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RC SERVO CONTROL VIA TPU

by Jeff Loeliger

Start ý The Answer Isý ý Designing Functions ý Demonstration ý Youýre in Control ý Sources and PDF

When dabbling in robotics, one of the hardest things encountered is obtaining the motors and geartrains. One solution is to use RC servosýthey are cheap, lightweight, hard wearing, come in many sizes, and have a standard interface. The drawback is that they are relatively hard to control.

There are a few options open for controlling servos. You can use the main micro (obviously takes up valuable CPU time), a separate control board (more expensive and cumbersome), or you can use a custom chip (again, more expensive).

This article covers implementing a fourth option that involves no extra cost and no loading to the CPU, and provides control of up to 16 servos.

PROBLEM DEFINITION

Servos have a three-wire interface: ground, power, and control. The input to the control line is a pulse-code modulated signal from which all servo timings and positions are derived. All servos have their own limits but convention states that applying a 1.5-ms signal holds the servo in neutral, a 1-ms signal turns it full counter-clockwise, and a 2-ms signal results in a full clockwise turn (see Figure 1). The signal must be repeated no less than every 30ý50 ms or the servo may start jittering and finally stop driving the output. The result would be the loss of active hold on the desired position. The use of timer hardware and software algorithms is therefore essential to allow the accurate control of timed events.

Figure 1ýThe control signal for a servo should be repeated every 30ý50 ms. By convention, 1.5 ms signal is centered.

As I mentioned earlier, there are three options currently available for servo timing control. The first method is the main microcontroller. When controlling the servo with software alone, itýs often hard to obtain an accurate output of the control signal. If the CPU is overloaded, the signal may be less accurate which causes the servo to jitter. This form of control also places extra loading on the CPU. Timer hardware can be used on the main microcontroller to eliminate jitter. However, the number of channels available is usually limited.

The second method, using an external board for servo control, adds to the cost of the system and takes up valuable space (a commodity not usually in abundance in robotic systems). Servo control with an external board requires a serial signal via a UART interface. The main micro-controller must therefore have a spare UART interface to talk to the servo controller. Coordination problems can also arise because the speed of the UART interface determines how fast the CPU can update the servo positions.

The third and final option is the custom chip. This form of control has the same UART issues as described in the external board solution. Although less of an issue on size, it can significantly add to the overall system cost.

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