Working on a recent Pi project, I needed to use some servo’s. This blog post discusses a servo controller project that can be used for toy and model cars, planes, model robots and so on. There are existing controllers out there, but I didn’t have one, and besides some of them seemed expensive – I didn’t want to spend $15+ on something I could make for $2. Also this was an opportunity to try to design something hopefully better (in some ways) than existing off-the-shelf controllers, and also explore how to create a custom hardware peripheral for attaching to the Pi. It isn’t hard, anyone can do it : )
The aim was to build something that could connect between the Pi (or any other single board computer or micro-controller, such as the Beaglebone, Arduino, and so on), and the servos, as shown in red here. It is explained in more detail below, but in brief, it would allow the Pi, or any micro-controller of choice to communicate using a standard I2C interface, and the servo controller would generate the correct pulse width modulation (PWM) signals to control multiple servos, which would create the motion for the desired task.
But first, it is good to understand the Servo, to know how to use it and its strengths and weaknesses.
When motors are running, it is hard to know how much they have turned, and when to shut off a motor precisely, to do accurate things like robotics. It would also be nice to adjust the voltage so that the motor can accelerate and get stuff done, but slow down as the robot approaches a human! Servo systems are different from standalone AC or DC motors in that they have feedback. The feedback can be used to precisely control the motion (position and speed). In industrial scenarios, servo motors are composed of an AC or DC motor (specially built to have very smooth rotation) and a rotary encoder (a device that generates pulses on a wire as a shaft is turned), and circuitry known as a servo amplifier (an amplifier just generates a voltage based on an input signal). This is connected up to the industrial machine controller or robot brains (using a digital interface) and position and speed commands are sent digitally to the servo amplifier. Much like any amplifier, it will generate a voltage, which drives the motor in either direction depending on the polarity of the voltage. The feedback from the encoder is used as an ‘error signal’; as it approaches the desired settings, then the servo amplifier output voltage is reduced, therefore allowing the motor to slow down and stop as it approaches the correct position. The machine cannot be knocked out of position, because the encoder will generate pulses that produce an error signal into the servo amplifier, which will generate a correction voltage to bring the machine back to where it was specified to be according to the original message sent from the machine controller.
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