# Active Disturbance Rejection Control (ADRC)

MATLAB, Simulink and Python implementation of Active Disturbance Rejection Control with Extended State Observer, Tracking Differentiator, input delay compensation, and control saturation.

__Note__: You can install the Python implementation using
``` bash
pip install adrc
```
You can look at [the project page on Pypi](https://pypi.org/project/adrc/1.0.0/) for more information.

__Note__: To be able to use all the Python codes, especially the demo script, you need to have the following packages installed:
- numpy
- scipy
- matplotlib
- python-control

---

## Theoretical Background

### System Representation

Consider an nth-order SISO system:

$$y^{(n)} = f(y, \dot{y}, \ldots, y^{(n-1)}, w(t)) + b_0 u$$

where:
- $y$ is the system output
- $u$ is the control input
- $b_0$ is the control gain (nominal)
- $f(\cdot)$ represents the total disturbance (internal dynamics + external disturbances)
- $w(t)$ represents external disturbances

### State-Space Form

Define state variables: 
$$x_1 = y, \quad x_2 = \dot{y}, \quad \ldots, \quad x_n = y^{(n-1)}$$

The system can be written as:

$$\begin{aligned}
\dot{x}_1 &= x_2 \\
\dot{x}_2 &= x_3 \\
&\vdots \\
\dot{x}_n &= f + b_0 u \\
y &= x_1
\end{aligned}$$

Basically, Active Disturbance Rejection Controller (ADRC) sees the whole system as a multi-integrator plus disturbance. The states are estimated using an observer. Because the total disturbance is estimated as well, the observer is referred to as Extended State Observer (ESO). The diagram below shows the block diagram of ADRC.

![alt text](<images/ADRC Diagram.jpg>)

### Extended State Observer (ESO)

To estimate both the states and the total disturbance $f$, we augment the state vector with $x_{n+1} = f$ (assuming $\dot{f} \approx 0$):

$$\begin{aligned}
\dot{x}_1 &= x_2 \\
\dot{x}_2 &= x_3 \\
&\vdots \\
\dot{x}_n &= x_{n+1} + b_0 u \\
\dot{x}_{n+1} &= \dot{f} \approx 0 \\
y &= x_1
\end{aligned}$$

In matrix form:

$$\begin{aligned}
\dot{\mathbf{x}} &= \mathbf{A} \mathbf{x} + \mathbf{B} u \\
y &= \mathbf{C} \mathbf{x}
\end{aligned}$$

where:

$$\mathbf{A} = \begin{bmatrix}
0 & 1 & 0 & \cdots & 0 & 0 \\
0 & 0 & 1 & \cdots & 0 & 0 \\
\vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\
0 & 0 & 0 & \cdots & 0 & 1 \\
0 & 0 & 0 & \cdots & 0 & 0
\end{bmatrix}_{(n+1) \times (n+1)}, \quad
\mathbf{B} = \begin{bmatrix}
0 \\ 0 \\ \vdots \\ b_0 \\ 0
\end{bmatrix}, \quad
\mathbf{C} = \begin{bmatrix}
1 & 0 & \cdots & 0 & 0
\end{bmatrix}$$

The ESO is designed as:

$$\dot{\hat{\mathbf{x}}} = \mathbf{A} \hat{\mathbf{x}} + \mathbf{B} u + \mathbf{L}(y - \hat{y})$$

where $\mathbf{L} = [l_1, l_2, \ldots, l_{n+1}]^T$ is the observer gain vector.

### Observer Gain Selection

The observer gains are selected using bandwidth parameterization. We place all observer poles at $s = -\omega_o$ where $\omega_o$ is the observer bandwidth:

$$\det(sI - (\mathbf{A} - \mathbf{L}\mathbf{C})) = (s + \omega_o)^{n+1}$$

For different system orders:

**First-order system (n=1):**

$$\begin{aligned}
l_1 &= 2\omega_o \\
l_2 &= \omega_o^2
\end{aligned}$$

**Second-order system (n=2):**

$$\begin{aligned}
l_1 &= 3\omega_o \\
l_2 &= 3\omega_o^2 \\
l_3 &= \omega_o^3
\end{aligned}$$

**Third-order system (n=3):**

$$\begin{aligned}
l_1 &= 4\omega_o \\
l_2 &= 6\omega_o^2 \\
l_3 &= 4\omega_o^3 \\
l_4 &= \omega_o^4
\end{aligned}$$

