Control Systems
What is cascade control?
Cascade control puts one feedback loop inside another. The outer loop watches the thing you care about, like an angle, and turns its error into a target for the inner loop, like a speed. The inner loop drives the motor to meet that target. Splitting the work lets each loop do one job well: the inner one fights disturbances fast, and the outer one only decides where to go.
One loop, two jobs
A satellite dish must turn to 90° and stay there in the wind. A single PD loop on the angle has to decide how fast to turn and fight the wind at the same time. It does neither well: it whips round at about 180° /s, and once the wind arrives it holds the dish 12° off for good.
Split the jobs
The outer loop turns the angle error into a speed order: 20° off orders 60° /s, and 60° off would order 180 if the clamp didn't hold it to 90. The inner loop holds that speed with feedforward, the order itself, since this motor turns about 1° /s per unit of drive, plus P and I on the speed error.
Live experiment
One loop, or a loop inside a loop
A dish aims at 90° while the wind pushes from 6 s. One PD loop whips round and the wind holds it 12° off; the cascade turns at a steady speed, dips 3° in the wind and comes back. Then take the cascade apart: no clamp, or no I term in the inner loop.
“A loop inside a loop” is lesson 3 of 6 in Balance Bluffs, Control Systems World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
The wind meets the inner loop first
A push changes the speed at once, long before the angle has moved much. So the inner loop's I term sees the wind straight away and pushes back within a fraction of a second. The dish dips about 3° and comes straight back. Take the inner I term away and the wind holds it 10° off instead.
A speed limit for free
Because the clamp sits on the order between the loops, the dish can never be told to turn faster than 90° /s, however far off it is. Far from the target it cruises; close in, the order shrinks and it eases onto 90°. Without the clamp it whips round at over 200° /s.
The inside must be faster
The inner loop has to deliver each order before the outer loop changes it: five to ten times faster is the usual rule. Here the inner loop settles in about a tenth of a second and the outer one in a second or two. Tune from the inside out.
Where you'll meet it
Industrial servo drives stack three loops: current inside speed inside position, each roughly ten times faster than the one around it. A drone flies on a cascade too: where to go becomes a tilt, the tilt becomes a spin rate, and the innermost loops hold each motor. Robot arms, camera gimbals and 3D printers are built the same way.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“A loop inside a loop” is lesson 3 of 6 in Balance Bluffs, Control Systems World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 90° /s: a speed limit for free. However far off it is, the inner loop is never asked for more than 90° /s. The gears thank you.
- Why not “0”? A clamp limits the order's size; it doesn't cancel it. The dish still turns, at the limit.
- Why not “60° /s”? 60° /s is what a 20° error would order. At 60° off, the order is cut to the 90° /s limit.
- Why not “180° /s”? 180° /s is what the outer loop asks for. The clamp exists to stop that order getting through.
Show the answer
Answer: Several times faster than the outer one. The inner loop must deliver each order before the outer loop changes it. Five to ten times faster is the usual rule.
- Why not “Turned off”? Turned off, there's no cascade: the speed order would reach the motor with nothing holding the speed.
- Why not “The same speed”? At the same speed the two loops interact and can fight. The rule of thumb is five to ten times faster.
- Why not “Slower than the outer one”? A slow inner loop is still delivering one order when the next arrives, so the outer loop never sees its orders carried out.
Show the answer
Answer: On speedTarget, between the two loops. The clamp limits the order the outer loop gives, so the dish can never be told to go faster than 90° /s.
- Why not “On the integral”? Limiting the integral helps against windup, but it doesn't stop the outer loop ordering 180° /s.
- Why not “On pwm”? Clamping pwm limits the motor's drive, not the speed it's ordered to reach.
- Why not “On the angle”? The angle is a measurement. Clamping it would only hide how far off the dish is.
“A loop inside a loop” is lesson 3 of 6 in Balance Bluffs, Control Systems World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Questions people ask
What is cascade control used for?
Any process with a fast inner variable and a slow outer one: position and speed in a motor, temperature and coolant flow in a chemical plant, tilt and spin rate in a drone. The inner loop catches disturbances before they reach the variable you care about.
How do you tune a cascade controller?
Inside out. Tune the inner loop on its own until it follows changes in its target quickly with little overshoot. Then close the outer loop, which sees the tuned inner loop as part of the plant, and tune it more gently.
How much faster should the inner loop be?
Five to ten times faster, by the usual rule of thumb. If it's slower than that, the outer loop's orders change before they've been delivered, and the two loops can fight and oscillate.
Related topics
- PID control explainedWhat the P, I and D terms each do, shown on a drone climbing to 10 m.
- How to hold a motor's speed with an encoder and PICount ticks for rpm, add the drive you expect, and let P and I fix the rest, hill or no hill.
- What is feedforward control?Put in what you already know the system needs, and let feedback fix the rest.
- Open-loop vs closed-loop controlSet and hope, or measure and correct: the idea behind every controller.