Servo Modification

A word of caution:    What is stated here, and the modification described, applies to an ANALOG Servo Only!     I have not attempted this modification on a Digital Servo, it probably would not work and may even damage the servo!   I am also pretty certain the servo manufacturers would not like for you to do this to any of their servos.

Background:

I have been asked to update and elaborate on an article I wrote quite awhile ago that dealt with converting a normal servo into a Retract Servo.    I hope this article does that.

With that in mind then, and because of the possibilities this modification provides, I believe "Retract" is only one of the uses that might be filled by this conversion/modification.   Perhaps a better phrase to apply to one of these "modified" servos would be "Speciality" servo, because this modification increases the output rotation to almost any amount from the original 60 degrees all the way up to 180 degrees and beyond!    (It's even possible to make the servo act as a varible speed motor from zero to about 30 RPM that will run in one direction for as long as you wish, then stop, or change direction depending upon the transmitter stick position.   This would be useful for controlling the "tracks" on a Radio Controlled model Tank, or for controlling it's gun turret ...   but that's another article for another time.)   The most practical application of this modification is to achieve more output arm rotation than you can get from the "standard out of the box" servo, but maybe not the full 180 degree rotation of a Retract type of servo.

Before we get to the conversion proceedures, let's discuss servos in general:

The difference between a Retract Servo and a "Normal" Servo is, basically, the amount of rotation of the output shaft.   Agreed, there may be other differences, such as the physical size, or the output torque, or the configuration needed to fit into a wing, and etc., but still, the basic difference is the output arm travel.    Typically, a Retract Servo's output shaft will rotate a full 180 degrees when you "flip" the Retract Switch on your transmitter, whereas a "normal" servo's output shaft would rotate only about 60 degrees under the same circumstances.    You are, no doubt, aware that you can connect a Retract servo to a channel other than the "Retract" channel and that if you should choose to do that, the "Retract" Servo will operate in the same manner in which the "Normal" servo operates EXCEPT THAT THE TOTAL OUTPUT ROTATION WILL BE CLOSE TO 180 DEGREES (Almost 90 degrees on each side of "Neutral") INSTEAD OF THE 60 DEGREES OF A "NORMAL" SERVO!   So, as stated earlier, the basic difference between a Retract Servo and a Normal Servo is about 120 degrees more shaft rotation for a "full transmitter input command".

The point of this modification, however, is to increase a Normal Servo's output rotation to more than the "standard" 60 degrees (for full Transmitter stick deflection) for the purpose of greater control surface deflection!   If we could do that and use it on say ...   the rudder channel, the greater output arm travel for a given transmitter stick movement will result in a greater deflection of the rudder which is sometimes needed on aerobatic planes.   Or, if you want to incorporate flaps on your model and need a larger deflection angle from an ordinary servo than you can get by juggling the linkages and control horns, this modification could solve your problem.

In any case, Certain servos can be easily altered to provide just about any amount of output shaft rotation from the "normal" 60 degrees up to 180 degrees (almost 270 degrees, actually) for whatever application or use you wish to make of them.

Theory:

The reason this modification works is because of the design of the typical analog servo.    Most of the analog servos in use today are of similar design and type.

Inside the case of an analog servo, there are three major components:

• A small electric motor that turns a "gear train" which, ultimately, rotates the output shaft.
• An electronic circuit board to control the direction and amount of rotation of that motor based on the transmitter's stick deflection for that channel.
• A potentiometer coupled to the servo's output shaft which provides information to the electronic circuit board.

The potentiometer is rotated by the servo's output shaft and is part of a "timing circuit" that generates a "train" of pulses based on "where" the servo's output shaft is.   The Microprocessor (on the circuit board) compares these pulses to pulses being received from the transmitter so that the micro knows where the servo output shaft actually is in relation to where it is supposed to be!   The resulting comparison causes the DC Motor to either continue rotating, reverse it's direction, or stop, so that the servo output shaft will faithfully follow the steering movement at the transmitter.   So ...   if you move the aileron stick slightly to the left and hold it there - the servo output arm will rotate slightly to the left and hold it's position until you move the stick again.   This comparison of pulses is made at a rate of about 1000 times per second.    As you can see, at that sampling rate, the servo can rotate the output shaft extremely quickly and precisely.

