Let's Make Robots!

Continuous rotating/normal servo - all in one

Rotates continuous or moves to desired position

This is a short tutorial how you get a servo for continuous running AND normal servo mode for a few bucks. You can switch between the modes in setting an additional pin high or low.

1) Open the servo and desolder the PCB. Remember the connections for motor and potentiometer

2) Remove the mechanical stop on the gear

3) Remove the potentiometer and modify it according to picture below. At least Hitec servos have this kind of potentiometer. Assemble the potentiometer and put it back on its original place

4) Prepare the servo as following

5) Make a small additional perfboard according to this circuit diagram and connect everything accordingly

I am using a TAKAMISAWA A5W-K PCB relay. They are very tiny, just like a 10 pin DIL chip and they are DIL socket compatible:

This is how my first prototype looked like:

6) You are done. Using a Sharp IR or ping on your servo, your robot can now not only look to the front, left and right, it can also look to the back!

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Unpowered or powered without a control signal? I have seen some servos that react differently to the latter condition (stay where they are, center or twitch) and my analog servos offer little resistance to being moved when unpowered. When they are still on a plane, I believe they are powered and receiving a steady control value. 

Here is a quote from the reviews:

I bought this servo and just received it. Compared to other 0.08s servo nothing much different. but for price this servo wins a lot. Quite fast and the rotation can do 360 degree when it off. The only bad points is this servo does not come with heat sink. after 10min of use this servo become hot. But overall, Great servo for money.

I believe "off" means unpowered (or unplugged) and rotated by hand. As we all know from Anton's magnetic servo sensor (http://letsmakerobots.com/node/18615) it has no stop, so perhaps the manufacturers did not include a hard stop for the gears (usually used to protect the potentiometer from breaking). Also, if one looks at the "exploded" picture from the HobbyKing site, the PCB of the servo looks different than for a regular servo. So I was thinking "what if..."

A regular servo has a built in oscilator, who's period is adjusted by the potentiometer. This period is fed to a comparator that compares it to the control signal. The difference is fed to the motor so it spins one way or the other until the signals match. Now think about this for a little. If the range of the built in generator is fixed (say from 1ms up to 2ms), feeding an out of range signal would make the motor spin continuously, as there will always be a difference. It's as simple as that. So theoretically, it should work, but it all depends how the board was designed.


Edit: While going to work I was still thinking about this and figured that as the servo horn rotates over 360 degrees, the sensor will adjust the internal oscilator period and I figured the motor will start to turn the other way, but I was wrong. A greater control singnal period will never be matched by the internal oscilator, so the motor will still be spinning. But the speed might be affected if the motor's speed is proportional with the difference between the 2 signals.


So here are the things that need to be addressed:

- clamping of the internal oscilator period so it works only in the 1ms-2ms range;

- adding an extra comparator(s) that looks at the period of the control signal and if it is in the 1-2 ms range it compares it further with the internal oscilator period, or it compares it further with a fixed value, say 1.5 ms period and feeds the difference as a proportional speed to the motor.

The last part is still blurry in my mind on how can be implemented with comparators, a microcontroller can simplify things a lot, but it will result in something similar with Anton's method. I'll keep on thinking, perhaps some of you guys can chip in some ideas...