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Phillip Giliver | Summer 2016

Background In Professor Chris Gerdes's vehicle dynamics laboratory, researchers are trying to develop control theory techniques that allow autonomous vehicles to behave like the very best drivers in the world. Currently, the lab has three fully operational vehicles that operate at the limits of traction.

Project MARTY, a highly modified 1981 DeLorean DMC-12, is a vehicular testbed that allows researchers to study the limits of sustained oversteer, a driving technique known as “drifting.” In this driving style, being able to balance acceleration, braking, and steering in such a way that allows for predictable control is imperative for safety applications in future vehicles. This summer, I worked to integrate a “brake-by-wire” system (a system that allows for computer-controlled wheel braking rather than purely hydraulic actuation), designed by brake manufacturer Brembo, into MARTY.

NOTE: Much of the project is currently protected under a non-disclosure agreement. As such, particular details have been omitted where necessary.

Why Brake-by-Wire?

Brake-by-Wire System Design Goals In the Coming Weeks…

Sources

Brake-by-Wire System Design in Project MARTY

A professional driver exhibits sustained oversteer in his drift car.

•  New brake lines need to be installed in the car.

•  All hydraulic lines need to be pressure tested.

•  New brake calipers need to be installed.

•  Brake-by-wire hardware mounting panels need to be manufactured.

•  Brake-by-wire hardware needs to be secured in the car.

•  Final electrical harnesses for the four corners need to be manufactured.

•  Code which was written for the testbench setup needs to be adapted to work for all four corners.

•  Test the brake-by-wire system at the track!

•  Code developed in Simulink in MATLAB 2012b

•  Dallas News, “Formula Drift event comes to Texas Motor Speedway,” http://bizbeatblog.dallasnews.com/files/2013/09/IMG_4153.jpg, August 2014

•  Ars Technica, “Stanford teaches a self-driving Delorean to drift,” http://cdn.arstechnica.net/wp-content/uploads/2015/10/Stanford_MARTY_029-640x360.jpg, October 2015

•  All photos of MARTY are my own

•  Physical packaging: With only CAD files as reference, I worked to package the braking hardware in the car in discrete, functional locations. Key design criteria were certain mounting orientations, maximum distances between certain pieces of hardware, cable strain relief, vibration isolation, and accessibility for maintenance.

•  Hydraulic system design: In addition to integrating new hardware units that allow for brake-by-wire functionality, new calipers, rotors, and pads will also be introduced. As such, the hydraulic brake lines needed to be replaced to accommodate for these changes. A key feature of the hydraulic system is the use of quick-disconnect fittings that allow for the brake-by-wire system to be easily bypassed, if necessary.

•  Establishing electrical / code functionality: In order for MARTY to command braking forces from the brake-by-wire system, a structurally robust electrical connection must be wired into the hardware units, as specified by Brembo. Once wired correctly, the communication protocol must be established. The brake system talks to the car through a protocol called CAN (Controller Area Network), which we program through a MATLAB software package called Simulink, a symbolic programming environment. While we only had time to establish working code for a testbench setup (pictured) this summer, we hope to build on the work done this summer to program the full brake system for the vehicle.

Brake-by-wire allows for precise control of braking forces in each of MARTY’s wheels. In diagram A, throttle and steering are the only two precision-controlled inputs that determine vehicle trajectory and sideslip (angle of the drift) during a driving maneuver. In diagram B, however, it is clear that braking force at the wheel causes a force on the car and induces a moment on the vehicle. By having this third input, MARTY will have an additional, precise mode of generating longitudinal tire force. Ultimately, this allows for more flexibility when performing driving maneuvers and could be applicable in a variety of emergency situations.

A) A free-body-diagram of MARTY without controllable braking forces (throttle and steering are the only inputs).

B) A free-body-diagram of MARTY with controllable braking forces, made possible through brake-by-wire.

From left to right: i) The mounting location of the rear brake-by-wire units, as seen from the back of the car. ii) A closer look at the rear mounting location, with hydraulic lines passing through a laser-cut duron prototype of the brake-by-wire hardware panel. iii) The front brake-by-wire mounting location in the front trunk of the car (actual hardware units redacted, due to an active non-disclosure agreement). PhD candidate shown for scale.

From left to right: i) The rear brake lines, in “non-bypassed mode.” In this operating regime, the brake-by-wire system can actuate the brakes. ii) The same rear brake lines in “bypassed mode.” In this operating regime, the brake-by-wire system is inactive, and braking happens through traditional hydraulic actuation at the master cylinder.

i) The MARTY brake-by-wire testbench, where we worked to establish electrical and code functionality (hardware units have been redacted). One of the laptops we use to communicate with the car’s main computer is pictured in the foreground.