> For the complete documentation index, see [llms.txt](https://yams.yassrobotics.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://yams.yassrobotics.com/api-reference/c++-reference/math/lqr-controller.md).

# LQRController

**Namespace:** `yams::math` **Java equivalent:** [Java LQRController](/api-reference/java-reference/math/lqr-controller.md)

`LQRController` wraps a `LinearSystemLoop` and exposes a unified `Calculate` interface for arm, elevator, and flywheel mechanisms.

***

## Constructor

```cpp
explicit LQRController(LQRConfig config)
```

***

## Methods

### Configuration

| Method         | Signature                                           | Description                                                         |
| -------------- | --------------------------------------------------- | ------------------------------------------------------------------- |
| `UpdateConfig` | `void UpdateConfig(LQRConfig config)`               | Replace the internal system loop at runtime                         |
| `GetType`      | `LQRConfig::LQRType GetType() const`                | Returns the configured mechanism type                               |
| `GetConfig`    | `const std::optional<LQRConfig>& GetConfig() const` | Returns the config if constructed from `LQRConfig`, otherwise empty |

### Reset

| Method  | Signature                                                                  | Description                                         |
| ------- | -------------------------------------------------------------------------- | --------------------------------------------------- |
| `Reset` | `void Reset(units::radian_t angle, units::radians_per_second_t velocity)`  | Reset the Kalman observer state for arm or flywheel |
| `Reset` | `void Reset(units::meter_t distance, units::meters_per_second_t velocity)` | Reset the Kalman observer state for elevator        |

### Calculate

| Overload           | Signature                                                                                                           | Description                                                  |
| ------------------ | ------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------ |
| Arm                | `units::volt_t Calculate(units::radian_t measured, units::radian_t position, units::radians_per_second_t velocity)` | Compute output voltage for arm control                       |
| Elevator           | `units::volt_t Calculate(units::meter_t measured, units::meter_t position, units::meters_per_second_t velocity)`    | Compute output voltage for elevator control                  |
| Flywheel (angular) | `units::volt_t Calculate(units::radians_per_second_t measured, units::radians_per_second_t velocity)`               | Compute output voltage for flywheel angular velocity control |
| Flywheel (linear)  | `units::volt_t Calculate(units::meters_per_second_t measured, units::meters_per_second_t velocity)`                 | Compute output voltage for flywheel surface speed control    |

***

## Example

```cpp
math::LQRConfig cfg;
cfg.WithType(math::LQRConfig::LQRType::ARM)
   .WithArmSystem(frc::DCMotor::Falcon500(1), 0.5, 50.0)
   .WithQElems({0.01745, 0.08727})
   .WithRElems({12.0})
   .WithStateStdDevs({0.015, 0.17})
   .WithMeasurementStdDevs({0.01})
   .WithMaxVoltage(12_V);

math::LQRController controller{cfg};

// In Periodic():
units::radian_t measured = GetArmAngle();
units::volt_t output = controller.Calculate(
    measured,
    targetAngle,
    targetVelocity
);
motor.SetVoltage(output);
```

{% hint style="info" %}
Call `Reset` whenever the mechanism is re-enabled or its state is known (e.g., at initialization) to prevent the Kalman filter from tracking stale state.
{% endhint %}