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Unformatted text preview: R/C SERVO TESTER This pocket sized servo signal emulator makes it a breeze to test and setup your servos. It features digital accuracy and is easy to build and use. Introduction I recently found myself having to test a large number of R/C servo controlled devices. My normal routine is to grab my spare receiver, plug in the servo and battery, then fire up the transmitter. As simple as this sounds, it is inconvenient for me, especially when the "spare" Rx is installed in a plane. When the transmitter nearly fell off the workbench one day, I decided enough was enough. I thought about buying a R/C servo signal emulator, but I am a do-it-yourself sort of guy. The various projects I saw on the internet did not turn me on. What I found were circuits that were usually based on LM555 timers. They all seemed to use too many components for too few features. My goal was to use a cheap PIC microcontroller and do everything I could to minimize the component count. The PIC would ensure precise R/C signal generation and I figured that features could be easily added in firmware rather than by more components. In the end I met all my expectations. And although I used junk box parts, the cost to duplicate my work is well under $10. The R/C Servo Tester (RCST) is easy to use. Just plug in a 4-cell battery and your servo. The variable pot allows you to set any servo position you wish, within the 1.0mS to 2.0mS range of a modern R/C system. With the press of a button, you can find the precise center to your servo. Flick a switch and the servo will cycle (run back and forth ) at your chosen speed. Servo Control Basics The servo signal is a simple digital pulse. It spends most of its time at a logic low (0 V). About every 20mS it goes logic high (3 to 6 VDC) and then quickly goes low again. It is this tiny window of logic high time, called the pulse width, that gets the attention of the servo. Please refer to the drawing. The period labeled "A" is called the frame rate. In the example it is repeated every 20mS (50 times per second), which is quite typical for most radio systems. Modern servos define center as a 1.5mS pulse width, as shown by detail "B" in the drawing. Full servo rotation to one side would require that this pulse width be reduced to 1.0mS. Full rotation to the other side would require the pulse width to increase to 2.0mS. Any pulse width value between 1.0mS and 2.0mS creates a proportional servo wheel position within the two extremes. The frame rate does not need to change and is usually kept constant. The servo will not move to its final destination with just one pulse. The servo amp designers had brilliantly considered that multiple pulses should be used to complete the journey. This little trick reduces servo motor current draw and it helps minimize erratic behavior when an occasional corrupt signal is received. To move the servo, you must repeat the pulse every few milliseconds, at the chosen frame rate. Modern R/C systems use a 40Hz - 60Hz frame rate, but the exact timing every few milliseconds, at the chosen frame rate....
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This document was uploaded on 02/08/2012.
- Winter '09