The ABS discussion thread

[kapilvgupta]

Originally Posted by DWind

Are we sure of this? I always thought there are 2 things to it.

1. ABS module becomes alive as soon as brake pedal is applied regardless of locking or non locking scenario and monitors continuously till break pedal is released fully.
2. This module calls another when it encounters a near lock situation to redistribute braking force force to other wheels / remove braking force to wheels encountering a lock.

Now based on the above, not all is lost. If release of braking force from pedal is more than locking threshold of the wheel then the 2nd module goes back to sleep and we are at the same braking power as a non-ABS car. No?

@DWind - I am not sure about the source of your info or I am not able to understand your classification of 'modules' properly.
Basically an ABS has multiple parts (controller, modulator & wheel sensor) and all of these work together. The wheel sensor senses wheel lock, the modulator distributes/regulates brake fluid and the controller actuates/deactivates the modulator based on input from the wheel sensors. The ABS system would not function properly if either of these systems are disabled or damaged.

A good ABS system is able to cycle upto 15 times a second and this means it can release and actuate the brakes upto 15 times per second....I can assure you that this is more than what 99% of human drivers can. Granted that there would be experts who can better an ABS system but for the general driver an ABS performance is better for most cases.

peace,
~kg

[anujmishra]

ABS is one of the mandatory and nice feature to have in cars. There is no doubt. But what I am reporting here is something unusual.

In many of the ownership thread of cars with ABS, owners experience ABS within 10 days of ownership in a very hard way. How come this is possible for those many owners experience hard braking with in 10 - 15 days of ownership.

Something like, I was going through one road and suddenly something came in front and I braked hard some sound I heard and I was saved in great way otherwise it was disaster. This is so much of co-incidence with all ABS car owners.

[Thad E Ginathom]

That is a mystery. An equally mystery is those who post that ABS caused them to have a collision, and they could have stopped in time without it. It is not possible that they could have driven that car, on that surface, in those circumstances, without ABS, so how could they possibly know?

An exception to this is the small number of conditions such as driving on fine grit, where ABS is acknowledged to increase stopping distance.

[DWind]

Originally Posted by kapilvgupta

Basically an ABS has multiple parts (controller, modulator & wheel sensor) and all of these work together. The wheel sensor senses wheel lock, the modulator distributes/regulates brake fluid and the controller actuates/deactivates the modulator based on input from the wheel sensors.

Agreed! My point is that there is more than one module with the controller deciding when to release and not release based on wheel sensor module

Originally Posted by kapilvgupta

A good ABS system is able to cycle upto 15 times a second and this means it can release and actuate the brakes upto 15 times per second...

Yes this 15 (or whatever is the system designed value) times per second is the maximum capability and will occur only when the controller encounters wheel lock signal again after a release and not continuously regardless of wheels locking up or not .

[adnaps]

Originally Posted by DWind

If release of braking force from pedal is more than locking threshold of the wheel then the 2nd module goes back to sleep and we are at the same braking power as a non-ABS car. No?

Good point ! Dont want to trust ABS - brake without locking up wheels - ABS or no ABS you are fine (Most drivers do that instinctively anyway).

Therefore when the ABS is activated , the wheel/s have lost their traction and you are already in the danger zone. In the danger zone , go gentle on the brakes - wheels unlock - resume traditional braking and it makes no difference if you have ABS or not.

Disclaimer _ The situation is a lot different in loose snow , mud etc.

[Thad E Ginathom]

Originally Posted by DWind

Are we sure of this? I always thought there are 2 things to it.

1. ABS module becomes alive as soon as brake pedal is applied regardless of locking or non locking scenario and monitors continuously till break pedal is released fully.
2. This module calls another when it encounters a near lock situation to redistribute braking force force to other wheels / remove braking force to wheels encountering a lock.

Now based on the above, not all is lost. If release of braking force from pedal is more than locking threshold of the wheel then the 2nd module goes back to sleep and we are at the same braking power as a non-ABS car. No?

ABS does not redistribute brake force to other wheels. I think there is another acronym which does that.

ABS does not reduce brakeing power: it aims to maintain it at it's most efficient point which is just before lock-up.

ABS does nothing until it is needed, and when it is, you are already in an emergency situation.

[druva]

Originally Posted by Thad E Ginathom

ABS does not redistribute brake force to other wheels. I think there is another acronym which does that.

ABS does not reduce brakeing power: it aims to maintain it at it's most efficient point which is just before lock-up.

ABS does nothing until it is needed, and when it is, you are already in an emergency situation.

EBD helps distributing the braking force uniformly.

EBD and not ABS helps in reducing the braking distance. ABS helps only in 'Anti Locking'

[Thad E Ginathom]

EBD --- that's the one! An abbreviation, of course, not an acronym: my mistake.

[dadu]

Originally Posted by druva

EBD helps distributing the braking force uniformly.

EBD and not ABS helps in reducing the braking distance. ABS helps only in 'Anti Locking'

Anti locking and helps steering away, if you keep the brakes pressed and Steering in the same straight line position (which is usually what happens during panic braking), you will hit the obstacle in front of you(and blame ABS).

