Re: The ABS discussion thread Quote:
Originally Posted by Sutripta Determining that a wheel hes locked/ about to lock in real time is easier said than done. This requires a multipronged approach. ... if one knows the vehicle speed with confidence (and confidence increases with data), as a starting point, and as a cross check, one can check individual wheel speeds against it.
Of course it is not the only strategy. In fact it is a very minor part of deciding whether a wheel is locking up or not. But vehicle speed is computed.
What you are saying, if I have understood you right, is that the ABS algorithm never ever computes vehicle speed. Is that correct? ... |
Sure, a lot of different approaches can be taken, especially ones that feel academically sound. What is actually implemented practically are simple plausibility checks - quite simply in the interest of keeping costs down. One would definitely use vehicle speed for comparison IF there are multiple point associated with vehicle speed for specific ABS behavior. That is, IF ABS behaves differently at different speeds. It doesn't.
With 4 signals coming in, AND plausibility check on each sensor being done continuously, one doesn't need to. Even ASR / ESP etc. are able to work without vehicle speed. To put it simply, these systems act iteratively, and cross checks action versus effect. Simpler methods prevent cascading of faults (KISS principle) - in this field liability cost is humongous! Quote:
Originally Posted by im_srini If I understand this right, the ABS kicks in just when the wheel is about to lock, correct ?
That is, when it's angular velocity ( as determined from pulses from the reluctor rings on the individual wheels ) approaches 0. ... |
No. "Approaching 0" is distinctly different from "has a deceleration profile that can result in wheel lock". Quote:
Originally Posted by im_srini ... In a very naive implementation, won't the ABS relieve brake pressure just as the wheel ( & the vehicle ) comes to a stop ?
So how do you decide when to stop ABS-ing, I mean, at some point you have to decide to stop ABS-ing otherwise you're never going to stop, correct ?
Is 'all wheels not rotating' a good enough indication that the vehicle has come to a stop ( seems an invalid assumption to me ).... |
No. It is continuously releasing pressure (release-hold-release-hold...) till that point. Well, it doesn't know that the wheel and the vehicle has come to a stop. Yes, "all 4 wheels coming to a stop" is a good enough indication (apart from the driver taking foot off the pedal) - with a couple of riders:
* All 4 wheels stopping could mean all 4 wheels locking, so it is not a reliable deduction method
* The braking action does result in changes in the deceleration rate (it is continuously computed), which indicates to the ABS that it is no longer required to act Quote:
Originally Posted by im_srini ... Also, does the ABS motor actually modulate brake pressure ( as in threshold braking ) or is it that it just pulses ( relieves pressure & then pressurizes ) at some pre-defined frequency ? |
No 'motor' - it is just a set of electronically controlled hydraulic valves.
The latter. More precisely, relieves pressure then does nothing (the line pressure is still high as the brake pedal is still pressed). There is a predetermined frequency - but duty cycle changes based on wheel speed. As the wheel slows down, the hold period increases to improve braking. Difficult to figure out from the pedal pulsation, since circumstances are not conducive for accurate observation when ABS acts! Quote:
Originally Posted by Viju ... in relation to a reference velocity. ... |
Missed this earlier. Is there a "reference velocity v/s wheel slip" map somewhere? The usual algorithms don't need any. For cars (I haven't checked for bikes), the knee points (at what d(WheelDeceleration)/dt to do what) is heuristically calculated and stored at track testing time - different for different cars - but nothing to do with slip and velocity.
Last edited by DerAlte : 17th August 2015 at 21:01.
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