Electro-mechanical Brake Actuator

I'm working on a next-generation solar car and am investigating
electro-mechanical braking.
Reasons are numerous but include the ability to integrate ABS for
maximum effectiveness of brakes and to preven tspectacular results
during vehicle performance testing at scrutineering. :-)
The approximate actuator force required is 2kN if the numbers on the
back of the envelope are right.
Low power consumption and mass are primary constraints; after the
obvious safety one!
A geared motor with worm drive uses no power unless it's moving, so
that's good.
Coils will require much more current but can act faster.
I'm open to suggestions as to the type of actuator to use.
Force feedback on the pedal would be wasteful of energy. I'm
thinking of using accelerometers to measure actual deceleration and
match that to the pedal position by varying the braking force; at
least until the onset of brake lockup.
A progressive spring on the pedal should give enough "feedback";
though I can imagine using the dive under braking to increase the
base return force at the pedal.
Reply to
Bernd Felsche
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I'm missing something. An average hydraulic brake might apply 800 psi to a slave of about 1 sq in piston surface area. That's human power. Modulating that braking with ABS involves bypassing the slave. This seems like a lower power faster method than your electromechanical mode. So I am missing some part of your requirement. Wonder what it is?
Brian W
Reply to
Brian Whatcott
If this vehicle has a human cargo, or operates in the presence of people then fail safe is mandatory. You will need proportional control of the force, unaffected by wear. If you skid and start facing sideways your accelerometer will get the wrong idea. For ABS, you monitor wheel speed, and look for the very rapid deceleration of a wheel locking up. If you went for electrical actuators, you would have to design a spring applied break with electrical relive, so designed that cutting the electrical supply would allow the spring to apply the break.
Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Reply to
Jonathan Barnes
[snip]
Sorry; I don't think in psi... the piston diameter will determine the force if hydraulic pressure is used as a guide. A quick check on pressure conversion and typical area puts your clamping force at about 10kN - which is by my reckoning is way too much in this application because of virtually assuring wheel lockup whenever the brake is applied.
The clamping force on the caliper needs to be around 2kN for the desired braking effect.
ABS requires a secondary power source to re-apply pressure when the wheel starts psinning again. That means some sort of pump and pressure accumulator.
That pump and accumulator weigh quite a bit; I doubt I'd be able to get anything under 2kg in mass. That'd be almost 1% of kerb mass and a 1% increase in rolling resistance; all the time. 1% extra energy required.
I'm not even considering the electrical power consumed by the pump because I have a hunch that the electro-mechanical brake will consume about the same amount of power as the ABS pump.
The actuator I'm looking for isn't hydraulic or electro-hydraulic, but electro-mechanical; e.g. a fast stepper motor on a worm gear.
Conti-Teves and Bosch have been working on electro-mechanicalk brakes for a number of years now, but the literature appears scant on detail.
Reply to
Bernd Felsche
{top-posting fixed]
[snip]
I'm allowing for independent brake circuits for power; and some NiCd cells near the actuator to allow for a couple of stops if main power goes.
In the first instance, I'm controlling via the output variable, which is vehicle speed change.
That is what you'll get by equating brake pedal travel to deceleration. There's the proportional bit; unless you're interested in making a system overly complex by trying to compensate for the dynamic effects ahead of the corresponding error signal (deceleration != set-point) being detected.
A fail-safe mode of the controller will have to determine nonsensical data from the accelerometer and fall back to a mode where more pedal pressure means brake harder and less pressure means brake less. Similar to what you have in hydraulic brakes in a conventional motor vehicle of the past century; sans pedal feel.
Only if there's only one accelerometer. There have to be at least two in order to account for the slope of the road. If there's any camber on the road as well, then you need three. A yaw-rate sensor is desirable.
Acceleration in the original direction of travel cannot be discontinuous. It means working out two main vectors; that of gravity and that of vehicle velocity change. If the initial vehicle direction changes, then without compensation, the system will tend to apply more braking force because the deceleration component in that direction will be reducing.