[arun4242]

Let me attempt to explain how an ABS system works

The sensor - a simple mechanical sensor might consist of a hollow cylinder of say 4 cm by 0.5 cm fitted on the rim of a wheel. A solid metal ball would be placed inside the cylinder.
During normal forward rotation of the wheel, the ball would occupy the rear end of the cylinder (possibly attached by a low tension spring). During normal braking too, the ball would continue to occupy the rear end of cylinder.
However, during heavy braking, when the wheels lock up, the ball would move forward to the opposite side ie the forward end of the cylinder where it would complete an electrical circuit and send a signal to controller that wheels have locked up.

The controller - receives input from sensor and passes instruction to module.

The module - on receiving input from controller, it bypasses the manual input to brakes (in form of pressed brake pedal) and removes the hydraulic pressure in the brake lines. This results in the wheel lock up ending and the tyres start rolling again.

Repetitions of the above cycle would constitute braking under ABS.

The above explanation is a highly primitive ABS system. Nowadays, ABS uses far better tech like failsafe sensors, four individual modules, reverse cancelers etc. Nevertheless, the basic principles remain the same.

The quoted capability of 15-200 cycles per second of ABS systems seem highly exaggerated and may refer to capabilities of controller only and not of entire ABS system.

What does ABS do ???

Four things
a) Shorter braking distance compared to non ABS system (I know this is a highly debated point in this thread but shall post another reply justifying this)
b) Braking in straight line - this is because in a non-ABS car whose wheels have locked up, the car may drift or yaw and is not fully under the control of steering wheel. Whereas in ABS, the wheels continue to roll most of the time and ensure the car continues to brake in a straight line or in the direction controlled by steering wheel.
c) Steering control under heavy braking
d) Simple technique for panic braking ie press the pedal fully without worrying about lock up or pumping brakes

I shall continue in another post to avoid an unduly long post.

[im_srini]

Originally Posted by arun4242

The sensor - a simple mechanical sensor might consist of a hollow cylinder of say 4 cm by 0.5 cm fitted on the rim of a wheel. A solid metal ball would be placed inside the cylinder.
During normal forward rotation of the wheel, the ball would occupy the rear end of the cylinder (possibly attached by a low tension spring). During normal braking too, the ball would continue to occupy the rear end of cylinder. However, during heavy braking, when the wheels lock up, the ball would move forward to the opposite side ie the forward end of the cylinder where it would complete an electrical circuit and send a signal to controller that wheels have locked up.

This mechanism appears too complicated, for this to work you would need some kind of a slip-ring, no ? In almost all the ABS mechanisms that I've seen wheel speed is usually monitored through a toothed wheel (attached to the driveshaft or wheel hub) & a fixed coil-based sensor on an adjoining suspension member.

Originally Posted by arun4242

Nowadays, ABS uses far better tech like failsafe sensors, four individual modules, reverse cancelers etc.

Could you explain "fail-safe sensors" & "reverse cancelers" ? Thanks...

[arun4242]

ABS stopping distance versus Non ABS systems:

An ABS system or a Non ABS system under wheel lock up, both are dependent on sliding friction (or Kinetic friction). Static friction and rolling friction play negligible roles.

ABS system - the braking force generated would be a direct function of the area of brake pads, the coefficient of friction between the brake pads and rim surface(where sliding friction occurs), the brake pressure applied and speed of the wheel.

NON ABS system - under normal braking conditions the braking force generated would be the same as for ABS.
In wheel lock up conditions, the braking force would be a function of the footprint area of tyres, the coefficient of friction between the tyres and the ground (where sliding friction occurs), weight of the vehicle, and speed of the vehicle.
Vehicles (ABS and non ABS) are designed such that the braking force generated during normal braking is always higher than under wheel lock up conditions.

Wheel lock up - This occurs when the braking force generated exceeds the forward momentum of the car and the brake pads lock on to the rim preventing it from rolling. The forward momentum of a car would be dependent on its speed.
Practically if I press the brake pedal to the floor at a speed of 20 km, I would definitely lock my wheels whereas if I did the same at 100 kmph, the vehicle would slow down to maybe 40 kmph before wheel lock occurs
The point I am trying to make here is that brake pressure required for a wheel lock up is not a constant but varies with speed.

An ABS system by its inherent nature automatically keeps adjusting brake pressure with reducing speed so that lock up does not occur and maximum braking is achieved. It thus achieves anywhere between 95-99% of the max braking effect (translates to minimum stopping distance) achievable for that particular car.

The only way a non ABS system can achieve a shorter stopping distance is if the driver at first applies applies the correct amount of brake pressure just short of wheel lock ensuring the maximum braking force is generated. Then, as the car slows down, he keeps reducing the brake pressure proportional to the deceleration all the while maintaining the car just short of wheel lock.
If at any time he applies more than required pressure, lock up would occur resulting in a longer stopping distance than ABS, while if he applied any less pressure, he would be braking inefficiently and again have a longer stopping distance. (If a driver actually could do this, I would say "hats off Mr Schumacher")
Also the commonly discussed method of pumping brakes to reduce stopping distance is nothing but to manual attempt to replicate ABS effect. A human can never match the cycle times of an ABS and hence would not be able to achieve shorter stopping distances.