**Fourth-order system (n=4):**

$$\begin{aligned}
l_1 &= 5\omega_o \\
l_2 &= 10\omega_o^2 \\
l_3 &= 10\omega_o^3 \\
l_4 &= 5\omega_o^4 \\
l_5 &= \omega_o^5
\end{aligned}$$

The observer bandwidth is typically chosen as:
$$\omega_o = k_{ob} \cdot \omega_c$$

where $\omega_c = -s_{cl}$ and $s_{cl} = \frac{-4}{T_{settle}}$ (for the first-order system) is the desired closed-loop pole location, and $k_{ob}$ is a multiplier (typically 5-20). $s_{cs}$ is usually selected as to make the closed-loop system critically damped.

### Discrete-Time ESO

The continuous ESO is discretized using exact discretization:

$$\mathbf{A}_d = e^{\mathbf{A} \Delta T}$$

$$\mathbf{B}_d = \int_0^{\Delta T} e^{\mathbf{A} \tau} d\tau \cdot \mathbf{B} = \mathbf{A}^{-1}(\mathbf{A}_d - \mathbf{I})\mathbf{B}$$

For numerical stability, we compute:

$$\mathbf{B}_d = \sum_{i=1}^{n} \frac{(\Delta T)^i}{i!} \mathbf{A}^{i-1} \mathbf{B}$$

The discrete observer gains are computed using pole placement in the z-domain. For a pole at $z = e^{s \Delta T}$ where $s = -\omega_o$:

$$z_{eso} = e^{-\omega_o \Delta T}$$

The discrete gains $L_d$ are selected such that the characteristic polynomial matches $(z - z_{eso})^{n+1}$.

**For n=1:**

$$\begin{aligned}
L_{d,1} &= 1 - z_{eso}^2 \\
L_{d,2} &= \frac{(1 - z_{eso})^2}{\Delta T}
\end{aligned}$$

**For n=2:**

$$\begin{aligned}
L_{d,1} &= 1 - z_{eso}^3 \\
L_{d,2} &= \frac{3(1 + z_{eso})(1 - z_{eso})^2}{2\Delta T} \\
L_{d,3} &= \frac{(1 - z_{eso})^3}{\Delta T^2}
\end{aligned}$$

**For n=3:**

$$\begin{aligned}
L_{d,1} &= 1 - z_{eso}^4 \\
L_{d,2} &= \frac{(1 - z_{eso})^2(11 + z_{eso}(14 + 11z_{eso}))}{6\Delta T} \\
L_{d,3} &= \frac{2(1 - z_{eso})^3(1 + z_{eso})}{\Delta T^2} \\
L_{d,4} &= \frac{(1 - z_{eso})^4}{\Delta T^3}
\end{aligned}$$

**For n=4:**

$$\begin{aligned}
L_{d,1} &= 1 - z_{eso}^5 \\
L_{d,2} &= \frac{5(1 - z_{eso})^2(1 + z_{eso})(5 + z_{eso}(2 + 5z_{eso}))}{12\Delta T} \\
L_{d,3} &= \frac{5(1 - z_{eso})^3(7 + z_{eso}(10 + 7z_{eso}))}{12\Delta T^2} \\
L_{d,4} &= \frac{5(1 - z_{eso})^4(1 + z_{eso})}{2\Delta T^3} \\
L_{d,5} &= \frac{(1 - z_{eso})^5}{\Delta T^4}
\end{aligned}$$

The discrete ESO update equation:

$$\hat{\mathbf{x}}[k] = \mathbf{A}_d \hat{\mathbf{x}}[k-1] + \mathbf{B}_d u[k-1] + \mathbf{L}_d (y[k] - \mathbf{C}_d \mathbf{A}_d \hat{\mathbf{x}}[k-1])$$

Simplified form:

$$\hat{\mathbf{x}}[k] = (\mathbf{A}_d - \mathbf{L}_d \mathbf{C}_d \mathbf{A}_d) \hat{\mathbf{x}}[k-1] + (\mathbf{B}_d - \mathbf{L}_d \mathbf{C}_d \mathbf{B}_d) u[k-1] + \mathbf{L}_d y[k]$$

### Control Law

The control law compensates for the estimated disturbance and tracks the reference:

$$u_0 = \frac{1}{b_0} \left[ r^{(n)} - \sum_{i=1}^{n} k_i (r^{(i-1)} - \hat{x}_i) - \hat{x}_{n+1} \right]$$

where $r$ is the reference signal and $k_i$ are the state feedback gains.

