Bi-axial test machine overhaul

The system upgrade for the university included a complete overhaul of the machine and integrating a new control system. The bi-axial machine uses a traditional cruciform specimen for testing various materials; such as steel, composites and aluminium used in applications such as nuclear waste disposal, airframe testing, and measuring friction and stress loads in landing gear. A team from Moog’s facility based in Solihull which specialises in test and simulation, completed this upgrade that included the installation and calibration of the machine as well as training of the university staff. The University of Sheffield has seven portable test controllers in operation and a further system has since been ordered from Moog.

The test machine upgrade incorporated a four-channel Moog portable test controller. Stuart Bibb, manager of test systems, says: “The complex control loops required for this customer’s test were configured using the advanced features and tools provided by the Moog systems. This is the kind of flexibility and support which makes the Moog Portable Test Controller stand out from the competition.”

Bi-Axial testing is complex because the specimen has to be held in the absolute centre of the machine, so that the specimen loading is perfectly symmetrical. This means the system needs to be controlled in both displacement and force, to ensure that the specimen centroid is maintained. The portable test controller achieves this through user defined channels and pseudo channels (calculation channels).

The Moog system features a host of functional upgrades including the ability to create virtual control channels that represent a defined force and translation through kinematic conversion. “Kinematic conversion makes it possible to provide a set point per DOF (Degrees of Freedom) so it becomes easier to create spectrum,” says Bibb.

“The test controller can synchronise scenarios through multiple channels and create a virtual DOF. It provides engineers with greater flexibility to create movement and improved control of the test mechanism.”

Engineers are able to configure the control loop and create feedback from the test specimen, on aspects such as average force and position. It can also create a sine wave concentrating on the end goal in less than one hour.

Mike Rennison, experimental officer for the Department of Mechanical Engineering at University of Sheffield says: “We chose the Moog Portable Test Controller because it is much more versatile and flexible than others on the market, enabling us to push the boundaries further during testing. It has also enabled us to upgrade all our equipment to meet the challenges brought to us by our customers. The portable test controller has been highly successful and we are ordering additional equipment from Moog.”

Bi-axial machine configuration

The Bi-axial machine at University of Sheffield is configured as follows. The Moog Portable Test Controller is set up to have an additional four user channels Force 1, Force 2, Translation 1 and Translation 2 in addition to the four hardware channels.

The user channels can then be used in the same manner as hardware channels to provide the facility to position the actuators for specimen mounting and apply the correct force distribution. The translation channels are used to keep the specimen location in the desired position and the force channels are used to apply the dynamic load to the specimen in perfect symmetry. Using the Translation mode, Translation 1, for the ‘y’ axis and Translation 2, for the ‘x’ axis, the position of both actuators can be adjusted by using the setpoint command for that axis. This will maintain the distance between the two actuators, moving both the actuators simultaneously, and by the same amount.

Bi-axial machine user channel configuration

The force channels are used to apply the dynamic load to the specimen, Force 1 for the ‘y ‘axis and Force 2 for the ‘x’ axis. The dynamic command signal can be applied to both axes simultaneously with a phase lag if required.