Thread: Disco 2 SLS Resurrection. Help needed.

Originally Posted by joel0407

Yeah, I'm waiting on my Nanocom to arrive. I know I can do those thing individually but I'm interested to know how the system does it automatically?

Have a look at the schematic on page 64-20 (Rear Suspension > Operation).

This is the flow when the system is inflating the bags:


And when deflating:


The schematic shows both control valves and the exhaust valve share a common manifold with the compressor outlet. If that is the case it would be possible to equalise pressure between bags by opening both the air control valves, which depending on the pressure in the bags might translate to inflating one whilst deflating the other.

The system determines how much the pressure needs to be adjusted by monitoring left and right height as measured by the height sensors.

cheers
Paul

If you want the most comprehensive description of how the system works and don't mind wading through convoluted language that makes RAVE sound like a children's bedtime story you can always read the WABCO patent application for the SLS....

You have been warned


That said it's not too bad as far as patents go and is pretty interesting reading. The "Preferred Embodiment" section is the description of the system that is covered by the patent. The Preferred Embodiment is usually stated in fairly broad terms as the intent is to prevent competitors making minor variations to avoid infringing the patent.

Originally Posted by OffTrack

The schematic shows both control valves and the exhaust valve share a common manifold with the compressor outlet. If that is the case it would be possible to equalise pressure between bags by opening both the air control valves, which depending on the pressure in the bags might translate to inflating one whilst deflating the other.

The system determines how much the pressure needs to be adjusted by monitoring left and right height as measured by the height sensors.

cheers
Paul

That's some awesome information you've got there. Thanks very much.

Just on the equalising the pressure though. If one side was down because extra weight had been applied to that side then I would expect it to be lower in height and higher in pressure. If that was the case then equalising would make the situation worse.

I just printed the WABCO patent to have a read.

Happy Days.

Some things are explained and questions answered in that Patient.

Abstract on page 1 states:

"Uneven support of the vehicle represents one cause of vehicle inclination, for example, when one wheel of the vehilce axle is standing on the sidewalk. In order to aviod air losses, a vehicle is not compensated for while the vehicle is stopped. Instead, the existing inclination is maintained and is entered into the control system as a target level. All changes which occur due to changes in load of the vehicle are compensated for by means of this prescibed target level."

Happy Days.

So after reading the Patent I am a little bit wiser but still not completelyknowledgable about the SLS.

There are 4 different phases of which the regulator (read SLABS) adjusts thebags differently. For example, it won’t adjust the suspension height duringbraking whilst in travelling phase, it won’t change the inclination of thevehicle during stopped phase, plus various other conditions specific todifferent phases.

My biggest question is what happens when one side is height and the otherlow. I could not find a definitive answer to this in the patent. It doesn’t specificallystate that the regulator will attempt to raise the low side before lowering thehigh side but it merely written in that order in the patent. It is firstly writtenthat, if load is removed and a height sensor goes out of predetermined rangethen the system will vent air. Then in the next paragraph it is written that ifload is added and a height sensor goes bellow a predetermined range, it willintroduce air.

It is also worth noting that it has a buffer which is different for eachtype of vehicle that the system is fitted to but an example of 5mm is used. I believeit is to stop the system running in a continuous loop. As the system can onlyvent or introduce air but not both, if it was to raise the right side, the leftside would also rise but at a lesser extent, at this time left side couldexceed its predetermined height. The next step would be to lower left side butthis would lower the right side but to a lesser extent. The next step would beto raise the left side. I think you are getting the idea. So to avoid thisloop, each side has a buffer to the predetermined height. Whichever side isadjusted first to the predetermined height and whilst this side will vary asthe second side is adjusted it will not move the extent of the side beingadjusted and hopefully not outside the buffer measurement where no adjustmentwill be made. If the none adjusted side does happen to move outside the bufferdistance, the loop will repeat however the system will not exceed apredetermined number of opposing control attempts.

Also interesting quote “the regulating processes performed during travellingphase take place with a comparatively greater delay, because dynamic influencewhich occur, for example, in turns, are advantageously suppressed.

I suppose the only way I’m going to answer my question is get the system working,stand on one side, wait until it adjusts then quickly move to the other sideand see if it inflates the low or deflates the high.

Happy Days.

Not sure if you picked this up, but the only time you get inclination adjustments is when the load is altered while stopped.

The patent states that in any travelling phase, which includes shunting (0-30kph) and travelling (30kph upwards), only even target height adjustments are made. So once you are moving the SLS will only adjust both sides equally.

What the SLS does is in effect is adjust the inflation of the airbags on each side of the vehicle so the springing rate/deflection is the same on both sides regardless of load distribution. Once the pressures are adjusted the bags behave in the same way as coil springs.

cheers
Paul

If I understand from the previous posts, if you load the vehicle unevenly, then at rest the system will adjust pressures to adjust the heights side to side. But, as above while underway it only adjusts both side simultaneously to maintain a target height (which the nanocom will allow you to alter).

To me this in in conflict as pressure alterations made simultaneously would mean that L & R solenoids are open at the same time and so pressure would balance out between the two sides. Added to this from the schematic I cannot understand why item 3, a flow control device, links the feed from the pump (item 7 to the R of the dryer, item 8) and the exhaust line.

Originally Posted by OffTrack

Not sure if you picked this up, but the only time you get inclination adjustments is when the load is altered while stopped.

Yep. Got that. The alterations are to the target heigh of each side independantly. The target heights for each side are set when the vehicle is stopped and the system will attempt to maintain these targets in response to applied load.

Originally Posted by OffTrack

The patent states that in any travelling phase, which includes shunting (0-30kph) and travelling (30kph upwards), only even target height adjustments are made. So once you are moving the SLS will only adjust both sides equally.

The way I understand this is not that the changes are made simultaneously but the target heights will be equal on both sides. This differs to the stationary phase where target heights can be different either side, such as when the vehicle is stopped on uneven ground.

Happy Days.