LoadControl Custom Task

A custom task run a load control simulation based on calculations contact or reaction forces. .

Introduction

This CustomTask tries to achieve the load requested from a provided user defined function by adjusting the velocity of a block of rigid material points. The control is done using a non-linear PID control algorithm.^[1]

Specifying the Force

A LoadControl custom task is based on a block of RigidBC or RigidContact particles. These particle are typically placed on the edge of the specimen. The following steps are used to setup all simulations with load control. First pick a direction as 1, 2, or 3 for x, y, or z direction. Load control is limited to be along grid axes. Next create rigid particles to apply the load:

Define a RigidBC to control velocity is the specified direction or define a RigidContact material type. Do not use any setting functions in the rigid material definition.
Place a block of those material points using that defined rigid material on the edge to be loaded.
Include displacement of the defined rigid material in the global archive using dispx, dispy, or dispz for the chosen direction.
When using RigidBC particles, include reaction force for the defined rigid material in the global archive using reactionx, reactiony, or reactionz for the chosen direction.
When using RigidContact particles, include contact force for the defined rigid material in the global archive using contactx, contacty, or contactz for the chosen direction.

In the custom task, define the following parameters

Set the custom tasks direction parameter to 1, 2, or 3 for x, y, or z direction loading. Load control is limit to be along grid axes.
Set the custom task's material to the material number for the defined RigidBC or RigidContact.
Set the custom task's Load parameter to a user defined function that defines desired load as a function of time.

Prior Documentation

It does that by dynamically estimating the relationship between the displacement and load and then uses a PID control algorithm^[2] to adjust the velocity.

A LoadControl custom task needs either global contact or global reaction for the specified Rigid material depending on if contact for that material is enabled. This task will not work if rigid materials have a velocity function assigned, because these functions over-ride the load control velocity.

Theory

This custom task tries to dynamically estimate the relationship between load and displacement of the rigid material using the following equation:

[math]\displaystyle{ A_t = \text{arg}\min_{A} \left[ (1-\alpha) \frac{(L-Ax)^2}{0.5(L^2+(A_{t-1}x)^2)}+\alpha\frac{(A-A_{t-1})^2}{A_{t-1}^2}\right] }[/math]

where [math]\displaystyle{ A_t }[/math] is the estimate the relationship between load and displacement (or the stiffness) at time t, x is the displacement, L is the recorded load and [math]\displaystyle{ \alpha }[/math] is the smooth parameter. The minimization problem has the analytical solution:

[math]\displaystyle{ A_t =\frac{\lambda L x + \gamma A_{t-1}}{\lambda x^2 +\gamma} }[/math] where [math]\displaystyle{ \lambda =\frac{2(1-\alpha)}{L^2+(A_{t-1}x)^2} }[/math] and [math]\displaystyle{ \gamma =\frac{\alpha}{A_{t-1}^2} }[/math].

The smoothing provides an interpolation between [math]\displaystyle{ A_t=L/x }[/math] and [math]\displaystyle{ A_t=A_{t-1} }[/math] as [math]\displaystyle{ \alpha }[/math] goes from 0 to 1 or to interpolate between current stiffness or no change in stiffness, which controls how fast the stiffness can change. Next, the error between requested load L(t) and recorded load L is smoothed and converted to velocity as:

[math]\displaystyle{ e_t =(1-\alpha)\frac{L(t)-L}{A_t}+\alpha e_{t-1} }[/math]

The integrated error is calculated as:

[math]\displaystyle{ Ie_t =\frac{L(t)-L}{A_t}+ Ie_{t-1} }[/math]

The derivative error is calculated with Brown's double exponential smoothing as:

[math]\displaystyle{ e{\scriptstyle 2}_t =(1-\alpha)e_t+ \alpha e{\scriptstyle 2}_{t-1} }[/math]

and

[math]\displaystyle{ De_t = \frac{1-\alpha}{\alpha}(e_t-e{\scriptstyle 2}_t) }[/math]

then finally the velocity is set using the PID algorithm:

[math]\displaystyle{ V_t = K_p e_t + K_i Ie_t + K_d De_t }[/math]

Note: If the velocity has a magnitude larger than MaxVelocity than it is given that magnitude and the integrated error for that time step is not accrued.

Task Scheduling

In scripted files, a LoadControl custom task is scheduled with the following block:

CustomTask LoadControl
Parameter velocity,(number)
Parameter material,(int)
Parameter direction,(int)
Parameter MaxVelocity,(number)
Parameter Load,(function)
Parameter Kp,(number)
Parameter Ki,(number)
Parameter Kd,(number)
Parameter smooth,(number)
Parameter UpdateTime,(number)
Parameter Archive,(file_name)

In XML files, these task options are scheduled using <Schedule> elements, which must be within the single <CustomTasks> block:

<Schedule name='LoadControl'>
   <Parameter name='velocity'>(number)</Parameter>
   <Parameter name='material'>(int)</Parameter>
   <Parameter name='direction'>(int)</Parameter>
   <Parameter name='MaxVelocity'>(number)</Parameter>
   <Parameter name='Load'>(function)</Parameter>
   <Parameter name='Kp'>(number)</Parameter>
   <Parameter name='Ki'>(number)</Parameter>
   <Parameter name='Kd'>(number)</Parameter>
   <Parameter name='smooth'>(number)</Parameter>
   <Parameter name='UpdateTime'>(number)</Parameter>
   <Parameter name='Archive'>(file_name)</Parameter>
</Schedule>

These are the parameters that this custom task needs to operate:

velocity - This is the starting velocity of the simulation for the specified Rigid material in velocity units. The sign of this parameter is probably more important than its value, as long as it is something reasonable. It is needed for initializing the load control.
material - The material number of the Rigid material to be controlled (must be entered by number).
direction - An integer (1, 2, or 3) to choose the direction (x, y, or z) to contrtol for velocity and load.
MaxVelocity - A positive number that limits the magnitude of the velocity of the controlled material (default is 1000 in velocity units).
Load - A user defined function that gives the desired load (in force-time units). This task doesn't work well with discontinuous functions.
UpdateTime - The time interval between runs of the load control algorithm (in alt time units). Should be equal to or greater than the global archive time. To run control algorithm on every time step, set global archive time to zero and omit this parameter.
Archive - This command gives a file name (with extension) such as "_PID_data.txt". When used, control calculations for A_t, error, integrated error, derivative of error, and final velocity will be written to a plain text file. Plotting the results can help choose control parameters.

These following parameters are for the control algorithm. The provided default parameters don't seem to really work. Try increasing Kp until it overshoots the target, then increase Ki to lessen the overshoot.

Kp - Proportional gain factor for PID algorithm (default 0.1).
Ki - Integral gain factors for PID algorithm (default 0.001).
Kd - Derivative gain factors for PID algorithm (default: Kd = 0.25Ki).
smooth - Exponential smoothing parameter in the range (0,1). Controls how fast the dynamic relationship between distance and load is updated and also how much smoothing is done on the load values (default 0.95).

To be developed

Contact forces are corrupted with fat tail noise. A median filter would help reduce the effect of this noise.

Self-tuning parameters.

References

[1] Wikepedia.com, "PID Controller"

[2] Wikepedia.com, "PID Controller"

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