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Why Control Loop Rate Matters in Material Testing Machines

When engineers evaluate a material testing system, they often look at actuator type, force capacity, or sensor resolution. But hidden beneath those specs is a critical factor that governs accuracy, stability, and fidelity: the control loop rate.

This parameter determines how often the controller samples sensor feedback (force, displacement, strain), command and updates actuator commands. It’s the heartbeat of the machine’s closed-loop system.

What Is Control Loop Rate?

  • 1 kHz loop → 1 ms between updates.
  • 5 kHz loop → 0.2 ms between updates.
  • 10 kHz loop → 0.1 ms between updates.

Each update computes the corrective action based on PID or other control algorithm used. The shorter the loop period, the closer the system behaves to an analog controller.

From control theory:

  • The closed-loop bandwidth (the highest frequency at which the machine can track input without unacceptable lag or amplitude loss) is typically ~1/20 to 1/50 of the loop rate.
  • Thus, a 5 kHz loop can realistically support ~100–200 Hz command fidelity.

For material testing, this matters because fatigue and dynamic tests often run in the 10–100 Hz range, with waveforms that must be followed precisely.

PID-controller

Why Loop Rate Affects Performance

  1. Tracking Accuracy
    The higher the loop rate, the less lag and amplitude attenuation.
    • At 1 kHz, a 50 Hz waveform may show noticeable phase lag and reduced amplitude.
    • At 5 kHz, the same waveform is tracked almost perfectly, as the system responds within 0.2 ms per cycle.

  2. Stability
    Slow loops add effective time delay, reducing phase margin. This can lead to overshoot or even oscillation near system resonances. Faster loops preserve stability up to the physical limits of the actuator.

  3. Responsiveness to Nonlinearities
    When a specimen yields, fractures, or changes stiffness, the load-displacement curve shifts abruptly. A high-speed loop reacts within microseconds, limiting overshoot and protecting both machine and specimen.

  4. Numerical & Noise Limits
    Going “too fast” can backfire. Above ~10 kHz, digital controllers may amplify sensor noise or run into coefficient quantization issues. Vendors like MTS note that beyond ~5 kHz, gains diminish and fidelity can even degrade if latency increases.

Industry Practice

  • Servo-hydraulic systems (MTS, Instron, Moog): Typically 2–5 kHz, sometimes 10 kHz for specialized fatigue tests. These systems are limited by hydraulic resonance (~50–100 Hz), so higher loop rates bring little extra benefit.

  • Electromechanical systems: Nested loops are common. Motor current loops often run at 5–20 kHz, while outer force/position loops run at 1–5 kHz. Vendors advertise 8–10 kHz control rates for high-fidelity fatigue.

  • Standards: ASTM E467 and ISO 4965 require accurate reproduction of cyclic loads up to 100 Hz. To meet this, a controller must typically operate at least in the multi-kHz range.

Rule of thumb: 5 kHz is the “sweet spot” for most test machines; 10 kHz provides a performance ceiling, while 1 kHz is insufficient for modern dynamic testing.

TACTUN: Configurable Loop Rates up to 100 kHz

Most legacy controllers lock you into fixed loop frequencies. TACTUN is different.

With an FPGA-based architecture, TACTUN allows machine builders to configure control loop rates anywhere from low kHz to an industry-leading 100 kHz maximum:

  • Select 1–2 kHz for static or creep tests where noise rejection matters more than speed.
  • Use 5–10 kHz for high-fidelity fatigue tests requiring accurate reproduction of complex waveforms.
  • Push up to 100 kHz in scenarios demanding ultra-fast response — such as fracture tests, high-speed servo-hydraulic rigs, or integration with advanced control strategies.

This flexibility means you decide the optimal rate for your machine’s characteristics — not the controller vendor. You balance stability, accuracy, and responsiveness based on the physics of your system.

Conclusion

Control loop rate is not just a number on a spec sheet—it’s a key factor that defines the quality of every test result. With TACTUN, machine builders gain full control over this parameter, ensuring their systems can meet both today’s ASTM/ISO standards and tomorrow’s testing challenges.

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