218 GDS Helpsheet - TraditionalCell Loadframe Control Parameter Tool - Instructions
SOFTWARE
TraditionalCell Loadframe Control Parameter Setup Tool
Adjusting the Control Parameter options for Loadframe Velocity Control
This helpsheet describes the parameters that can be adjusted to modify velocity control feedback for Loadframe based systems where the ini file is using TraditionalCell
This feature is only available for GDSLAB v2.7.3.0 24/07/2020 or later
If control issues are experienced when performing Stress based tests with a traditional load frame using velocity feedback control, there are some parameters that can be adjusted within the control feedback loop to improve the stability and accuracy of control. This is applicable if your ini file calls for the TraditionalCell cell type.
These parameters can be accessed from the GDSLAB main menu as shown below by clicking on Modify Loadframe Velocity Feedback. This launches a stand-alone tool which allows the control parameters to be modified.
When launched, the Loadframe Control Parameter tool displays the following window:
The effect of each control parameter is described below:
Station - select the station number that you would like to edit the control parameters for from the drop-down list. Each test station has independent control parameter settings
Current Calculated Stiffness – this shows the current calculated value of stiffness is kN/mm. The calculation is updated when there has been a change in both load and displacement greater than their Deadband values to reduce the effect of any noise in the signal.
Use Adaptive Control - double click and edit to "True" to turn adaptive ON or to "False" to turn adaptive OFF. The velocity target sent to the load frame is based on how far away the target load or displacement is from the current read value. The Velocity Factor and Velocity Cap values are applied after the control algorithm has generated an initial velocity command estimate.
If Use Adaptive Control is set to False; the relationship between distance from target and velocity set on the load frame will act the same way as it did before the adaptive control functionality was implemented in 2017, assuming both Velocity Factor and Velocity Cap are set to default values of 1.
If Use Adaptive Control is set to True then the velocity sent to the load frame will be factored according to the Current Calculated Stiffness value. A sample stiffness value of 1kN/mm will result in the same relationship as if adaptive control were not being used. If the calculated stiffness value is greater than 1kN/mm then the adaptive control will reduce the velocity to make control softer to avoid oscillation on a stiffer test specimen. If the calculated stiffness value is less than 1kN/mm then the adaptive control will increase the velocity to make control more aggressive to more quickly reach the target on a softer test specimen.
Velocity Factor – the velocity factor changes the calculated velocity rate by a constant user-defined factor. The default value is 1. Lower values will decrease the output velocities (should stabilise the control but will take longer to reach targets). Higher values increase the output velocity (will reach targets faster but may cause instability).
The Velocity Factor will act whether Use Adaptive Control is set to either True or False and will also act under either displacement or load-based control feedback.
The ideal velocity factor will likely be quite different for a given sample when comparing between a GDS LF50 and an older Triscan load frame. This is because the newer USB LF50s are significantly more responsive and have lower backlash than the older frames, and so may require a lower velocity factor value.
Velocity Cap – the velocity cap applies a maximum velocity limit to the load frame in mm/min. Decrease this velocity cap to limit the maximum velocity at which the load frame can move. The default value is 1 mm/min.
This cap can be useful to prevent control oscillation due to overshoot at higher frame velocities, while still allowing the frame control to be more responsive at lower velocities close to the target.
The Velocity Cap will act whether Use Adaptive Control is set to either True or False. it will only act under load-based control feedback.
Control Loop Deadband - double click to set the adaptive control loop dead band. The default is 0. This gives the option to set a small range around the current target within which the load frame will stop moving. The load frame will only continue to seek the target value if the difference between the current value and the target value is greater than the specified control deadband. If the control loop deadband is set to 0 then the load frame will always seek towards the target value.
Allow Forward Commands - as default set to True. If set to false this prevents the load frame from moving in the positive direction.
Allow Reverse Commands - as default set to True. If set to false this prevents the load frame from moving in the negative direction.
Compliance Deadband (mm) - double click to set the adaptive control calculation dead band for displacement control. The default is 0.003mm. The ideal value is just greater than the transducer electrical noise fluctuation. This value is only used for the stiffness estimator and is not a control deadband.
Compliance Deadband (kN) - double click to set the adaptive control calculation dead band for load control. The default is 0.003kN. The ideal value is just greater than the transducer electrical noise fluctuation. This value is only used for the stiffness estimator and is not a control deadband.
When a value has been updated a message in red will show at the bottom confirming that the change has been saved to the system registry.