Also remember that there's a driver on board. It's not an autonomous vehicle.
Yes. You can also look at the deceleration to check that it's not beyond a level where lockup can be expected. And you can use the acceleration figure, integrated over time, to determine vehicle speed during braking to control the maximum amount of slip. Those are all options.
No need for that. Conventional hydraulic brakes in motor vehicles have no such _undesirable_ facility.
Redundancy and reliability is how safety is achieved.
Consider that if a car had failsafe hydraulic brakes that locked up on line failure; on one wheel at speed.
Reply to
Bernd Felsche
As you like :-) ( I have sniped a bit )
what movement is required
Very heavy, and how is the rotary output used to produce the force on the break pad ?
A couple of stops ?? if you louse power the car must stop and stay stopped till the problem is fixed.
No it's not... your output variable is wheel torque resistance. this may or may not effect vehicle speed, the relationship between vehicle speed and applied break force is not predictable in the real world.
the relationship includes tyre grip, slip ratio and lots of other terms you would be hard pressed to measure under controlled laboratory conditions. in the real world you have no chance.
There's the proportional bit; unless you're interested
Real smart.... just like a computer game, except " game over " is a bit different.
and fall back to a mode
BIG assumption that the vehicle is travelling in the direction it's pointing
a BIG button to tell the computer to F**k off, lets hope the driver remembers to press it in an emergency..... but then how does it link the break pedal sensor to the actuators ?
Lock up expected... How the hell do you know,,, a little patch of water on the road and your system gets it all wrong.
And you can use the
You had better armour all your signal cables to be as tough as hydraulic lines, make any joint very waterproof, and have redundancy.
You forgot mechanically robust, and in the case of electrics, able to keep water out.
Electronics have curios failure modes, unpredictable, and software is worse.
Look at the air breaks on lorries, have you seen the great streaks on the road a trailer with a failed line leaves.
Hydraulics on cars are both dual system, and required to have a mechanical back up ( hand break )
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Reply to
Jonathan Barnes
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800 pounds per square inch bearing on one square inch is 800 pounds so 800 lb X 0.455 kg/lb X 9.81 newton/kg = 3600 N about....
Take a look at the usual hand lever operated hydraulic master on a motorbike, feeding a disk calliper/slave cylinder.
If this won't come out light enough, a Bowden cable is the next lighter approach, I suppose....
Brian W
Reply to
Brian Whatcott
Not true. ABS reduces the effectiveness of brakes to allow an unskilled driver to steer while braking.
The scrutineer would be an unskilled driver who is also unfamiliar with the car, right?
If you assume disc brakes.
Discussion so far clearly indicates the vehicle has relatively little mass. I'd take a hard look at a severely cut-down version of electric trailer drum brakes, with a mechanical backup pushing the same shoes. They are naturally self- energizing; a coil fixed to the primary brake shoe attracts and drags on an iron disc attached to the inside of the drum face. The resulting force on the coil housing pushes the shoe into the drum, and the reaction force from that pushes the secondary shoe into the drum. When they're new and clean, they work okay. As they wear, the magnets get covered with iron filings and dust. How long does it have to work well?
Interrupt the electrical power to modulate the brake. Caution; magnetic remanence (I think that's the right word) will keep the brake on for some time after the power is cut, limiting the ABS cycle frequency.
Given the ABS algorithm and sensor requirements that have only been sketched out here, I'd be inclined to adapt an existing control computer, rather than homebrewing one.
It also stays in position when you remove the power, which may not be so good.
Just use a _really_ strong return spring on the pedal, hook it to a rheostat, and forget the ABS.
-Mike-
Reply to
Mike Halloran
Sorry; I didn't notice the tiny piston size in your posting. Caliper pistons are usually a bit bigger than 1 square inch. The piston's I've seen have been about 40mm diameter for a motorcycle.
