First-Order System Model Fitting

Overview

Many physical systems (thermal, electrical, mechanical) exhibit first-order dynamics. This skill explains the mathematical model and how to extract parameters from experimental data.

The First-Order Model

The dynamics are described by:

tau * dy/dt + y = y_ambient + K * u

Where:

• y = output variable (e.g., temperature, voltage, position)
• u = input variable (e.g., power, current, force)
• K = process gain (output change per unit input at steady state)
• tau = time constant (seconds) - characterizes response speed
• y_ambient = baseline/ambient value

Step Response Formula

When you apply a step input from 0 to u, the output follows:

y(t) = y_ambient + K * u * (1 - exp(-t/tau))

This is the key equation for fitting.

Extracting Parameters

Process Gain (K)

At steady state (t -> infinity), the exponential term goes to zero:

y_steady = y_ambient + K * u

Therefore:

K = (y_steady - y_ambient) / u

Time Constant (tau)

The time constant can be found from the 63.2% rise point:

At t = tau:

y(tau) = y_ambient + K*u*(1 - exp(-1))
       = y_ambient + 0.632 * (y_steady - y_ambient)

So tau is the time to reach 63.2% of the final output change.

Model Function for Curve Fitting

def step_response(t, K, tau, y_ambient, u):
    """First-order step response model."""
    return y_ambient + K * u * (1 - np.exp(-t / tau))

When fitting, you typically fix y_ambient (from initial reading) and u (known input), leaving only K and tau as unknowns:

def model(t, K, tau):
    return y_ambient + K * u * (1 - np.exp(-t / tau))

Practical Tips

1. Use rising portion data: The step response formula applies during the transient phase
2. Exclude initial flat region: Start your fit from when the input changes
3. Handle noisy data: Fitting naturally averages out measurement noise
4. Check units: Ensure K has correct units (output units / input units)

Quality Metrics

After fitting, calculate:

• R-squared (R^2): How well the model explains variance (want > 0.9)
• Fitting error: RMS difference between model and data

residuals = y_measured - y_model
ss_res = np.sum(residuals**2)
ss_tot = np.sum((y_measured - np.mean(y_measured))**2)
r_squared = 1 - (ss_res / ss_tot)
fitting_error = np.sqrt(np.mean(residuals**2))