High Torque Servos By James Goss      I thought it would be interesting to compare the radiation generated by a servo for low and high torque servos. I consider anything over 50 ounces of torque to be a high torque servo and when it gets up to 100 ounces and above we are beginning to really get serious. Servos as high as 200 and 300 inch ounces of torque are available. Everyone that has been in the hobby for a while understands about keeping the servos away from the antenna and receiver when possible. This is due to the radiation produced by the servos and being picked up by the receiver, amplified again and inducing a glitch in the servo.      The radiation I am referring to is a result of current flow in the servo and servo pigtails. When current flows in a conductor a magnetic field is generated. It starts at the center of the wire and expands outward into the area around the wire. How far out from the wire it travels depends on the amount of current in the conductor. When the servo is at neutral it is pulling minimum current and the magnetic field is at minimum also. When the servo is commanded to move, a larger current value will flow through the dc motor and its torque will move the control surface. At this time the radiated magnetic field expands and covers a larger area surrounding the servo. The higher the mechanical load is on the servo the more current it needs to generate the required torque. High torque servos do require more current for their operation, even at neutral. This higher current creates a larger radiation field around the servo and its wires.      A while back I decided to run an experiment and measure the radiation generated by a standard servo (44 oz) and a high torque servo (108 oz). I set my scope up and used a 220 mh coil as a pickup sensor. I wasn't trying to measure the noise field by any standard; instead I wanted to compare the two servos and the radiation generated in microvolts across the 220 mh coil.  With both servos at neutral it was easy to see that the high torque servo had more radiation and for a much greater distance. When a mechanical load was placed on the servos the distance really increased. For the standard torque servo a distance of three inches gave a reading of 50 microvolts. For the high torque servo it produced the same 50 microvolts, but at eight inches away from the servo. The mechanical load on the servo was my hand, not calibrated very well, but did place an unknown load on the servo. The actual load on the servo was not important for this experiment. It is easy to prove that high torque servos do have more radiation. I think this will hold true for all brands of servos.      You may wonder why it is important to know about radiation from servos so here is an example. In my quest to improve the Postage Stamp planes I decided to replace the standard servos for the elevons with some high torque, high speed, coreless servos. The results was amazing, the plane was all over the sky. Glitch after glitch, I only had half control of the plane, the other half of the control was coming from the servos. I flew it a few more times with the same results. Moving the antenna didn't solve the problem; the receiver was picking up the radiation. The plane flies fine with standard servos so this is just another case that shows high torque servos radiate more energy than low torque servos.      If you plan to use some high torque servos on an upcoming project be sure to keep this in mind and watch where you place your servos. A lot of sport planes today have their servos in the tail section so be careful about running the antenna within a few inches of high torque servos.