Creating a digital servo

MAKING ONE GOOD SERVO OUT OF TWO NOT-SO-GOOD ONES

The TowerPro MG995 servo (see the MG995 review) is, from a precision and reliability perspective, one of the worst servos ever made.

The combination of badly mismatched electronics and mechanics has produced a servo with massive amounts of overshoot and oscillation around center - but all is not lost.

A while ago I found myself with two useless servos: a TowerPro MG995 and a Hitec HS425BB that had developed a fault in its electronics.

"I wonder..." I thought to myself. "Could I put the servo amplifier from the MG995 into the HS425 and make a half-decent digital "standard" servo out of these two useless units?

Well that's exactly what I did -- and two years later, that servo is still providing excellent service in the tail of a 40-sized 3D profile plane that gets a real hammering every weekend.

Now I'm very much aware that there are a lot of people who have bought MG995 servos and been so disgusted with their performance that they've simply tossed them in a box under the bench and forgotten about them. Well here's your chance to recover a little value from that purchase... now you can convert some of your cheap "standard" servos to digitals without spending another penny.

What You'll Need

In order to fit the guts of an MG995 into another standard-sized servo you're going to need the following:

• A soldering iron (25W or so)
• Some fine rosin-core solder
• Small screwdrivers
• Sidecutters for cutting/stripping wire
• A modest amount of dexterity and good eyesight

First thing to do is dismantle the MG995 an other servo (in this case an HES Super-Servo I bought from Radical RC a long time ago and which suffered from a dead servo-amp) by removing the four screws that hold the three sections of the cases together.

Pull the bottom section of the case free, this will expose the motor and the servo amplifier circuit-board.

Now remove the two screws that hold the small circular board to the feedback pots so that these can come away as shown in the picture below.

Note carefully the orientation of the wires that go to the motor. Most servos will have a red dot on one motor terminal or will have a red wire.

Now, on both servos, remove the small screw that holds the feedback potentiometer in place. A gentle pull will allow the feedback pot to come free and this will leave only the motor wires joining the amplifier to the servo.

Unsolder the motor wires from both servos and solder the wires from the MG995 amp to the motor in the other servo -- making sure you're soldering the positive wire to the positive motor terminal.

Now you can push the feedback pot back into the case -- note that it has a flat portion on the shaft and this must line up with the slot in the gear into which it fits.

The next step is to screw the pot back into place using the two screws that were removed earlier.

Now the amplifier from the MG995 can be positioned in the case as shown in this picture.

The bottom of the case can now be replaced.

Now, BEFORE you plug this new now-digital servo in and try it, remove the top portion of the case to expose the gears of both servos and count them.

If both servos have an even or odd number of gears then you can put the top back on but if one had an odd number and another has an even number you're going to have to go back and reverse the wires on the motor.

Why is that?

Well the odd or even number of gears determines which way the servo's output shaft turns in relation to the motor's direction of rotation. If the rotation is the wrong way then the servo will simply run to the end and stall because the amp will be driving the motor the wrong way to correct the output arm position.

If you're at all worried about this you can try testing the servo with one of the gears removed and make sure that it's going to turn the right way before you put the whole thing back together.

Once you're happy, screw it all back up and enjoy your new really cheap digital servo.

Note that you may still experience a little overshoot but since the motor and gears of most standard servos are a lot lighter than those in the MG995 mechanics, that overshoot will be far less severe and the servo will show much greater precision and accuracy.

And, with the cheapest digital servos (such as the Hitech HS5475) costing around US$35 each, the combination of the servo amp from a $10 MG995 servo and the mechanics of a low-cost $15 standard servo make for a much cheaper option. Plus, you'll have the satisfaction of having put it together your self.