Doesn't provide ABS. Bowden cable is also NOT reliable at the sort of forces required to stop a car unless it's very strong cable and the lever is moved a long distance (as in hand-brake lever).
The reason for ABS is because the tyres for a solar car are very thin to minimise hysteresis (rolling resistance); not much rubber at all so any significant lock-up destroys the tyre immediately.
P.S. Please use Usenet standards when quoting. Putting /// in quoted text is very unconventional.
Reply to
Bernd Felsche
Hmmm...so bowden cable isn't reliable enough for stopping cars except when it's used for stopping cars. Gotcha! :-)
Good luck with the project!.
Brian W
Reply to
Brian Whatcott
In this case, ABS is deployed to avoid wearing through the thin rubber. I thought that was obvious from the solar car application, but maybe not to those who have not thought in that paradigm.
Irrelevant. Scrutineers *never* drive. A scrutineer would never risk damaging a team's vehicle because they could easily trash a million bucks worth of hardware.
One of the team drivers operates the vehicle during the performance testing. The driver who's quite experienced and best at braking, usually.
Yes; hopefully less than 250kg, including driver.
8 to 9 hours between maintenance. Usually little application of brakes.
I'm just a little worried about the binary nature of the trailer brakes. :-)
And a lot worried about the current drawn!
Then there's the mass of the brakes to consider. I don't think our little team is going to have the resources to bake our own carbon brake drums, so steel is the next best thing. I may well be able to shrink an aluminium casing onto a thin "braking ring" of steel (or steel into aluminium) for heat dissipation and stiffness.
Corrosion would have to be watched; but the life-cycle of the vehicle is short and it'd operate in "dry" conditions most of the time.
Existing ones rely not only on a large number of sensors that aren't exactly light, but are usually designed for 4-wheel applications. The better ones use yaw sensors and accelerometers as well.
And to cap it off, the conventional ABS is designed to operate valves and a pump that maintains pressure in an accumulator.
Good for a parking brake. :-)
A backup battery, equivalent to the ABS's accumulator holds reserve power for several full brake operations. The car would never use the reserve for motive power.
Which would mean that I'd have to risk stopping to change a tyre if a wheel locks up during braking; or use tyres that have higher rolling resistance.
Reply to
Bernd Felsche
Parking brakes are only used to stop a car in an emergency; when main brakes have failed.
0.2g stop is all that's required to be delivered by a parking brake and only from lower speeds, IIRC which means an "eternity" in an emergency from high speed. Not that the solar car will be going very fast; but it'll still have to do better than typically 0.38g (average) stops.
We had enough trouble meeting that requirement with the non-designed brakes of an earlier vehicle.
Typical hand-brake movement is 10 to 20 cm, which results in about 1 to 2 cm of cable movement. Foot brakes can work with less "stroke" because the force applied can be greater.
Cable stretch with repeated use is problematic partly because of the limited effective displacement of the cable.
Does anybody remember cable brakes in the early VW Beetle? I've heard nothing good about them; requiring constant adjustment.
Reply to
Bernd Felsche
Will do.
BTDT.
Some are OK... well; I've only found one that's really a candidate and that's the Magura Gustav M which has a steel disc and is the only one approved for tandem MTB.
As with the motorbike hydraulics, it doesn't make ABS easy to implement. I've download a few SAE papers to absorb over the next couple of days.
We definitely locked up; real good. Finished a bit sideways. If you were there in October, you might be able to figure out which vehicle with the cheeky grin did that! :-)
Rear brake will be separate; Bowden cable acting on drum. Fail-safe in case of front brake failure. Yes; I know that that presents a stability issue if the driver gets too heavy-handed.
ADAMS? Wassat? I'd bear it in mind if I knew what it was. :-)
Yes... and perhaps snap the chain or break the drive-shaft in the process. :-( Well, maybe that's an exaggeration, but braking torque can be far greater than drive torque from a "3kW" motor.
I had thought of it. Great if you have a wheel motor. Our total budget may just buy a wheel motor.
