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Need some assistance with "active" suspension

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truck man

New Member
Hi Guys

I'm not very knowledgeable in electronics and would love some help. I did a quick search here but no luck.

It's not really active suspension. What I have is an air-bag rear suspension system in a light truck. There is an on-board air compressor and tank. There are solenoids that when triggered, add or release air from the bags and the ride height changes. Solenoids are currently manually activated with a switch.

What I'd like to do is have a very simple mechanism to monitor & adjust ride height as the load varies.
It needs to be:
a. Basic
b. Reliable
c. Withstand under truck environment (can be boxed I guess)

After lots of reading, I've settled on this idea. I'd use 2 hall effect switches (one slightly above the other) mounted to the chassis and a magnet mounted to a bracket off the axle. As the load increases and the chassis drops, the magnet activates the top hall effect switch which activates a relay which turns on the solenoid that lets more air into the bags > the chassis rises > magnet moves away from hall effect switch > relay turns off.

As the load is removed, the reverse happens. Chassis rises, magnet activates the lower hall effect switch which activates another relay which turns on the solenoid that lets more air out of the bags > the chassis drops > magnet moves away from hall effect switch > relay turns off.

However I can see a problem. As the truck is driving around, the bumps in the road (and sweeping corners) will be triggering both the switches and the relays will be constantly activating the solenoids - not good. What I think I need is a delay circuit.....something like this:

The hall effect switch is triggered & it activates a timer. If the timer reaches a pre-set time (somewhere between 5 - 20 secs, ideally adjustable initially untill I settle on a time) and the switch is still triggered, the timer activates the relay which then activates the solenoids.

There are already height control systems for air bags around but they are much too expensive and much too fancy. I want this to be a very basic & reliable "set & forget" system - no user input required.

I was thinking of using something like an Allegro A1101-A1104 Hall-effect switch
and maybe a 555 timer with a power on delay circuits like one of these (maybe the bottom pulse one?)

Problem is, I don't know enough about electronics to piece the circuits together or if they are even suitable. Building the board, soldering, etc is not a problem, just need some input on whether the idea & circuits will work or if there is another simple & reliable option I didn't think off.
 

mneary

New Member
Since your load should not be changing while the vehicle is in motion (or you are in big trouble) you could activate the adjusting system only when the brake is set.
 

truck man

New Member
Sure, sounds OK to me.

But what about the actual circuit design to make the "adjusting system" work? Does what I proposed with the hall effect switch & the timer sound possible?
 

Mikebits

Well-Known Member
The air bag system should be making these adjustments for you, perhaps you should contact the airbag company and see if they have a software upgrade that is more suited for your driving conditions. I suspect anything you try will be futile at best. This is a software issue and no simple hardware modification will suffice. I suggest you contact the airbag manufacturer.

And, no, I doubt your mods would work very well.
 
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mneary

New Member
To automate a manual system, sealed switches like you might find on doors, and standard auto relays, could do the trick. No need for delays if the switches are far enough apart and the shocks aren't bouncing too much.

A simple disable on the parking brake switch makes sure it isn't on while driving, but I wouldn't necessarily leave it on all the time.

I tend to spend hours on smart solutions, but sometimes a few relays are better, especially in a vehicle.

If you can't find sealed door switches, try "burglar alarm" reed switches. You would definitely not want to leave this on all the time since these switches are NC and would draw power all the time.

Do you have to deal with salt spray?
 

truck man

New Member
Currently it's a manual system with switches. I'd like to automate it.

I don't want to put the new sensors too far apart as it will not trigger untill there is too much weight in the truck. I was thinking the chassis would move no more than 5 mm before the sensor would activate the valves to raise it back up again. I think it would move 5mm during normal driving. I watch other vehicles going over bumps & around corners and the body easily moves 5 mm. That's why I think a delay is necessary.

It's the circuit I provided links to that I'm hoping some of you electronics experienced guys could take a look at and tell me if it would work. I'm pretty sure the components would handle the under truck environment.
 

mneary

New Member
Simple timers like the ones you posted aren't adaptable to handle the intermittent pulses you'll get while driving. I really can't imagine any circuit that could handle this, actually. That's why I suggested simply turning it off using something handy like a switch on the parking brake to disable it.

If you really think a simple timer can handle the driving environment, maybe you should describe in detail what it would do. Given such detail, there are many of us who could easily come up with a schematic.
 

truck man

New Member
Like I said in the original post, "I'm not very knowledgeable in electronics" so maybe what I have in mind isn't as easy as I thought. I tried to describe "in detail" what I was hoping to achieve but I'll have another go at it.........

There are 2 x Hall effect sensors, mounted one above the other, spaced a certain distance apart -distance to be determined - maybe as little as 10mm (3/8"), maybe as much as 50mm (2")

There is a magnet mounted nearby, lined up half way between the 2 Hall effect sensors.

The Hall effect sensors output is high until the magnet approaches it, then it gets pulled to low - see the Allegro A1101-A1104 Hall-effect switch chart in my post above

The low output is IMO enough to trigger the coil of a tiny relay which can then activate my valve solenoids (solenoids draw about 120mA when triggered)

I think this simple idea will work, it's just I think it will react too quickly to changing road conditions and play havoc with my solenoids, valves and bags.

