slew-rate-limiter.rst

# Slew Rate Limiter

A common use for filters in FRC|reg| is to soften the behavior of control inputs (for example, the joystick inputs from your driver controls). Unfortunately, a simple low-pass filter is poorly-suited for this job; while a low-pass filter will soften the response of an input stream to sudden changes, it will also wash out fine control detail and introduce phase lag. A better solution is to limit the rate-of-change of the control input directly. This is performed with a slew rate limiter - a filter that caps the maximum rate-of-change of the signal.

A slew rate limiter can be thought of as a sort of primitive motion profile. In fact, the slew rate limiter is the first-order equivalent of the :ref:`Trapezoidal Motion Profile <docs/software/advanced-controls/controllers/trapezoidal-profiles:Trapezoidal Motion Profiles in WPILib>` supported by WPILib - it is precisely the limiting case of trapezoidal motion when the acceleration constraint is allowed to tend to infinity. Accordingly, the slew rate limiter is a good choice for applying a de-facto motion profile to a stream of velocity setpoints (or voltages, which are usually approximately proportional to velocity). For input streams that control positions, it is usually better to use a proper trapezoidal profile.

Slew rate limiting is supported in WPILib through the SlewRateLimiter class ([Java](https://github.wpilib.org/allwpilib/docs/development/java/edu/wpi/first/math/filter/SlewRateLimiter.html), [C++](https://github.wpilib.org/allwpilib/docs/development/cpp/classfrc_1_1_slew_rate_limiter.html), :external:py:class:`Python <wpimath.filter.SlewRateLimiter>`).

## Creating a SlewRateLimiter

Note

The C++ SlewRateLimiter class is templated on the unit type of the input. For more information on C++ units, see :ref:`docs/software/basic-programming/cpp-units:The C++ Units Library`.

Note

Because filters have "memory", each input stream requires its own filter object. Do not attempt to use the same filter object for multiple input streams.

Creating a SlewRateLimiter is simple:

.. tab-set-code::

  ```java
  // Creates a SlewRateLimiter that limits the rate of change of the signal to 0.5 units per second
  SlewRateLimiter filter = new SlewRateLimiter(0.5);
  ```

  ```c++
  // Creates a SlewRateLimiter that limits the rate of change of the signal to 0.5 volts per second
  frc::SlewRateLimiter<units::volts> filter{0.5_V / 1_s};
  ```

  ```python
  from wpimath.filter import SlewRateLimiter
  # Creates a SlewRateLimiter that limits the rate of change of the signal to 0.5 units per second
  filter = SlewRateLimiter(0.5)
  ```

## Using a SlewRateLimiter

Once your filter has been created, using it is easy - simply call the calculate() method with the most recent input to obtain the filtered output:

.. tab-set-code::

  ```java
  // Calculates the next value of the output
  filter.calculate(input);
  ```

  ```c++
  // Calculates the next value of the output
  filter.Calculate(input);
  ```

  ```python
  # Calculates the next value of the output
  filter.calculate(input)
  ```

### Using a SlewRateLimiter with DifferentialDrive

Note

The C++ example below templates the filter on units::scalar for use with doubles, since joystick values are typically dimensionless.

A typical use of a SlewRateLimiter is to limit the acceleration of a robot's drive. This can be especially handy for robots that are very top-heavy, or that have very powerful drives. To do this, apply a SlewRateLimiter to a value passed into your robot drive function:

.. tab-set-code::

  ```java
  // Ordinary call with no ramping applied
  drivetrain.arcadeDrive(forward, turn);
  // Slew-rate limits the forward/backward input, limiting forward/backward acceleration
  drivetrain.arcadeDrive(filter.calculate(forward), turn);
  ```

  ```c++
  // Ordinary call with no ramping applied
  drivetrain.ArcadeDrive(forward, turn);
  // Slew-rate limits the forward/backward input, limiting forward/backward acceleration
  drivetrain.ArcadeDrive(filter.Calculate(forward), turn);
  ```

  ```python
  # Ordinary call with no ramping applied
  drivetrain.arcadeDrive(forward, turn)
  # Slew-rate limits the forward/backward input, limiting forward/backward acceleration
  drivetrain.arcadeDrive(filter.calculate(forward), turn)
  ```