Equivalently:

$$u_0 = \frac{1}{b_0} \left[ \mathbf{K}^T (\mathbf{r} - \hat{\mathbf{x}}) \right]$$

where:

$$\mathbf{r} = \begin{bmatrix} r \\ r^{(1)} \\ \vdots \\ r^{(n-1)} \\ r^{(n)} \end{bmatrix}, \quad
\mathbf{K} = \begin{bmatrix} k_1 \\ k_2 \\ \vdots \\ k_n \\ 1 \end{bmatrix}$$

### Controller Gain Selection

The controller gains are selected by pole placement. All closed-loop poles are placed at $s = -\omega_c$:

$$\det(sI - (\mathbf{A} - \mathbf{B}\mathbf{K}^T)) = (s + \omega_c)^n$$

where $\omega_c = \frac{-4}{T_{settle}}$ for a desired settling time $T_{settle}$.

For an nth-order system:

$$k_i = \binom{n}{i} \omega_c^i, \quad i = 1, 2, \ldots, n$$

**Examples:**

For n=2: $k_1 = 2\omega_c$, $k_2 = \omega_c^2$

For n=3: $k_1 = 3\omega_c$, $k_2 = 3\omega_c^2$, $k_3 = \omega_c^3$

### Tracking Differentiator (TD)

When reference derivatives are not available, a Tracking Differentiator is used to estimate them from the reference signal. Five TD methods are implemented:

#### 1. Euler-based TD
Low-pass filter with numerical differentiation:

$$\begin{aligned}
y_f[k] &= a \cdot y_f[k-1] + (1-a) \cdot r[k] \\
\dot{y}_f[k] &= \frac{y_f[k] - y_f[k-1]}{\Delta T}
\end{aligned}$$

Parameter: $a \in (0,1)$ (filter coefficient)

#### 2. Time-Optimal Differentiator (TOD)

$$\begin{aligned}
\dot{v}_1 &= v_2 \\
\dot{v}_2 &= -r \cdot \text{sign}\left( v_1 - r + \frac{v_2 |v_2|}{2r} \right)
\end{aligned}$$

Parameter: $r > 0$ (convergence rate)

#### 3. Linear Differentiator (LD)

$$\begin{aligned}
\dot{v}_1 &= v_2 - \frac{v_1 - r}{\lambda} \\
\dot{v}_2 &= -\frac{2(v_1 - r)}{\lambda}
\end{aligned}$$

Parameter: $\lambda > 0$ (bandwidth)

#### 4. Robust Exact Differentiator (RED)

$$\begin{aligned}
\dot{v}_1 &= v_2 - \lambda_1 |e|^{0.5} \text{sign}(e) \\
\dot{v}_2 &= -\lambda_2 \text{sign}(e)
\end{aligned}$$

where $e = v_1 - r$

Parameters: $\lambda_1, \lambda_2 > 0$

#### 5. Improved Nonlinear TD (INTD)

$$\begin{aligned}
\dot{v}_1 &= v_2 \\
\dot{v}_2 &= -r^2 \tanh\left( \frac{\beta v_1 - (1-\alpha) r}{\gamma} \right) - r v_2
\end{aligned}$$

Parameters: $\alpha \in [0,1]$, $\beta, \gamma, r > 0$

All TDs are integrated using forward Euler with sample time $\Delta T$.

### Input Delay Compensation

When the system has input delay $\tau$, the actual plant input is $u(t-\tau)$. The ESO is modified to use the delayed control signal:

$$\hat{\mathbf{x}}[k] = \mathbf{A}_d \hat{\mathbf{x}}[k-1] + \mathbf{B}_d u[k-1-d] + \mathbf{L}_d y[k]$$

where $d = \lfloor \tau / \Delta T \rfloor$ is the delay in samples.

The controller computes the current control signal, while the ESO uses the control signal from $d$ steps ago, which is the signal currently affecting the plant.

