Introduction to System Identification

What is «System Identification?»

System identification is the process of determining a mathematical model for the behavior of a system through statistical analysis of its inputs and outputs.

This model is a rule describing how input voltage affects the way our measurements (typically encoder data) evolve in time. A «system identification» routine takes such a model and a dataset and attempts to fit parameters which would make your model most closely-match the dataset. Generally, the model is not perfect - the real-world data are polluted by both measurement noise (e.g. timing errors, encoder resolution limitations) and system noise (unmodeled forces acting on the system, e.g. vibrations). However, even an imperfect model is usually «good enough» to give us accurate feedforward control of the mechanism, and even to estimate optimal gains for feedback control.

In FRC, the most common system that we’re interested in characterizing is the permanent-magnet DC motor, which closely obeys the following «voltage-balance equation» (for more information, see this paper):

\[V = kS \cdot sgn(\dot{d}) + kV \cdot \dot{d} + kA \cdot \ddot{d}\]

where \(V\) is the applied voltage, \(d\) is the displacement (position) of the motor, \(\dot{d}\) is its velocity, and \(\ddot{d}\) is its acceleration (the «overdot» notation traditionally denotes the derivative with respect to time).

We can interpret the coefficients in the above equation as follows:

kS is the voltage needed to overcome the motor’s static friction, or in other words to just barely get it moving; it turns out that this static friction (because it’s, well, static) has the same effect regardless of velocity or acceleration. That is, no matter what speed you’re going or how fast you’re accelerating, some constant portion of the voltage you’ve applied to your motor (depending on the specific mechanism assembly) will be going towards overcoming the static friction in your gears, bearings, etc; this value is your kS. Note the presence of the signum function, because friction force always opposes the direction-of-motion.

kV describes how much voltage is needed to hold (or «cruise») at a given constant velocity while overcoming the electromagnetic resistance in the motor and any additional friction that increases with speed (known as viscous drag). The relationship between speed and voltage (at constant acceleration) is almost entirely linear (with FRC® components, anyway) because of how permanent-magnet DC motors work.

kA describes the voltage needed to induce a given acceleration in the motor shaft. As with kV, the relationship between voltage and acceleration (at constant velocity) is almost perfectly linear for FRC components.

Once these coefficients have been determined (SysId uses an ordinary least-squares regression), we can then take a given desired velocity and acceleration for the motor and calculate the voltage that should be applied to achieve it. This is very useful - not only for, say, following motion profiles, but also for making mechanisms more controllable in open-loop control, because your joystick inputs will more closely match the actual mechanism motion.

Some of the tools in this toolsuite introduce additional terms into the above equation to account for known differences from the simple case described above - details for each tool can be found below:

The WPILib System Identification Tool (SysId)

The WPILib system identification tool consists of an application that runs on the user’s PC and matching robot code that runs on the user’s robot. The PC application will send control signals to the robot over NetworkTables, while the robot sends data back to the application. The application then processes the data and determines model parameters for the user’s robot mechanism, as well as producing diagnostic plots. Data can be saved (in JSON format) for future use, if desired.

Included Tools

Nota

With a bit of ingenuity, these tools can be used to accurately characterize a surprisingly large variety of robot mechanisms. Even if your mechanism does not seem to obviously match any of the tools, an understanding of the system equations often reveals that one of the included routines will do.

The System Identification toolsuite currently supports:

  • Simple Motor Setups

  • Drivetrains

  • Elevators

  • Arms

Several of these options use identical robot-side code, and differ only in the analysis routine used to interpret the data.

Simple Motor Identification

The simple motor identification tool determines the best-fit parameters for the equation:

\[V = kS \cdot sgn(\dot{d}) + kV \cdot \dot{d} + kA \cdot \ddot{d}\]

where \(V\) is the applied voltage, \(d\) is the displacement (position) of the drive, \(\dot{d}\) is its velocity, and \(\ddot{d}\) is its acceleration. This is the model for a permanent-magnet dc motor with no loading other than friction and inertia, as mentioned above, and is an accurate model for flywheels, turrets, and horizontal linear sliders.

Drivetrain Identification

The drivetrain identification tool determines the best-fit parameters for the equation:

\[V = kS \cdot sgn(\dot{d}) + kV \cdot \dot{d} + kA \cdot \ddot{d}\]

where \(V\) is the applied voltage, \(d\) is the displacement (position) of the drive, \(\dot{d}\) is its velocity, and \(\ddot{d}\) is its acceleration. This is the same modeling equation as is used in the simple motor identification - however, the drivetrain identification tool is specifically set up to run on differential drives, and will characterize each side of the drive independently if desired.

The drivetrain identification tool can also determine the effective trackwidth of your robot using a gyro. More information on how to run the identification is available in the track width identification article.

Elevator Identification

The elevator identification tool determines the best-fit parameters for the equation:

\[V = kG + kS \cdot sgn(\dot{d}) + kV \cdot \dot{d} + kA \cdot \ddot{d}\]

where \(V\) is the applied voltage, \(d\) is the displacement (position) of the drive, \(\dot{d}\) is its velocity, and \(\ddot{d}\) is its acceleration. The constant term (\(kG\)) is added to correctly account for the effect of gravity.

Arm Identification

The arm identification tool determines the best-fit parameters for the equation:

\[V = kG \cdot cos(\theta) + kS \cdot sgn(\dot{\theta}) + kV \cdot \dot{\theta} + kA \cdot \ddot{\theta}\]

where \(V\) is the applied voltage, \(\theta\) is the angular displacement (position) of the arm, \(\dot{\theta}\) is its angular velocity, and \(\ddot{\theta}\) is its angular acceleration. The cosine term (\(kG\)) is added to correctly account for the effect of gravity.

Installing the System Identification Tool

The system identification tool (also referred to as sysid) is included with the WPILib Installer.

Nota

The old Python characterization tool from previous years is no longer supported.

Launching the System Identification Tool

The system identification tool can be opened from the Start Tool option in VS Code or by using the shortcut inside the WPILib Tools desktop folder (Windows).