Motor Control and Mechanism Design

Lesson Goals

By the end of this lesson, students should be able to:

• Understand proportional control (P) and why integral (I) and derivative (D) are often unnecessary for typical FRC mechanisms.
• Understand the purpose of the static friction constant (kS) and gravity constant (kG).
• Understand the role of the velocity constant (kV) in mechanisms where velocity is important.
• Understand how a control loop can regulate both velocity and position.
• Understand the difference between voltage and current.
  • Voltage is related to motor speed.
  • Current is related to motor torque.
  • Both are important for motor performance.
• Understand the difference between stator current and supply current.
• Understand the importance of stator and supply current limits.
• Understand the mechanical, electrical, and programming considerations involved in designing an FRC mechanism.

1. Open-Loop vs. Closed-Loop Control

Before learning how to tune a motor, it is important to understand the difference between open-loop and closed-loop control.

Open-Loop Control

Open-loop control means that a motor is given a command without using sensor feedback to correct its behavior.

For example:

m_motor.setControl(new VoltageOut(6.0));

This tells the motor to apply 6 volts. However, the motor does not know whether it has reached a desired position or velocity.

Closed-Loop Control

Closed-loop control uses sensor feedback to compare the actual state of a mechanism with its desired state.

For example, imagine a flywheel that needs to spin at 50 rotations per second.

• Target velocity: 50 rps
• Actual velocity: 35 rps
• Error: 15 rps

The controller uses this error to determine how much additional output the motor needs.

Once the flywheel reaches 50 rps, the controller adjusts its output to maintain that velocity despite changes in battery voltage or external load.

Position vs. Velocity Control

A control loop can regulate different properties of a mechanism.

• Position control: Attempts to reach and maintain a desired position.
• Velocity control: Attempts to reach and maintain a desired velocity.

For example:

• An elevator generally uses position control.
• A flywheel generally uses velocity control.
• An intake roller may use open-loop control or velocity control, depending on its requirements.

The important distinction is that closed-loop control allows the motor controller to adjust its output based on feedback rather than simply applying a fixed output.

CTRE Reading

CTRE Phoenix 6: Closed-Loop Control 

2. Understanding PID

PID stands for:

• Proportional
• Integral
• Derivative

A PID controller calculates its response based on the error between the target and actual value.

Proportional (P)

Proportional control responds to the current error.

A larger error results in a larger correction.

For example, if an elevator is 10 rotations below its target, a proportional controller will apply more output than if it were only 1 rotation below its target.

Why do we usually use P?

• It is straightforward to tune.
• It responds directly to the error.
• It is often enough to get a mechanism to a target position.
• It can make a mechanism respond quickly.

Too much P can cause oscillation, overshoot, or instability.

Integral (I)

Integral control responds to accumulated error over time.

If a mechanism consistently falls short of its target, I can gradually increase the output until the remaining error is eliminated.

Why do we often avoid I in FRC mechanisms?

• It can cause overshoot.
• It can accumulate error while a mechanism is physically unable to move.
• It can make tuning more difficult.
• Many mechanisms can reach their target with P and feedforward alone.

However, I is not inherently bad. It can be useful when a mechanism has persistent error that P and feedforward cannot correct.

Derivative (D)

Derivative control responds to how quickly the error is changing.

It can act as damping, helping reduce overshoot.

Why do we often avoid D?

• Sensor noise can produce undesirable output.
• It can cause jitter.
• It adds another parameter to tune.
• Many FRC mechanisms can achieve acceptable performance without it.

D can still be useful when a mechanism has significant overshoot or oscillation that needs damping.

Takeaway: P is commonly the starting point. I and D are not universally unnecessary; they should be used when the mechanism benefits from them.

CTRE Reading

CTRE Phoenix 6: Manual PID Tuning 

3. Feedforward: kS, kG, and kV

PID reacts to error. Feedforward anticipates the output needed based on the desired motion and the mechanism’s physical characteristics.

This is particularly useful for FRC mechanisms.

kS — Static Friction Constant

kS is the output needed to overcome static friction and get a mechanism moving.

For example, a motor may need a small voltage before an elevator begins moving. Once the elevator is moving, that initial resistance is no longer the same.

• Too little kS: The mechanism struggles to start moving.
• Too much kS: The mechanism may jerk when starting.

kG — Gravity Constant

kG is the output needed to counteract gravity.

This is especially important for elevators and arms.

An elevator must continuously apply force to prevent itself from falling. An arm must continuously produce torque to hold its position against gravity.

kG provides that compensation.

