Control; it’s all around us.  We use it every time we move a finger, take a step and with every breath of our lungs.  Nature has plenty of automatic control systems, like the body’s temperature, heart rate or even the feeling that you need to eat.  Other control systems are learned like keeping your balance while walking or riding a bicycle. 

Open control

There are many types of control systems.  An open control system is one without feedback.  Think of it as telling your robot’s motors to run at 70% without actually checking if they’re turning that fast.  It’s blind control.  Your motors will receive the same amount of power no matter what.  What will happen if your robot encounters rough terrain or an incline?  The motors will slow down, despite you having set the speed to 70%.  This is because your control system doesn’t measure desired speed versus actual speed.

Closed control

A closed control loop is one where the actual result of the controlling action is measured and compared to a desired result.  The desired result is also known as the “Set Point”.  The difference between the Set Point and the actual result is known as the error. This error is fed into a system that calculates the motor speed to be sent to the motors based on this error.  Cruise control is a great example of this kind of system.  You set the Set Point to 80 km/h, and the control system will calculate the right amount of power the engine should generate.  On the normal flat road this will be a fairly constant amount.  However, when you hit a hill, your engine will encounter more resistance and the car will slow down to 70 km/h.  The new slower speed is compared to our Set Point, 80 km/h and an error of 10 km/h is calculated.  The control system calculates the new (higher) power the engine should generate and make it run faster.  The car’s new speed is checked against the desired speed and if there is still an error, the control system will perform additional corrections until the desired speed is reached once again.  This loop will repeat until you switch off the cruise control or you run out of petrol!

The tricky part is how to calculate by how much the engine’s output should be increased to match the desired speed.  There are various methods for doing this but the most commonly used one is the proportional-integral-derivative control (a.k.a PID-control).  In the next article I will discuss what PID-control is, how it works and how you can use it to follow a line.

– Xander