# 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.