Control Engineering

Paul Grayson

The Four Driving Functions - Brakes
December 26, 2007

Illustration by Continental Automotive Systems

BRAKES  

A company called Technical Services came up with this description of truck air brakes at work:
Brakes are a heat engine, they convert kinetic energy (motion) into thermal energy or heat.  A 400 horsepower engine can accelerate an 80,000 lbs. truck to 40 mph in 1/4 mile (1320 feet). The brakes should be able to stop the truck in 133 feet at this speed and weight, making them 10 times more powerful than the engine!  A full stop from 60 mph may raise brake drum temperatures to 600 degrees F, about the limit for safe operation.  If the brakes aren't set right, or the load is not distributed properly, some drums might reach to 800 - 1000 degrees F, definitely dangerous.  Brake drums increase in diameter about 0.01" per 100 degree temperature rise.  At 600 degrees the brake drums are 0.055" larger than at 50 degrees. This will increase the brake actuator stroke about 0.40", call it half an inch. The question is: Do you have that much additional stroke available? 

You can see how some people can makes a life's work out of studying brakes, how they work, and how to make them better so that I can just buy them when I need them.  They obviously are a part whose design has evolved for quiet some time, use special materials, and have a shape based on function.

First thing you want an automatic guided vehicle to be able to do is stop, so careful consideration of the brake system is a good place to start. While there are several brake systems you may find on the vehicles that you automate.  One of the most clever designs coming into use soon is the electronic wedge brake (EWB).  Continental Automotive Systems acquired the design when they bought Siemens VDO.  In the electronic wedge brake a tiny 12 VDC motor drives a wedge plate that in turn captures the vehicle's momentum to power the brakes.  This is a design that had its roots in aircraft brakes and has been adapted and refined for highway brake-by-wire systems.  It has just finished cold region testing on modified production vehicles and should start showing up in car show rooms soon.  Compared to other brake systems the electronic wedge brake system seems to have all of the advantages.   Air brakes on trucks in comparison work a bit differently.  AIM's technology demonstration vehicle is currently outfitted with its original hydraulic drum brakes and the team is looking at what braking technology is available to upgrade them to something better. 
 
PARKING BRAKE
Once the vehicle is no longer moving, you want to keep it that way so a parking brake is a good idea.  The electronic wedge brake can function as both the service brake and the parking brake.  Each wedge brake operates individually so in the case of a vehicle with six wheels, this gives you six independent ways of stopping the vehicle and holding it.  Once the wedge brake is set it does no take any power to keep it that way.   On AIM's vehicle the parking brake is set either manually or by a 600# spring.

BRAKES KEY TO OTHER FEATURES
With only four driving functions available to control a vehicle, all the clever things that an intelligent vehicle does boils down to some combination of those four functions working together.  It turns out the brakes are a major player in many of the systems that are producing new vehicle behaviors.  Stability control, traction control, ABS, and many other of the modern safety features on a car are actually things the brakes are doing.  Now, with newer brake control designs, brakes can be controlled independently and rapidly by computer in response to new sensors included on board.  Tone rings in the wheels tell the computer if an individual wheel is turning and how fast.  The first tire to discover black-ice (suddenly loose traction) signals the brake to slow that wheel down and alerts the engine control to go to idle - in spite of the fact that the driver may have pushed the accelerator to the floor as was done in a ride along demonstration of the system I was being given.  Individual wheel braking can be done in a pattern that will prevent roll over, can steer the vehicle in the direction the wheels are pointed, and a variety of other clever behaviors that are helpful in maintaining control of the vehicle under adverse circumstances.

BRAKES / THROTTLE 
Brakes and throttle can each be applied from 0% to 100% but need to work opposite of each other.  Rather than drive with "two feet", which would leave the possibility of conflict, these two functions can be de-conflicted by combining them into one control that goes through a range of 100% brakes to 0% (brakes and throttle) to 100% throttle.  


Driverless vehicle at the Nov. 3, 2007 DARPA Urbn Challenge, Victorville, CA.  Photo by AIM Team members Linda and Tom Graham.

GO ROBOTS !

Paul F. Grayson - Chief Engineer
AMERICAN INDUSTRIAL MAGIC, LLC
Racing to build technology that saves soldier's lives.
390 4-Mile Rd. S.
Traverse City, MI 49686-8411
(231) 946-0187, (231) 883-4463 Cell
pgrayson@aimagic.org
http://aimagic.org
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Posted by Paul Grayson on December 26, 2007 | Comments (0)