Reply to
Bernd Felsche
Check out go-kart brakes
Check out hydraulic brakes on motorbikes.
Do not go near mountain bike disc brakes.
I am surprised to hear you are worried about locking your tyres, I don't remember that as an issue on our solar cars (there again we retired with brake failure once). If you are braking all wheels then use a brake proportioning valve (or some other scheme) to keep the force low in the rear, then you'll get understeer if the dummy locks them.
If you were entering the World Solar Challenge Engineering Excellence Race then it would be worth developing an ABS, maybe. If you are desperate to have ABS then bear in mind that ADAMS comes with an ABS module which you could use to set up your control algorithm.
Of course if you are feeling cunning you could use your drive motor to prevent that wheel from locking up, sort of traction control in reverse.
Cheers
Greg Locock
Reply to
Greg Locock
"Bernd Felsche" wrote in message
If all you want the system for is to prevent flat spots...
1, You do not need full ABS, just a system to relive break pressure when a wheel rotation stops, ( but not when they all stop :-) ).
2, An inductive sensor can pick up teeth on the edge of a disk, this can detect a wheel stop, a few extra components including a sounder a big red LED and a P3 battery would provide a lock up alarm.
An alarm system would only need to weigh a couple of hundred grams, and would have no safety problems.
Could the driver react quickly enough to prevent a flat spot with this aid?
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Reply to
Jonathan Barnes
Well, that's the biggest risk. I observe that in a panic that very few will actually try to steer around an obstacle to exploit the strength of ABS; rather the exploit its weakness which is the greater than ideal stopping distance.
You mean vent the fluid? I can't think of anything more frightening than the pedal suddendly going to the floor under emergency braking.
Well, nothing more frightening pertaining to braking:-)
A stop is already too late. You have to reduce braking when the wheel decelerates too quickly.
You mean pump the pedal like crazy if it goes all way to the "floor"? Hmmm... do I have enough people who think ill of me?
Yes; the method could work but it won't be popular.
Reply to
Bernd Felsche
I've got it! The low-mass, low-energy anti- flat spot solution, that is.
Assuming you've got spokes or wheel teeth or barcode on the rims, and optical or magnetic prox sensors or something to detect them, you use simple hydraulic brakes, and program a PIC to watch the bars and stuff go by, and when it detects an incipient lockup, say as a rapid reduction in edge rate, it just pumps a few milliamperes through the driver's thigh muscle to reduce the applied pressure.
"Here, son, shave your leg and apply these, er, anti- lockup patches.."
Okay, there might be some regulatory hurdles if you want to put it in production, but you're just making the one car, right?
If there's no existing rule that bans it, there will be the second time you try to use it, but it's good for one race...
;-)
-Mike-
Reply to
Mike Halloran
...
gotta keep the imagination fired up: venting to a sprung accumulator that takes a few ccs of fluid with volume proportional to applied pressure say 200 - 800 psi. A solenoid opens the path, a one way valve returns the fluid around the valve.
Brian W
Reply to
Brian Whatcott
If they don't steer round ( given that that's an option :-) ), they probably don't have the skill to do better than ABS in a straight line.
Neither can I.... block the pedal and then *reduce* the pressure on the break side, Just like ABS, but without power to re pressurise. Fix it so it resets if the driver reduces the pressure on the pedal, that way if he pumps the pedal you have a crude human powered ABS.
I meant to suggest a lock up detector ( rapid wheel deceleration ) and a stop detector combined, same sensor, just a slight difference in the logic components.
No, just if you overdid the breaking an alarm sounds... It's then the drivers choice, back of the breaks or risk a flat spot. The alarm would have no connection to the break system.
Of course you could wire the system to an electric pad to produce a muscle spasm in the drivers leg :-)
Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Reply to
Jonathan Barnes
Why not use the electric motor(s) to brake? Or is this for backup? If so, why the need for ABS?
Reply to
Pat

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