Tech Articles

Tuning SDI Braking Scale Factors in Adams/Chassis


Tech Articles ID    KB8018855
Status:    Published
Published date:    10/28/2009
Updated:    07/12/2014
Reported In:   Adams (Not Release Specific) - Adams Chassis
 

Abstract (Question)

How do I determine appropriate values for the sdi_braking_scale parameter used for SDI simulations in Adams/Chassis?

Description (Answer)

In order to obtain reasonable controller performance when using the Smart Driver controllers in Adams/Chassis, it is important to tune the sdi_braking_scale parameter in the braking subsystem. This must be tuned such that when Smart Driver requests 100% brake torque, the model is returning the brake torque expected by the controller.

 
This article will outline a general procedure that can be followed to obtain a good estimate of what this scale factor should be set to in order to get the expected controller performance. Since there are two braking models employed in A/Chassis, we will discuss each independently.
 
Default Brakes
 
When using the default brake model, the brake torque (per wheel) is generally defined by the following equation:
 
            brake_torque = 0.5*sdi_braking_scale*max_sd_output*brake_bias*wheel_radius
 
where:
            brake_torque = torque applied at individual wheel
            sdi_braking_scale = user input to scale brake torques
            max_sd_output = max braking applied by smart driver (usually 100)
            brake_bias = percentage of brake torque applied to front axle
            wheel_radius = radius of wheel
 
The objective is to set the sdi_braking_scale such that the brake_torque equals the max_brake_torque input to the smart driver vehicle parameters array.
 
max_brake_torque_front = brake_bias*total_brake_torque
max_brake_torque_rear = (1-brake_bias)*total_brake_torque
 
Note that total_brake_torque is currently set by default to 2.5e6. This number can be adjusted by customization to the smart driver test rig if the user has calculated or measured this data themselves.
 
We can see that the scale factor per axle will be:
 
brake_bias*total_brake_torque = 0.5*sdi_braking_scale*max_sd_output*brake_bias*wheel_radius
 
(1-brake_bias)*total_brake_torque = 0.5*sdi_braking_scale*max_sd_output*(1-brake_bias)*wheel_radius
 
We can simplify this into a single equation since the brake_bias term cancels out:
 
            total_brake_torque = sdi_braking_scale*max_sd_output*wheel_radius
 
            sdi_braking_scale = total_brake_torque/(0.5*max_sd_output*wheel_radius)
 
For the default value of total_brake_torque, we have found that a value of approximately 160 is a good starting point.
 
 
Simple Brakes
 
When using the simple braking model, the brake torque is defined by the brake pedal model used. However, the methodology employed is the same as for the default brakes. Determine the sdi_braking_scale required such that when full brakes are applied by smart driver, the max braking torque will match what is provided by Smart Driver.
 
Depending on the pedal model and options used, the equations will differ somewhat, but the following approximations will assist in calculating a reasonable starting value:
 
brake_torque = cyl_press*cyl_eff*cyl_area*brake_factor*pad_radius
cyl_press = brake_torque/(cyl_eff*cyl_area*brake_factor*pad_radius)
 
cyl_press = function(sdi_braking_scale,sd_max_output,pedal_scale)
pedal_scale = mcyl_eff*pedal_ratio/mcyl_area
 
 
where:
            brake_torque = torque applied at individual wheel
            cyl_eff = front/rear cylinder efficiency
            cyl_area = area of cylinder of interest
            brake_factor = 0.75
            pad_radius = distance from wheel center to brake pad application point
            cyl_press = master cylinder pressure
            mcyl_eff = master cylinder efficiency
            pedal_ratio = brake pedal ratio
            mcyl_area = master cylinder area
 
Using these equations, it should be possible to calculate the optimal value for sdi_braking_scale for the particular simple braking model used. Keep in mind that since the brake bias is not included in the above calculations that the braking scale calculated may differ for each axle.
 

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