• Elevator: Gravity compensation is generally constant.
• Arm: Gravity compensation changes with the arm’s angle.

Without kG, the PID controller must generate error before it applies the additional output needed to hold the mechanism.

kV — Velocity Constant

kV represents the output needed per unit of target velocity.

This is particularly important when velocity matters, such as:

• Flywheels
• Rollers
• Conveyors
• Drivetrains

For a flywheel, the motor needs a certain amount of voltage to maintain a given speed. kV estimates that required voltage based on the target speed.

The higher the target velocity, the higher the expected output.

CTRE Reading

CTRE Phoenix 6: Closed-Loop Control 

4. Voltage, Current, Speed, and Torque

This is one of the most important foundational concepts for mechanism design.

A useful simplified mental model is:

• Voltage (V): Related to motor speed.
• Current (A): Related to motor torque.

These are simplifications, but they are useful for understanding motor behavior.

Voltage

Voltage is the electrical potential applied to the motor.

Increasing voltage generally increases the motor’s achievable speed and its ability to accelerate, assuming the load and other conditions remain the same.

However, voltage does not directly equal speed. Actual speed also depends on the load, motor characteristics, and back-EMF.

Current

Current is closely related to torque.

When a motor needs to produce more torque, it generally draws more current.

For example:

• A motor spinning freely draws relatively little current.
• A motor accelerating a heavy elevator draws more current.
• A motor trying to move an elevator that is physically stuck may draw a very large amount of current.

This is why current is so important for mechanism design.

Stator Current vs. Supply Current

These two current measurements represent different parts of the electrical system.

The motor controller switches and regulates the power delivered to the motor. Therefore, supply current can be substantially lower than stator current.

If the battery supplies 12 V and the motor is receiving 6 V, an 80 A stator current corresponds to roughly 40 A of supply current.

This is why stator current is not the same thing as battery current.

CTRE Reading

CTRE Phoenix 6: Improving Performance with Current Limits 

5. Current Limits

CTRE provides two types of current limits.

Stator Current Limit

A stator current limit restricts current flowing through the motor’s phases.

It can help:

• Limit motor torque.
• Prevent motor overheating.
• Protect mechanisms from excessive force.
• Reduce the risk of damage during a mechanism jam.

Supply Current Limit

A supply current limit restricts current drawn from the battery bus.

It can help:

• Prevent excessive battery loading.
• Reduce the risk of robot brownouts.
• Protect the robot’s electrical system.

A stator limit can also help prevent brownouts because it limits motor current demand, but it is not a complete substitute for supply current limits.

One important consideration is that a current limit set too low can negatively impact acceleration and mechanism performance.

Important: CTRE’s current-limit calculator considers supply current draw. Stator current and supply current should not be treated as interchangeable when selecting limits.

CTRE Reading

CTRE Phoenix 6: Improving Performance with Current Limits 

6. Designing an FRC Mechanism

Before programming a mechanism, you should be able to answer the following questions.

Mechanical Considerations

• What is the mechanism’s purpose?
• What is the required range of motion?
• What is the maximum load?
• What is the mechanism’s mass and inertia?
• What gear ratio is needed?
• How much torque is required?
• Is gravity acting on the mechanism?
• Are there hard stops?
• What happens if the mechanism jams?
• Is the mechanism structurally capable of handling the load?

Electrical Considerations

• Which motor should be used?
• How many motors are required?
• What is the expected current draw?
• What stator current limit is appropriate?
• What supply current limit is appropriate?
• Could the mechanism cause a robot brownout?
• How will the mechanism affect the other mechanisms on the robot?

Programming Considerations

• Does the mechanism need position control, velocity control, or open-loop control?
• What sensor will provide feedback?
• Does the mechanism need PID?
• Does it need kS, kG, or kV?
• What are the appropriate Motion Magic constraints, if applicable?
• What happens when the sensor disconnects?
• What happens when the mechanism reaches its limit?
• What should the mechanism do when disabled?

The main lesson here is that mechanism programming is not just writing commands to a motor. It is understanding how software interacts with a physical system.

Additional Resources

• CTRE Phoenix 6 Documentation 
• CTRE Phoenix 6: Closed-Loop Control 
• CTRE Phoenix 6: Manual PID Tuning 
• CTRE Phoenix 6: Control Requests 
• CTRE Phoenix 6: Improving Performance with Current Limits 

To Complete This Lesson

Nothing to turn in, Read it and make sure you have the material in this section pretty solid! After that, you’re ready to move on.