The Modification:

From what we have already discussed, we can say that the components on the circuit board do two things:

1. They use the train of pulses from the receiver (originating in the transmitter) to provide the direction and amount of rotation information for the electric motor driving the output shaft ...   and,
2. Using the potentiometer as the control,   they generate a series of pulses to be compared with the ones coming from the receiver to determine when it is time to stop the electric motor.

What this modification will do is fool those circuits so they make the motor run a little longer (hence the greater output arm travel) before the circuitry tells the motor to stop!

All that is necessary to accomplish this task is to add a resistor (of the proper value based on the amount of travel desired) to each outer terminal of the potentiometer. This will change the timing circuits delaying the motor turnoff.

I mentioned earlier, "certain" servos can be easily altered, there are some that are difficult to alter and others that can not be altered in this way.   This modification requires that the servo be one which has the output shaft directly coupled to the shaft of a potentiometer, and that the wiring is actual wires and not printed circuit or "surface mounted" components.   Most of the typical servos sold as a part of a radio package in the last few years are of this type and may be easily modified.   But if, when you open the case on your servo, you do not see what I have depicted in the accompanying diagrams, STOP!   Do not continue!   The servo's that I have successfully modified are: The Futaba S-148, The Airtronics 94102 and The Tower Hobbies "TS" series.   The example for this article is the Futaba S-148.

So! Let's get on to the modification:

Step 1:

Carefully open the servo case by removing the screws on each corner of the bottom.   If you do not see what is depicted in Figure 1, below, showing the two wires from the motor to the circuit board.   (Orange and Brown in the Figure, but color doesn't matter) STOP!   Your servo might not be one that can easily be modified.   (NOTE:   Your servo may, or may not, have the resistor depicted in Figure 1 across the motor terminals.)

        Figure 1



View after removing the bottom of the servo case.

If your servo is similar to that shown. then gently lift the circuit board to gain access to the potentiometer.    This should be easily done because the wires should be long enough to permit this.   When the circuit board has been lifed away you should see the potentiometer (similar to than shown in Figure 2).

Step 2:

Locate the two wires from the circuit board to the two end (side) terminals of the potentiometer .   In the older Futaba S-148 servo, the color of these wires were Green and Red, but the color is not important.   It is important to note which color is connected to which terminal!   We don't want to reverse them!    Carefully un-solder these wires from the potentiometer terminals and bend them out of the way.   Now, cut one lead of each resistor you intend to use very short (about 1/8") and solder the cut lead of one resistor to one of the terminals on the potentiometer (where you removed the wire), then solder the cut lead of the other resistor in the same manner to the other end terminal of the potentiometer.

Step 3:

Keep the leads of the resistor as short as possible.   Remember, all of this has to go back together without touching something else. I have found that there is usually room if you bend the resistor leads into a "J" (See Inset) before soldering it to the potentiometer and then install the resistor vertically below the level of the potentiometer.   (See FIGURE 4.)   Now, solder the wires removed from the potentiometer terminals to the other lead of each resistor that you have added. Be sure to connect the proper color wire to each resistor based on your note above!

That's all there is to it!   Put it all back together using tape or "spaghetti" to insulate the leads where necessary.    Figure 4 shows the placement of the added resistors before re-assembling the unit.   Nothing can be above the level of the potentiometer and the leads of the resistors must not touch the circuit board when it is replaced.

Depending on what value you selected for the resistors (they must be matched) the servo output arm will now rotate a greater number of degrees for a specified transmitter stick movement than it did before.   The value of the resistors will vary depending on the value of the potentiometer inside of your servo.   The one I modified had a 2000 ohm potentiometer and I found that using a pair of 470 ohm resistors would produce about 180 degrees of rotation, and a pair of 150 ohm resistors would produce about 90 degrees of rotation.   You can obtain these resistors from Radio Shack for about 50 cents for a pack of two.   You only need 1/4 watt resistors as there is very little current here.

Best of Luck! ...... Ron Roberts