Sponsored By Tormach

STEM Lesson: Hacking Hobby Servos For Continuous Rotation

July 9th, 2013 by

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.

 

Learning Outcomes

Programming

Soldering

Resistor Values

Ohm’s Law

Digital Volt Ohm Meter

Potentiometer

Resistors

Tools and Materials

Servo

Flat Sided Cutters

Screwdriver

Digital Volt Ohm Meter

Soldering Iron

Solder

3rd Hand

Resistors

Safety Glasses

 

Procedure

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.

20130708_114601

With the backing plate removed, you will find the circuit board shown here. It is delicate and should not be touched at this point.

20130708_133417

 

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.

20130708_115557

 

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.

 

20130708_115721

After removing the stopper from gear, it will probably need to be cleaned up a bit with sandpaper or a dremel for smooth movement.

20130708_133706

 

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.

20130708_120125

 

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.

20130708_125721

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.

 

20130708_125901

20130708_130230

 

We will need to get some data from the potentiometer for the next step, so clip the wires attaching it to the control board.

20130708_131849

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.

20130715_110326

 

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.

20130715_095054

 

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.

20130715_101624

 

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.

20130708_132646

 

Find the stop and clip it with the pliers.

20130708_132905

20130708_132914

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.

20130708_133349

20130715_153406

 

 

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.

360 test

 

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 

 

 

 

 

 

One Trackback

  1. […] motors modified for continuous rotation. A detailed STEM lesson on servo modification can be found here. Servos are generally packaged with several servo ‘horns’ which are mounted to the […]

Post a Comment

You must be logged in to post a comment.