Motion control for test rigs, testing machines and metrology

In short:Test and measurement machines need motion that is repeatable, measurable and traceable: actuators that follow a programmed load or waveform, feedback finer than the result you report, and data synchronised with motion. Pick the actuator technology from frequency and stroke first (screw actuators for slow, long-stroke life tests, linear motors and voice coils for fast, short-stroke fatigue), then size on RMS force and cycle life.

Typical requirements

Machinery we specify motion for

• Material fatigue testing machines
• Product life-cycle and durability rigs
• Steering and component pull testers
• Coordinate measuring machines
• Balancing machines
• Hardness testers
• Weighing and checkweighing systems
• Flying-probe testers
• Test benches with data acquisition
• Automotive and aerospace test rigs

What test and measurement demands from motion

A test machine is a measuring instrument that also applies load. It must apply exactly the force, displacement or waveform the procedure calls for, measure the result with known uncertainty, and repeat that thousands or millions of times without drifting. Motion components therefore get judged on three things: how faithfully they follow a command (bandwidth, smoothness, backlash), how well their feedback and the load measurement can be trusted (resolution, accuracy, calibration) and how long they last under continuous cycling.

The field is broad. FAULHABER lists hardness testing, 3D measurement, horizontal balancing machines checking small parts to milligram precision, viscosity testing and high-throughput weighing among its metrology applications. Curtiss-Wright Exlar describes GSX30 and GSX40 actuators chosen for material fatigue testing for their force and small package, and Tolomatic lists life testing of a power drill and life-cycle testing of an elliptical trainer with its RSA and GSA actuators.

Where motion components sit in test and measurement

Choosing the actuator technology for a test

The first decision is technology, and frequency and stroke make it for you.

These bands are rules of thumb. The reasons behind them are physical: a screw that reverses rapidly over a short stroke concentrates wear on a few millimetres of thread and builds heat, while electromagnetic actuators have no contact in the force path. TA Instruments’ ElectroForce 3550, for example, uses two moving-magnet linear motors in tandem for 15 kN dynamic force over 50 mm stroke at up to 50 Hz, from an ordinary three-phase supply. At the other extreme, a Moog structural test ran 83 hydraulic channels with a 1,200-channel data system.

Worked example: sizing a durability rig actuator

A seat-adjuster test needs 1.5 kN at 150 mm stroke, one full cycle every 4 s (0.25 Hz), for 100,000 cycles.

1. Speed: 150 mm out and back in 4 s, with dwell, means about 150 mm/s peak with a trapezoidal profile.
2. Force and duty: 1.5 kN at the specimen plus about 100 N for fixture mass and friction. The rig runs continuously, so size on RMS force: if 1.6 kN is held for half of each cycle, RMS is about 1.6 × √0.5 = 1.1 kN.
3. Screw: a 10 mm lead ball screw at 150 mm/s turns at 900 rpm, well within a mid-size rod actuator.
4. Life: 100,000 cycles × 0.3 m of travel is 30 km. Check the screw’s L10 life at the RMS load with the supplier; plan for at least two test programmes per actuator.
5. Duration: 100,000 × 4 s is about 111 hours, under five days of round-the-clock running.
6. Control: run force control from an in-line load cell, count cycles in non-volatile memory, and stop on a force or position deviation that signals specimen failure.

A guided rod actuator such as the Tolomatic GSA (published to about 4.2 kN) carries side load from the fixture; an unguided RSA with an external guide is the alternative. See how to size a linear actuator.

Measurement chain: feedback, load cells and calibration

Accuracy, repeatability and resolution are different things. Aerotech defines resolution as the smallest increment the control system can detect or command, repeatability as how closely repeated moves land, and accuracy as closeness to a known standard, and points to ISO 230-2 and laser interferometry for verifying positioning. Choose encoders with resolution at least several times finer than the smallest change you report, and mount them as close as possible to the measuring point.

Force is verified separately. ZwickRoell notes that ISO 7500-1 and ASTM E4 recommend recalibrating within 12 months and after any relocation, with at least five points between 20% and 100% of the calibrated range.

Rotating rigs and slip rings

Rotating rigs put sensors on the spinning part: gearboxes, wheels, shafts, rotors. An instrumentation slip ring then becomes part of the measurement chain. Michigan Scientific describes end-of-shaft rings up to 12,000 rpm using precious-metal contacts for low noise with strain, torque, temperature and vibration signals. For faster rigs, the High Speed Slip Ring is published for 800–15,000 rpm. Our page on slip rings for test rigs and the high-speed slip ring type page go further.

Environment and safety on test rigs

Test rigs often sit inside temperature or climate chambers, near oil and coolant, or beside specimens that fail suddenly. Check the actuator’s and encoder’s temperature ratings against the chamber range, not the lab, and keep motors outside the chamber where possible with the force passed through a sealed rod. A specimen that fractures releases stored energy, so guard the rig, limit force and speed in the drive, and use Safe Torque Off on access. Large rigs that run unattended overnight also need fault detection that stops the test, saves the data and leaves the rig safe for the morning shift.

What to send an engineer

• The test standard or internal procedure, and what is being measured.
• Specimen and fixture mass, and how the specimen is mounted.
• Force or torque (static and dynamic), stroke or angle, waveform and frequency.
• Number of cycles or hours, and how often the rig is reconfigured.
• Control mode (force or position) and the measurement accuracy you report.
• Data logging rate and the DAQ or controller you use.
• Environment: chamber temperature range, oil, dust, noise limits.

Typical pitfalls

1. Using a screw actuator for high-frequency, short-stroke fatigue and wearing it out in weeks.
2. Sizing on peak force when continuous cycling makes RMS force and screw life the limits.
3. Backlash in the load path, which appears as hysteresis in force-displacement results.
4. Running millivolt signals through noisy rings or alongside power circuits.
5. Losing the cycle count on a power cut and repeating or invalidating a test.

Send us the test procedure, loads, waveform and cycle count, and our specialist engineering and supply partners can propose actuators, motors, encoders, drives and slip rings for the rig.

Standards to have in view

• ISO 7500-1 (verification of static uniaxial testing machines: force measuring system)
• ASTM E4 (force verification of testing machines)
• ISO 6892-1 (tensile testing of metallic materials at room temperature)
• ASTM E466 (force-controlled constant amplitude axial fatigue tests of metallic materials)
• ISO 21940-11 (mechanical vibration, rotor balancing: procedures and tolerances for rotors with rigid behaviour)
• ISO 10360 series (acceptance and reverification tests for coordinate measuring systems)
• ISO 230-2 (determination of accuracy and repeatability of positioning of numerically controlled axes)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.