### Control Saturation

When control saturation limits $[u_{min}, u_{max}]$ exist, the control signal is saturated:

$$u = \begin{cases}
u_{max} & \text{if } u_0 > u_{max} \\
u_{min} & \text{if } u_0 < u_{min} \\
u_0 & \text{otherwise}
\end{cases}$$

The saturated control signal $u$ is fed back to the ESO to maintain consistency between the ESO's prediction and the actual plant input.

---
Here are some figures showing the controller in action, in presence of time-varying referece signal, input delay, input saturation, etc.

![alt text](images/ADRC_demo1.jpg)

![alt text](images/ADRC_demo2.jpg)

![alt text](images/ADRC_demo3.jpg)

---

## Code Documentation

Below, the MATLAB implementation has been explained. The Python implementation is very similar and hence its explanation has been skipped here. 

### TD Class

**File:** [TD.m](TD.m)

Object-oriented implementation of Tracking Differentiator with multiple methods.

#### Properties

**Private:**
- `method` (char): TD method ('euler', 'tod', 'ld', 'red', 'intd')
- `dT` (double): Sample time
- `params` (cell): Method-specific parameters
- `state` (struct): Internal state variables

**Public (Read-only):**
- `y` (double): Filtered output
- `yd` (double): First derivative estimate
- `ydd` (double): Second derivative estimate

#### Constructor

```matlab
obj = TD(method, dT, varargin)
```

**Parameters:**
- `method`: String specifying TD method
- `dT`: Sample time in seconds
- `varargin`: Method-specific tuning parameters
  - `'euler'`: a (filter coefficient, default 0.9)
  - `'tod'`: r (convergence rate, default 1)
  - `'ld'`: lambda (bandwidth, default 1)
  - `'red'`: lambda1, lambda2 (defaults 1, 1)
  - `'intd'`: alpha, beta, gamma, r (defaults 1, 1, 1, 1)

**Example:**
```matlab
td = TD('tod', 0.01, 20);  % TOD with r=20, dT=0.01s
```

#### Methods

**`update(ref)`**

Update TD with new reference value.

**Parameters:**
- `ref`: Current reference signal value

**Updates:** `y`, `yd`, `ydd` properties

**Example:**
```matlab
td.update(1.0);
filtered_ref = td.y;
ref_dot = td.yd;
```

**`reset(initialValue)`**

Reset TD to initial state.

**Parameters:**
- `initialValue`: Initial value (default: 0)

**`setParameters(varargin)`**

Update tuning parameters without recreating object.

**`setSampleTime(dT)`**

Update sample time.

---

### ADRC Class

**File:** [ADRC.m](ADRC.m)

Comprehensive ADRC controller with ESO, optional TD, delay compensation, and saturation.

#### Properties

**Private:**
- `n` (double): System order (1-4)
- `dT` (double): Sample time
- `Ld`, `K` (double): Observer and controller gains
- `Ad`, `Bd`, `Cd` (double): Discrete ESO matrices
- `b0` (double): Control gain estimate
- `Tsettle`, `kob` (double): Tuning parameters
- `Ke` (double): Error scaling gain
- `Xhat` (double): Extended state estimates [x₁...xₙ, f]
- `uPrev` (double): Previous control input
- `uMin`, `uMax` (double): Saturation limits
- `inputDelaySteps` (double): Delay in controller steps
- `uHistory` (double): Control history buffer
- `useTD` (logical): TD enable flag
- `TD_obj` (TD): TD object instance

**Public (Read-only):**
- `isInitialized` (logical): Initialization status
- `controllerOrder` (double): n+1 (extended state dimension)

#### Constructor

```matlab
obj = ADRC(systemOrder)
```

**Parameters:**
- `systemOrder`: System order (1-4)

**Example:**
```matlab
controller = ADRC(2);  % For 2nd-order system
```

#### Methods

**`initialize(varargin)`**

Initialize controller with name-value pairs.

**Name-Value Parameters:**
- `'Tsettle'`: Settling time (default: 1.0)
- `'kob'`: Observer bandwidth multiplier (default: 10)
- `'b0'`: Control gain estimate (default: 1.0)
- `'uMin'`: Lower saturation limit (default: -inf)
- `'uMax'`: Upper saturation limit (default: inf)
- `'dT'`: Sample time (default: 0.01)
- `'XhatInit'`: Initial extended state (default: zeros)
- `'uInit'`: Initial control input (default: 0)
- `'Ke'`: Error scaling gain (default: 1)
- `'inputDelay'`: Input delay in seconds (default: 0)
- `'TD_method'`: TD method or 'none' (default: 'none')
- `'TD_params'`: Cell array of TD parameters (default: {})

**Example:**
```matlab
controller.initialize('Tsettle', 1.0, 'kob', 10, 'b0', 1.6, ...
                     'dT', 0.01, 'uMin', -10, 'uMax', 10, ...
                     'inputDelay', 0.05, ...
                     'TD_method', 'tod', 'TD_params', {20});
```

**`u = step(reference, output, varargin)`**

Execute one control step.

