Open vs Closed Loop Control

Control theory is the study of how systems can be controlled to achieve a desired outcome.

In robotics, a controller determines how a robot should respond to achieve a goal.

For example:

Move a lift to a certain height.
Drive to a specific location.
Maintain a flywheel speed.
Follow an autonomous path.

The main difference between control systems is whether or not they use feedback.

Open-Loop Control

An open-loop control system sends commands without checking whether the desired result was actually achieved.

The controller assumes that the system behaves exactly as expected, with no way to compare the output with the desired result.

Example: Timed Motor Movement

Imagine you want a lift to move upward.

An open-loop solution might be:

motor.setPower(1.0);
sleep(500);
motor.setPower(0);

However, this assumes:

The battery voltage is the same.
The lift has the same load.
The motor always moves at the same speed.

However, real robots are not perfect. The lift may stop too high, too low, or at a different position each time because the controller never checks what actually happened.

Advantages of Open-Loop Control

Open-loop systems are useful because they are simple, easy to implement, and computationally inexpensive. They are often sufficient for simple tasks where precision is not important.

Examples:

Running an intake
Turning on a light
Setting a servo position

Closed-Loop Control

A closed-loop control system uses feedback to compare the current state of the system with the desired state.

The controller measures the error and adjusts its output to reduce that error.

The controller continuously asks: "Am I where I want to be?"

If the answer is no, it makes adjustments.

Example: Encoder-Based Lift

A closed-loop lift controller might have:

Target Position:

2000 encoder ticks

Current Position:

1500 encoder ticks

Error:

2000 - 1500 = 500 ticks

The controller uses this error to determine motor power.

As the lift gets closer to the target:

Error decreases.
Motor power decreases.
The mechanism slows down.

Eventually, the lift reaches the target position and stops.

Open Loop vs Closed Loop

Open Loop	Closed Loop
No feedback	Uses feedback
Simple	More complex
Less accurate	More accurate
Cannot correct errors	Corrects errors automatically
Good for simple actions	Good for precision control

Why Robotics Uses Feedback

Robots operate in the real world, where conditions constantly change.

Factors like battery voltage, friction, weight, mechanical wear, and field conditions make perfect prediction impossible.

Feedback allows robots to adapt to these changes. This is why almost every competitive FTC robot uses some form of closed-loop control.

The best website to learn advanced control theory is probably CTRL-ALT-FTC. I won't be explaining too much here, as this resource has done such a great job explaining this topic in depth.