The “Servo Motor” is an assembly with a small electric motor, gears to rotate a shaft left and right, and an electronic circuit which moves the shaft to the precise angle it’s told to by an electronic signal. All that is built into a small plastic box. These are at the heart of the Radio-Control (RC) hobby; They are used to steer or fly all RC models.
Servos are available large and small but most share a common design: The rotating shaft accepts a lever arm which can be attached to extend in any direction. Holes in the arm provide easy attachment points for push rods or wires that move back and forth moving whatever the linkage is connected to. Why not connect the linkage to a turnout?
The popularity of RC models has brought the cost of the smaller versions of these motors down to becoming quite competitive with older more common model railroad switch machines.
In order to use any servo motor the electronic controlling signal needs to be generated and adjusted. This makes each servo move just the way it’s needed. Controlling and adjusting the servo actions easily was the goal when developing our Servo Control Base. This base uses a simple four button Setup Remote and the lights of your own control panel to let you know what’s going on. And that makes adjusting each servo operation a snap.
Most servo motors share a common design with two mounting flanges at either end of the rectangular body. In a model airplane several servo motors might be set side by side between parallel mounting-rails built into the frame. When used in model railroading there are no standardized mounting techniques but a few are common:
- Direct links on the surface of the layout are easy to install and repair. We offer Servo Well Mounts for this purpose, but improvised mounts cut into a scenic surface or hidden in a scale structure work too.
- Under-table mounts are more hidden. We have developed a Servo Undertable Mount that uses the side to side lever wire of several popular switch machines. Several options along this line are being manufactured now.
- The push-pull nature of servo operation invites some of the “push rod and bell crank” techniques developed 70 or 80 years ago to manually operate turnouts. These techniques are a matter for another discussion, but a bit of exploration on the subject will show how much room there is to creativity.
As you can see, servo motors offer lots of possibilities with flexible mounting and setup. The same devices can be used differently in tight spots or hard to get to locations. Standard mounts can serve in most places, but custom mounts can solve problems in others.
Out of the Box Ideas
Servos can do anything that needs a little motion. Lowering a water tower spout, or opening a coaling gate. Tipping a log car, or turning a man’s head. Animation that used to be complicated becomes so simple when adjustments can be made later, literally with the push of a button.
Small servos can do a lot, but remember the servo control signal will control servos of any size. Lift a bridge, ring a bell, raise a flag. Think of that little thing you’ve always wanted to motorize.
Servos Move Around When First Powered
All servos can have unexpected movements on startup. These motions can be sudden and quite forceful. This “thrashing” about only lasts a moment, but often moves far outside the carefully adjusted range of travel. This is a characteristic of the servos, not the Control Base (the servo will do this with any controller). Whenever hooking up a servo and linkage be sure to consider what will happen when the servo moves far to the left or right in this moment at startup.
When planning your linkages to add in some springiness, that will accommodate these brief out of range excursions without damaging the track, servo, or mountings.
For the Well Mount servos, a simple Z or V shaped bend in a direct drive wire can be all you need.
Important Update: Users with our original under table mounts: Contrary to the original directions mount your servo with the shaft closer to the mounting base (and the table top) Attach the arm on the servo shaft with the arm pointing down, away from the mounting base (not pointing up toward the track). Install the wire as before with the Z bend through the arm, and the length of the wire running through the center hole in the base. Clip the upper end of the wire so it’s even or just above the top of the throw bar when the servo is centered, with the servo arm pointing straight down.
This updated arrangement lets the arm move safely at startup. When out of range the wire will extend safely above the track. There is a possibility with the arm pointing upwards that a far left or right motion will pull the operator wire out of the hole through the throw bar. This situation is at best a nuisance, and at worst could damage the turnout!
If you need new or additional operator wires to make these changes please let me know.
Power for Servo Bases
Because servo motors can gobble lots of electricity to just hold their position the Servo Base is not designed to “share” power with it’s neighbors, as with most of our other bases. This way the eight servos of a single base have access to up to two amps of 5v electricity that is available from a wall plug power supply. The DC input plug is mounted in the center of the Servo Base to allow multiple bases to be positioned more closely together in tight quarters while maintaining full access to the input and output pins.
One larger power supply can be used to power multiple Servo Bases as is detailed in another discussion All About Power.
Once you’ve mounted your servo motors they’ll need to be adjusted for best operation: All About Adjusting Servos.