Slew Rate Limiter

A common use for filters in FRC® 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 Trapezoidal Motion Profile 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, C++).

Creating a SlewRateLimiter

Nota

The C++ SlewRateLimiter class is templated on the unit type of the input. For more information on C++ units, see The C++ Units Library.

Nota

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:

// Creates a SlewRateLimiter that limits the rate of change of the signal to 0.5 units per second
SlewRateLimiter filter = new 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:

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

Using a SlewRateLimiter with DifferentialDrive

Nota

In the C++ example below, the filter is templated on double, rather than on a unit type, 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:

// 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);