Multi Axis Servo Hydraulic Test Rig

Posted on: October 26th, 2012 by admin No Comments

The highest category prize in the inaugural Ford APA (Asia-Pacific and Africa) Engineering Technical Excellence Awards was won by Ford Australia’s multi axis servo-hydraulic light truck FlexMAST durability test rig, because of its many unique features.


The FlexMAST (Flexing Multi-Axis Shaker Table) rig consists of a vehicle frame under loading from 8 actuators, which are controlled by 8 Control Cubes containing a total of 64 synchronized measurement channels. Special matrix code developed by Ford interfaces to our QanTiM® simulation software to reproduce service measured tri-axial accelerations, mounted on the 4 corners of the vehicle frame. The 8 actuators provide the traditional orthogonal translations and rotations in addition to providing twist and shear of the frame, thereby reproducing 12 measured acceleration signals.


The rig was designed to perform durability simulations on the frames, bodies, body mounting systems and load boxes of body-on-frame light truck vehicles. It works by animating the motions of the frame of a vehicle buck using the set of eight actuators operating in displacement control.


The rig has demonstrated a much greater level of simulation accuracy compared to conventional car industry rigs which use bell cranks, adjustable spherical bearings and three-stage valves. This is due in part to the more linear behaviour of the hydrostatic bearings, the reduced actuator friction, the absence of bell-cranks, and the greater actuator mounting stiffnesses. This greater linearity is easily seen through the QanTiM system modelling and results in easier drivefile development, as well as more accurate final results. As an added bonus, because there is virtually no rig wear, there is no degradation in test performance with rig operation hours.


This rig has proven valuable in areas beyond the original brief. In addition to the durability work which it was primarily built to do, in which it showed excellent correlation with on-road durability testing, the rig proved valuable during investigative work on driveline dynamics problems.


To eliminate noise problems caused by backlash in conventional test rigs, Ford designed this rig with Inova actuators rigidly front-mounted to elevated stanchions and with Team hydrostatic bearings on each end of connecting links between the actuators and the vehicle buck frame. Connecting links are tubular carbon fibre for high stiffness, low mass and good damping of cross-axis vibration.


To reduce the noise generated by the servo valves, multiple two-stage Star high-response servovalves with electric feedback, operating in parallel, were used instead of the more conventional option of single three-stage valves with two-stage mechanical feedback pilot valves.


To meet the demanding requirements for load, stiffness and cross-axis load of the actuators, Inova servo actuators with hydrostatic rod bearings were used.


Ford also developed special software, which decomposes roadload vibration data into the eight fundamental motions that the rig is able to reproduce, then calculates initial desired rig responses based on expected vibration at each link attachment point on the vehicle buck frame. This software is a set of subroutines which interfaces to Ford’s existing


CaTs3 QanTiM time domain simulation software and has allowed the rig to more faithfully reproduce roadload vibration profiles.


As mentioned above, to provide the required data capture by the control system, new CaTs3 Control Cube digital controllers with 8 channels of data acquisition per controller were used. With 2 channels per Cube used for actuator management, the rig is able to collect up to 48 channels of general data for roadload correlation and other purposes.


Hylec Controls assisted Ford with the design of this rig and supplied all the components mentioned above.


Ford hopes to incorporate some of these improvements, such the use of hydrostatic spherical bearings on links and actuator ends, on other rigs.