IntermediateTopic 64 of 101Signals, Control & Comms
Digital control
Controllers implemented in discrete time on microprocessors.
In depth
Digital controllers sample sensors, compute control law in software and output via DAC/PWM at period T. Delay, quantisation and aliasing affect design; sampling should be 10–30× closed-loop bandwidth. Design in discrete domain (z-plane) or discretise a continuous design.
Key points
- Zero-order hold
- Sample time selection
- Real-time constraints
ExampleA motor controller runs a current loop at 20 kHz and speed loop at 2 kHz.
What is Digital control?
Controllers implemented in discrete time on microprocessors.
Why is it important to understand digital control?
Digital control implements controllers on microprocessors, which is how almost all modern control is done.
What limits the performance of a digital controller?
The sampling rate, computation delay and quantisation, which add lag and noise that must be included in design.
Why is the sampling rate chosen carefully?
It should be fast enough to capture the dynamics, often ten to twenty times the closed-loop bandwidth, while not overloading the processor.
How is Digital control used in practice?
Typical applications include running a PID loop on a microcontroller at 1 kHz, implementing a state feedback controller in an embedded motor drive and filtering a sensor signal digitally before control.
What should I learn before Digital control?
Study "LQR" first (topic 63 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of Digital control?
Zero-order hold. Sample time selection. Real-time constraints.
Can you give an example of Digital control?
A motor controller runs a current loop at 20 kHz and speed loop at 2 kHz.
What comes after Digital control?
The next topic is "Discretisation methods", which builds directly on this one.
How difficult is Digital control?
It is classed as Intermediate in Signals, Control & Comms (topic 64 of 101). It is an intermediate topic. Work through the beginner topics in Signals, Control & Comms first.