**Parameters:**
- `reference`: Reference signal value
- `output`: System output (measurement)
- `varargin`: Optional reference derivatives [r', r'', ...] (vector or individual arguments)

**Returns:**
- `u`: Control signal

**Example:**
```matlab
u = controller.step(ref, y);  % Without derivatives
u = controller.step(ref, y, ref_dot, ref_dotdot);  % With derivatives
```

**`reset(XhatInit, uInit)`**

Reset controller state.

**Parameters:**
- `XhatInit`: Initial extended state (optional)
- `uInit`: Initial control input (optional)

**`setTD(method, varargin)`**

Configure or update tracking differentiator.

**Parameters:**
- `method`: TD method or 'none'
- `varargin`: TD parameters

**Example:**
```matlab
controller.setTD('tod', 15);  % Enable TOD with r=15
controller.setTD('none');     % Disable TD
```

**`setSaturation(uMin, uMax)`**

Update saturation limits.

**`setInputDelay(delaySec)`**

Update input delay setting.

**`updateTuning(Tsettle, kob)`**

Update controller tuning parameters and recompute gains.

**`Xhat = getEstimatedStates()`**

Get current extended state estimates [x₁, ..., xₙ, f]ᵀ.

**`f = getEstimatedDisturbance()`**

Get estimated total disturbance (last element of Xhat).

---

## Scripts

### ADRC_app.m

Interactive GUI application for ADRC simulation.

**Features:**
- System transfer function input (numerator/denominator)
- Controller tuning sliders (Tsettle, kob, b0)
- Input delay configuration (matched/unmatched)
- Control saturation limits
- Reference signal generator (Step, Sinusoid, Sawtooth)
- TD method selection with parameter adjustment
- Real-time plotting

**Usage:**
```matlab
ADRC_app
```

### demo.m

Comprehensive demonstration script with three examples:

1. **Basic ADRC**: Step response without TD
2. **ADRC with TD**: Sinusoidal tracking with TOD
3. **Input Delay Compensation**: Step response with input delay

---

## Implementation Notes

1. **Sample Time**: Controller sample time `dT` can differ from simulation time step `dt`. The controller should be called at rate `dT`.

2. **Delay Compensation**: Input delay is specified in seconds and internally converted to discrete steps. Delay buffer maintains control history.

3. **Initialization**: Always call `initialize()` before `step()`. The controller will throw an error if used uninitialized.

4. **TD Integration**: When TD is enabled, it is automatically updated within `step()`. Manual reference derivatives can still be provided via `varargin` to override TD estimates.

5. **State Estimation**: Access estimated states via `getEstimatedStates()` for monitoring or additional processing.

6. **Tuning Guidelines**:
   - Start with `Tsettle` = desired settling time
   - Set `kob` = 5-20 (higher = faster observer, more noise sensitivity)
   - Ensure `b0` matches nominal plant gain
   - Adjust `Ke` if using non-unity error scaling

---

## References

1. Han, J. (2009). "From PID to Active Disturbance Rejection Control". IEEE Transactions on Industrial Electronics.

2. Gao, Z. (2006). "Active Disturbance Rejection Control: A Paradigm Shift in Feedback Control System Design". American Control Conference.

3. Herbst, G. (2013). "A Simulative Study on Active Disturbance Rejection Control (ADRC) as a Control Tool for Practitioners". Electronics.

4. Zheng, Q., Gao, Z. (2010). "On Practical Applications of Active Disturbance Rejection Control". Chinese Control Conference.

5. Madoński, R., & Herman, P. (2015). Survey on methods of increasing the efficiency of extended state disturbance observers. ISA transactions, 56, 18-27.

6. Madoński, R., & Herman, P. (2015). Survey on methods of increasing the efficiency of extended state disturbance observers. ISA transactions, 56, 18-27.

