20K ohm scale to obtain this value. The minimum resistance was 900 ohms.
Step Seven: Replace the pot with a resistor network
We are going to render the potentiometer non-functional, so some resistors will be needed to take its place. From experience on these servos, we have found that around 2000 ohms works well for small robot motors. Therefore we need to use two, 1K ohm resistors and solder them to the circuit board. Your situation and the type of servos you use may require different values. Again this represents an opportunity for experimentation. Provide the class with a variety of randomly selected resistors. Students must learn how to read resistor values from a color chart in order to come up with a combination that gets close to their desired number. Suggest that students make use of the many apps available that can help sort out color codes.
There are three solder pads on the circuit board used by the potentiometer. Each of these will be used by the resistor array you will use to replace it. This means that whether you use two or three resistors (this is determined by your desired value and what resistors you have available), you will need to end up with three ‘legs’ that can be soldered on to each of the pads. Since resistors are not polar, orientation does not matter. Below are two 1Kohm resistors twisted on one end and ready for soldering.
Next, solder to the board as shown. Be careful not to create a solder ‘bridge’ that connects the pads to each other. There is plenty of room inside the case for the resistors, but try to keep them as compact as possible. Put the board aside when finished.
Step Seven: Remove Stop from Potentiometer
The potentiometer does double duty in this application, keeping the main gear in place as well as its usual job of controlling the motor through resistance. We therefore need to retain the potentiometer but will only keep its outer shell. Take a very small flathead screwdriver, and separate the two halves of the potentiometer case as shown.
Find the stop and clip it with the pliers.
Reassemble the potentiometer and make sure that it spins smoothly. Place it back in the case. Remember the retention screw. Put back the motor and circuit board. Finally, replace the back of the case and the four screws.
Step Eight: Testing
Since programs are a potential source of error, all students should test the mechanics of the servo using the same program. We use a modified version of the ‘sweep’ program contained in the arduino library. The only change is the delay is increased from 15ms to 45 ms.
Here is the result. An inexpensive, smooth and robust platform for any motion-related project. Helping students hack their own servos is a project based lesson that provides a wealth of knowledge and experience to help form a foundation for many future STEM endeavors.
Continuous Rotation Servo