To all those, who have managed to read this post all the way till here, I would say thanks and hope I have been able to convince you that ABS stopping distances are shorter.

[arun4242]

Originally Posted by im_srini

This mechanism appears too complicated, for this to work you would need some kind of a slip-ring, no ? In almost all the ABS mechanisms that I've seen wheel speed is usually monitored through a toothed wheel (attached to the driveshaft or wheel hub) & a fixed coil-based sensor on an adjoining suspension member.

You are absolutely right. I only used my example for illustrative purposes and not as something used in todays cars.

Nevertheless, the example used by me is a real one, used by the initial ABS systems designed in the 1950s for planes by the Russkies and used today in classroom teachings of the principles involved in ABS. You can obviously make out that 1) I was one the students at the lectures and 2) I was an attentive student

Originally Posted by im_srini

Could you explain "fail-safe sensors" & "reverse cancelers" ? Thanks...

Ooops, you caught me out there. I got in aviation terms into this thread unnecessarily assuming they are also used for cars.

Fail safe sensors - implies sensors with a very high degree of reliability. More importantly they ensure that even, if one of the sensors fails, the vehicle is still safe. For example, one of the planes I flew had an ABS system where if only two sensors told the controller that they had locked up, then the controller would automatically apply the instruction to release to all three wheels by assuming the third sensor had failed in detecting lockup. Even if the the instruction was wrong, the loss in braking efficiency would be negligible, whereas if the sensor had actually failed, this action would result in making the vehicle safe.
Reverse cancelers - they ensure ABS cutout while reversing a vehicle.

Anyway, thanks for reminding me to stick to ABS as used in cars for future posts.

[DerAlte]

Originally Posted by arun4242

... initial ABS systems designed in the 1950s for planes by the Russkies and used today in classroom teachings of the principles involved in ABS. You can obviously make out that 1) I was one the students at the lectures ...

If anyone is to be blamed, it should be the Indian academicians who are keeping students in dark ages!

There is a lot of difference between what the Soviets did to their aircraft and what Germans did to their cars for preventing wheel lock. Since the early 80's, automotive ABS has had completely different sensors (electrical) and computation / control (microprocessor based) than the systems you describe. All safety systems are designed to fail on the safe side (e.g. action on component failure will not cause system failure), and redundancy is limited to what makes sense economically. There is a lot of mathematics involved at design time, not at run time, so that the computation system does not need artificial intelligence to ensure safe conditions.

That the valves in an ABS system, whether it is from Bosch, Delphi or Denso act at 15-200 cycles per second is not an exaggeration - that is exactly what they are made to do. That is what you will feel in the braking foot when ABS acts. If you examine the maths behind it, you will understand why that is done, and the maths is rather simple. Just don't assume the sensors take the decision what is to be done.

The sensors just measure, e.g. wheel RPM, and the control unit checks 4 sensors to figure out if data from any one (could be more) of the sensors represents wheel lock. The sensor does not say so directly, just examining the rate of change of the wheel RPM data indicates that. This differentiates between coming to a halt under normal conditions, and wheel lock / slipping. What is to be done when it is the time to act is pre-programmed - rapidly cycle the ABS bleed valve of the locked wheel(s) until the wheel lock condition abates. ABS will also act if only one 1 wheel or one side (2 wheels) is locking. No wheel lock = wheels rolling / not slipping = a chance for the driver to steer to safety. If slipping / skidding, the car obeys physics but not the driver's steering.

A related control functionality (not EBD) uses the same sensing and similar logic to achieve Automatic Slip Regulation - to ensure maximum torque is transferred without the wheels losing grip. A slightly more complex system (with additional sensors) is the system that controls cornering without fish-tailing.

[Sutripta]

Hi Arun,
Interesting post. Some clarifications please.

Originally Posted by arun4242

[u]
The sensor - a simple mechanical sensor might consist of a hollow cylinder of say 4 cm by 0.5 cm fitted on the rim of a wheel. A solid metal ball would be placed inside the cylinder.
.....

Can't figure out the orientation of the sensor on the wheel rim. Also, the signal is yes/ no in nature?

Originally Posted by arun4242

...
ABS system - the braking force generated would be a direct function of the area of brake pads, the coefficient of friction between the brake pads and rim surface(where sliding friction occurs), the brake pressure applied and speed of the wheel.

NON ABS system - under normal braking conditions the braking force generated would be the same as for ABS.
In wheel lock up conditions, the braking force would be a function of the footprint area of tyres, the coefficient of friction between the tyres and the ground (where sliding friction occurs), weight of the vehicle, and speed of the vehicle.
.....
The point I am trying to make here is that brake pressure required for a wheel lock up is not a constant but varies with speed.

Can't figure out why braking force should depend on pad area.
More importantly, why should the force at which the wheel starts locking up depend on speed.

Regards
Sutripta