Steering Servo Torque

Actually, it will probably require more torque to steer when stopped than at any other time.

When we did our DARPA Grand Challenge vehicle, which was a Polaris ATV without power steering, we first used a spring scale to find out how much force was required to steer the thing manually when parked on a rubber mat, the worst case. From that, the torque was calculated and a motor (a large Faulhaber servomotor with an inline planetary speed reducer) drove the steering shaft directly. The steering wheel wasn't connected to the wheels; it drove only a pot and an encoder, so even when driven manually, the vehicle was entirely steer by wire.

We put a polyurethane spider coupling on the shaft to keep road shocks from hitting the gear train too hard. That's so that if you hit a curb, you don't break gear teeth.

I'd have to go back to the records to get the exact specs, but I know that we current-limited the motor to 11A at 24VDC, and that was always enough to get the job done. Lock to lock time for the steering (which had quite a wide range) was six seconds.

Our vehicle weighed just about a ton, so you should be able to do this with less power.

John Nagle

You might need to make a simple test rig and then spin the tires up to the rpm they will be turning at 130 mph. Then test the force needed to turn the tires.The tires will be gyroscopes and more force may be needed than you think to change their axs of rotation. Be careful, as things tend to explode when things are spun faster than their design specs.

Thanks for all the good advice. I was thinking the same thing that the force to turn the tires may be greater at speed than standing still.

I'll do some research on gyroscopic effects and forces they generate.

John used an 11A @24VDC motor to turn a 1 ton ATV. I was looking at some DC geared motors.

One was 18RPM with 35 in-lbs of running torque @ .5A, and 65 in-lbs stall torque @ 1.5A at 12V. The other was a little more powerfull. I believe it was a windshield wiper motor and has a running current of 1.5A at 40RPM.

These motors weigh in around 4 lbs, and I feel they would probably be an overkill considering even the largest RC car servos will only output around 350 in-oz of torque. The smaller DC geared servo motor I was looking at will do 1040 in-oz of torque.

I plan on building my own H-Bridge driver using some MOSFETS, and wiring into a standard RC car servo board along with an external pot for the feedback. I may just layout a whole new PCB with servo board electronics and MOSFETS all together. I'm still learning about speed controls and servo drivers, but those are minor things I'll work out.

Since my profession is wireless technology, I'll build my own radio controller with two way communications and as many channels as I need for steering, brakes, throttle, engine kill, parachute, and most importantly a fail safe feature that will actuate brakes, parachute, and ignition kill in case of lost communications. I'll add some diagnostics like MPH and engine RPM that can be displayed on the controller.

Thanks again for the information. And any more info or comments are greatly appreciated.

You will find that stiffness is important too - the interaction of a gyroscopic wheel with a bump may well set off interesting oscillations. This all becomes even more interesting if a closed servo loop is involved ...

I suspect that the turning force requirement will turn out to be a relatively minor aspect of the problem.

I'd look for a non-reversable stiff actuator with no play - something like a screwed rod type.

Careful testing is called for

Dave

Do people use linear actuators for steering mechanisms? I've seen them with stepper motors. Would a servo system with feedback be better on a screw and nut type linear system as you mentioned.

I've mainly been focusing on a servo system since I understand them well, and that's what I typically see for RC steering mechanisms. As far as steppers, haven't really played around with them much. I have a basic undestanding of steppers, but haven't studied the driver circuits. I know the steppers have a lot of holding power which is good, but I imagine a high torque DC motor (with gear reduction) should be able to have just as much holding power.

As far as the mechanicals for the steering system, it will be something very rigid with no play. I see where using a linear system may have a more rigid setup vs. a rotating arm onto some steering rods. Their will be a lot of testing and tuning to achieve the right combination.

But if anyone knows of a steering mechanism that has been tested and may suit my application better, please let me know.

Thanks.

Just as a point of reference you might want to take a look at

is a MUCH SLOWER vehicle, but you might find some of the information there in the steering section useful.

Best of luck with your project

Thanks for the link. My first attempt (and hopefully last) at the steering system will be similar to a go cart. Not sure what the exact terminology is, but it will have 2 rods with adjustable round ends that will house some sort of hard rubber bushing. One end will connect to an arm which is controlled by the servo, and the other end will connect to a knuckle/spindle assembly where the tire will mount. Since I will only be turning between 10 and 20 degrees, and mostly going straight, I feel this will give me a solid front end. It can be set up very stiff depending on how tight I crank everything down. And with a high power servo setup I'm hoping it will steer fine without any free play.

This may sound dumb, but the TV show Myth Busters (Discovery Channel) often use large RC servos.

There was an episode where Jamie Heinemann used a servo on a rocket propelled Chevy Impala (you just *have* to see it to believe it). He operated the serrvo with a regular RC controller (from a chopper) I don't know the specs, but you could email them and they might let you know what to use.

The servo was surprisingly small for that sort of job - a rocket-propelled car!

:-]

Dale

Terry wrote:

Hmmmm.... the choice of car might be telling. The Impala (at least the older models, which I've driven) had non-variable assist power steering with a feather touch. You could steer it with your fingertip.

I kinda doubt that a robotic car that weighs 50 lbs will itself have hydraulic power steering as an assist mechanism, so the motorized steering has to be strong enough on its own. That pretty much rules out even the largest of RC servos, which top out at around 400 oz-in and have a maximum swing of about 160 degrees.

In any case, Jamie writes for RC Driver magaxine and a couple others. Maybe he's talked about this techniques there.

-- Gordon

That's an interesting point.

The difference here in Australia is that even cars from the 1980s often don't have power steering, and rarely I have never heard of power steering in the locally made standard Fords and Holdens of the 60's and

70's.

Cheers

:-]

Dale

Gord>

I have seen a few remote control full size cars. Some use air power or hydraulics. I do watch mythbusters and have seen their remote control cars. For my 50lbs RC car, I plan on using a dc motor in the range of 800 to 1600 oz-in of torque. If I need more I can gear reduce it further, but I feel that this will be more than enough for my setup. And if I design the front it very stiff, it should be pretty stable at top speed. Of course the tire width will be a factor, so I plan on using the thinest contact patch I possibly can.

I am most likely going to use the MC33030 servo driver IC with my own circuit for a closed loop feedback and H-Bridge to handle the extra power. I have a decent setup that allows me to make my own PCB boards (2 layers) with 10mil min spacing, so I can really play around with my design untill I get it just right. Cost only being the 2 sided .063 copper boards.

The servo motor I was going to order is no longer available from the surplus store, so I'm looking at a few others. As soon as I purchase the motor, then I will build the circuit for the right amount of power and begin some testing.

An email to the mythbuster team sounds lke a good idea. I'll look into it.

Thanks.