Instron (Norwood, MA) manufactures materials testing equipment and accessories that are used to test samples ranging from components for jet engines to medical syringes. The company’s ElectroPuls systems are used for fatigue testing, which examines the behavior of materials under fluctuating or cyclic loads in the elastic regime. The E1000, E3000, and E10000 fatigue test systems are suited for biomedical/biomechanical research applications, and feature a wide dynamic performance range and low force characteristics. The all-electric systems use linear motor technology to eliminate the need for ball/lead-screws, and enable slow-speed static tests through to high-frequency dynamic tests at over 100 Hz.

The ElectroPuls fatigue test system testing a polyethylene sample.
Instron needed a reliable, highly accurate, absolute optical encoder that was compatible with BiSS® serial communications, and was non-contacting to eliminate friction/wear that leads to mechanical hysteresis errors. The company chose the RESOLUTE absolute optical encoder from Renishaw (Hoffman Estates, IL) to support ElectroPuls controllers offering linear resolutions of 1 nm and speeds beyond 20 m/s. The encoders were selected for both axes on the E10000 and new E3000 test frames.

Conventional servo-hydraulic test instruments use a linear variable differential transformer (LVDT), a type of inductive absolute encoder, to determine the linear direction of travel and position of the actuator. ElectroPuls is driven by a linear synchronous motor, and requires additional high-quality velocity feedback for precise dynamic control, due to the challenges of driving linear motors with lower inherent damping than traditional rotary designs. While suitable for position control, an LVDT suffers from relatively poor signal stability and cannot, therefore, produce accurate velocity measurements. LVDTs are also susceptible to magnetic fields produced within a linear motor, and exhibit increasing nonlinearity over longer axes.

Optical encoders have intrinsically better signal stability and higher accuracy, making them superior in this application. Instron determined that an absolute optical encoder, capable of providing absolute position information without homing, would be the best solution for their bi-axial systems, as it offers several advantages over servo mechanisms that use optical linear incremental encoders in conjunction with an LVDT.

An LVDT previously was used to provide the absolute position feedback needed to commutate a linear motor, and an incremental optical encoder to provide the required measurement and control accuracy. The RESOLUTE absolute linear encoder reduced assembly complexity and number of parts. The ElectroPuls E3000 linear-torsion machine requires both linear and rotary absolute encoders for the linear and rotational axes, respectively. The linear position is provided by a RESOLUTE readhead and RTLA tape scale with ±5 μm/m accuracy and 5-nm resolution. A second RESOLUTE reading a 115-mm-diameter RESA ring determines angular position, offering resolution of 0.019 arc second and an encoder accuracy of ±2.5 arc second.

The company recently launched an advanced bi-axial variant of its ElectroPuls E3000 All-Electric test instrument. The E3000 is a compact tabletop instrument comprised of a load frame, crosshead with combined linear/ torsion actuator, Dynacell load cell, and T-slot table for fixing samples. The new E3000 linear- torsion model is a smallerscale equivalent of the E10000 linear-torsion system, and includes a rotation axis with a standard range of ±135° as well as optional multi-turn capability for applications such as orthopedic bone-screw testing. An ElectroPuls bi-axial linear-torsion test can be conducted on most materials, and has found applications in testing inter-vertebral disc prostheses, various biomaterials, athletic footwear, and elastomeric components.

The RESOLUTE encoder’s data capture must be accurate and reliable, as sample tests, in many cases, cannot be repeated. Failures such as miscounting are unacceptable because they lead to erroneous readings, and potentially incorrect conclusions about a sample. RESOLUTE has high tolerance to dirt, scratches, and grease that can cause other encoders to miscount, and an independent position-checking algorithm also ensures that any potential problem is flagged long before it reaches the controller.

RESOLUTE is a single-track absolute optical encoder, which allows far wider installation tolerances than traditional multi-track solutions. Its integral set-up LED further simplifies installation during test frame manufacture and servicing.

This article was contributed by Renishaw. For more information, Click Here .

NASA Tech Briefs Magazine

This article first appeared in the October, 2015 issue of NASA Tech Briefs Magazine.

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