Control Systems
Open-loop vs closed-loop control
An open-loop controller sets its output and hopes: it never checks the result. A closed-loop controller measures the result, compares it with the target, and corrects its output. That measuring and correcting is feedback. A toaster on a timer is open loop; a thermostat is closed loop.
Open loop: set and hope
A drone that needs 9.8 units of thrust to hover is given a fixed 12 and left alone. It climbs, and keeps climbing, because nothing ever looks at its height. Give it exactly 9.8 and it would hover, until a gust, a heavier battery or a warm day changed what hover needs. Open loop only works as well as your prediction.
Closed loop: measure and correct
A closed loop has four parts, and the words are worth knowing:
- The setpoint is the target: 10 m, 21 °C, 1200 rpm.
- The sensor measures what's really happening.
- The error is the difference between them:
- The controller turns the error into an output, the push it gives the plant, the thing being controlled.
The output changes the measurement, which changes the error, which changes the output: a loop. Because it acts on what really happened, it corrects for things nobody predicted, a gust or an open window, without knowing what they were.
Live experiment
Set and hope, or look and correct
Set and hope fixes the drone's thrust at 12 and never looks: it climbs past 10 m and keeps going. On-off looks every step, thrust 14 below the target and 5.6 above, and bobs around 10 m.
“Open loop, closed loop” is lesson 1 of 6 in Feedback Falls, Control Systems World 1. Free with an account. It’s open as soon as you sign up.
The simplest closed loop: on-off
Below the target, full push; above it, low push. It's closed loop, since it looks every step, but it never settles. The drone has momentum, so it passes the target before the low thrust slows it, drops back and passes it the other way. It bobs, roughly right, never exactly. A thermostat does the same with temperature, usually with a gap between its on and off points so it doesn't click constantly.
Smoother than on-off
Instead of all or nothing, push in proportion to the error: a big push far away, a gentle one close in. That's the P of a PID controller. Add a term against speed to stop overshoot, and one that adds up the error to remove what's left, and you have the controller inside cruise control, drone autopilots, 3D printer heaters and industrial ovens. Most real systems also add a little open loop on top, feedforward, for the part of the job they can predict.
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.
“Open loop, closed loop” is lesson 1 of 6 in Feedback Falls, Control Systems World 1. Free with an account. It’s open as soon as you sign up.
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: Open loop: nothing measures the result. Nothing looks at where the drone ends up, so nothing can correct it. That's open loop. Closed loop would read the height and adjust.
- Why not “Closed loop: thrust is the loop”? A closed loop measures the result and adjusts. A thrust that never changes isn't being adjusted by anything.
- Why not “Feedback”? Feedback means the result is fed back to set the next push. Nothing is fed back here.
- Why not “A thermostat”? A thermostat measures and switches, which is closed loop. A fixed thrust never measures.
Show the answer
Answer: Open loop: it never checks the room. It sets the heat and hopes. A thermostat, which measures the room and switches the heater, is closed loop.
- Why not “Damped”? Damping pushes against how fast something is changing, which also needs a measurement. A timer has none.
- Why not “Proportional”? Proportional control adjusts the output in proportion to a measured error. The timer measures nothing.
- Why not “Closed loop”? A closed loop would measure the room and switch the heater; a timer never looks at the temperature.
Show the answer
Answer: Feedback. The result is fed back to decide the next push. That's what every controller in this world does.
- Why not “Open loop”? Open loop is the opposite: it never measures its own result.
- Why not “A timer”? A timer runs for a set time and measures nothing.
- Why not “Feedforward”? Feedforward acts on something known in advance, without measuring the result.
“Open loop, closed loop” is lesson 1 of 6 in Feedback Falls, Control Systems World 1. Free with an account. It’s open as soon as you sign up.
Questions people ask
What is the difference between open-loop and closed-loop control?
An open-loop controller sets its output and never checks the result, like a toaster on a timer. A closed-loop controller measures the result, compares it with the target and corrects the output, like a thermostat. Closed loop is also called feedback control.
What are examples of open-loop systems?
A toaster or microwave on a timer, a washing machine's fixed programme, a garden sprinkler on a clock, a stepper motor told to step 200 times. Each does what it was told without measuring whether it worked.
Why use open-loop control at all?
It's simpler and cheaper, needs no sensor, and can't become unstable. When the system is predictable and small errors don't matter, open loop is often enough, and it's commonly added to a feedback loop as feedforward.
Related topics
- PID control explainedWhat the P, I and D terms each do, shown on a drone climbing to 10 m.
- Bang-bang (on-off) control vs PIDFully on or fully off: the thermostat's controller, its band, and its limits.
- What is feedforward control?Put in what you already know the system needs, and let feedback fix the rest.
- What is hysteresis, and how does it stop flicker?On below one line, off above another: the gap that stops a noisy reading flickering.