Instron ElectroPuls linear-torsion tester uses Renishaw encoders

  • May 20, 2015
  • Renishaw Inc
  • Case Study

May 20, 2015 - Instron has built a reputation as a global market leader in the materials testing industry. The Norwood, MA, based company manufactures and services a comprehensive range of materials testing equipment and accessories for the research, industrial and academic sectors, and a variety of Instron systems 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.

Instron needed a reliable, highly accurate, absolute optical encoder that was compatible with BiSS serial communications, easy to set-up and install, and non-contacting to eliminate friction/wear that leads to mechanical hysteresis errors. The company found a solution with Renishaw's RESOLUTE, the only absolute optical encoder to support ElectroPuls controllers and satisfy all other criteria - while offering linear resolutions of 1 nm and speeds beyond 20 m/s. This resulted in RESOLUTE encoders being chosen for both axes on the E10000 and new E3000 test-frames.

An LVDT problem

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 exhibits increasing non-linearity 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.

“We previously used an LVDT 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,” said Graham Mead, Principal mechanical engineer for Instron ElectroPuls, at Instron's UK-based European headquarters. “With the RESOLUTE absolute linear encoder we are able to get all the functionality and accuracy from one device. The reduced assembly complexity and number of parts allows much simpler set-up and maintenance-free operation. The use of the RESOLUTE ring encoder for the rotary axis perfectly complements the linear encoder. It provides superior resolution, accuracy and mechanical simplicity over our previous belt driven, conventional rotary encoder.”

High performance solution

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 an impressive 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 smaller-scale 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.

RESOLUTE is well suited for mechanical test applications due to its reliability, resolution, accuracy and ease of installation. The 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.

Strain measurements on stiff samples demand position encoders with higher resolutions and RESOLUTE’s low jitter (less than 10 nm RMS) improves displacement sensitivity and allows more information to be gleaned from the sample.

The behavior of dynamic materials-testing machines must be precisely controlled with either strain feedback from an encoder or stress feedback from a load cell. Standards body ASTM International tolerates a deviation of up to 2 percent of the maximum applied load before a test becomes invalid. RESOLUTE’s high accuracy and excellent dynamic performance permit more detailed analysis of samples at failure, contributing to the accuracy of drawn conclusions.

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

Instron knew of the limitations of LVDT technology and were waiting for the right encoder solution to meet their needs. RESOLUTE with BiSS protocol provides the advanced capabilities that continue to support new developments in the groundbreaking ElectroPuls series. From evaluating the tensile strength of sutures to durability testing of fracture fixation devices, the ElectroPuls systems provide a uniquely versatile testing platform. Renishaw's close working relationship with Instron continues to drive future product innovation and development across both companies. 

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