I'd really like to put in a delay of some sort slow the signal down, just like I said in my original post above. I don't know if a simple timer would do the trick, if I knew, I wouldn't be asking for help from experience electronics guys. Maybe a simple timer will work, maybe it's overkill and all I need is a capacitor that charges up (or discharges), thereby delaying the signal. Maybe I need a microprocessor (whatever that is) to do it. I have no idea, none. Please help me design it but try to keep it simple.

I chose hall effect sensors because they are particularly suited to a vehicle environment and are used by many car makers all over the vehicle - inside engines, inside brake systems, steering sensors, etc, etc. They withstand hostile environments and are reliable. I don't want to use door switches, alarm switches, reed switches or the like. They will do exactly the same job anyway so why use something less reliable?
The circuit will only be active when the ignition is on. Whatever current the hall effect sensor will draw is totally negligible, it will have no effect - none whatsoever on anything else and I don't see the relevance. But I know little so maybe I'm missing something.....
 

truck man

New Member
This is the idea I referred to in my OP. Pulses come from the hall effect sensor, eventually C1 charges and pin 3 on the 555 triggers the relay.



This is probably not exactly what I need but I thought it's a starting point.
 

mneary

New Member
This circuit will delay until you have hit a certain number of bumps in the road and then turn the relay on forever. If you put in the optional resistor shown in dotted lines, the relay will eventually turn off, probably after the load is lifted off the springs.
 

rick1216

New Member
I have worked on fully automatic systems...like on early 90's Lincoln Continental That system uses a variable resistor to signal the controller. You may be able to do an analog system using the concept of a dampened fuel gauge system....where the gauge it has a current limiting resister across it that dampens the needle moment.....which as you know it would be wildly fluctuating without dampening....then you may be able to use this stabilized input to trigger an A/D converter or an op amp circuit.You would probably want to lock out operation unless it is in park or neutral.Rick
 

truck man

New Member
This circuit will delay until you have hit a certain number of bumps in the road and then turn the relay on forever. If you put in the optional resistor shown in dotted lines, the relay will eventually turn off, probably after the load is lifted off the springs.
OK, that won't work.

I did some more reading and searching through past posts and came across this thread:

http://www.electro-tech-online.com/threads/notc-relay.90779/

It's a delay circuit for a relay & sounds like it may work for me. Could you please have a look at the circuit in post #6, what do you think? Will that timer reset if the hall effect signal pulses?
 
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adamey

Member
I have worked on fully automatic systems...like on early 90's Lincoln Continental That system uses a variable resistor to signal the controller. You may be able to do an analog system using the concept of a dampened fuel gauge system....where the gauge it has a current limiting resister across it that dampens the needle moment.....which as you know it would be wildly fluctuating without dampening....then you may be able to use this stabilized input to trigger an A/D converter or an op amp circuit.You would probably want to lock out operation unless it is in park or neutral.Rick
I would suggest something like this. There are lots of height sensors around as there are many vehicles that use air suspension (Land Rover, Ford as an example).

You should only need one height sensor for the rear on a truck - attach it to the differential. Height sensors like this are really just your normal everyday potentiometer in a rugged case.

Take the voltage output from the sensor and use a circuit to average the voltage out over time (I'm more of a digital guy, so someone on here would know a good circuit to do this). This will prevent the solenoids from constantly adding and removing air while you're driving over bumps. I'd also use another POT as your height control in the vehicle. You compare the voltage from your control to the sensor, and when there's a difference you activate the appropriate solenoids. You should have some hysteresis built in so that you need a minimum voltage difference before they are activated.
 

truck man

New Member
You suggestion certainly sounds sensible, it's just I have no idea how to do what you are describing.

If I can't get anywhere, I'll do what mneary originally suggested and somehow try to wire it up so it only works in Park. Not sure how though.....

Any feedback on this circuit linked to above? This one:




Apparently the purpose of the circuit is to delay a relay switching on. Do you think it will work for me?
 

mneary

New Member
The circuit is good. It would delay the turn-on of the relay (L1) about 20 seconds, and shut it down immediately when the load is balanced. If the switch is only on 99% of the time, the relay would not come on. You would put a circuit on each relay. Each relay would in turn operate a solenoid. It's probably impractical to try and drive the solenoids without a relay; what do they draw?

Clever circuit. There should also be a diode across the relay (anode connects to M1 Drain). It could be 1N400x. M1 should be bigger than 2N7002; you can use the cheap but popular IRF520 or BUZ101, also available locally.

What model truck? Getting a wiring diagram for a vehicle shouldn't be hard.
 

truck man

New Member
Thank you for your help. I just have a few Q's...

How would I shorten the delay of 20secs? Could I put a 2meg pot in place of R1 to vary the timing?

I measured the solenoids current draw - I think they draw 120mA when active, maybe there is a spike when they are triggered which I couldn't measure.... do you think M1 could drive them directly instead of the relay? I could measure the solenoids coil resistance if necessary.

The Hall effect sensor I was going to use has an output that switches low (turns on) when a magnetic field triggers it. After turn-on, the output is capable of sinking 25 mA and the output voltage is VOUT(sat) 215mV. It appears the circuit above is triggered when power is applied to it, but this is not the case in my situation, is it? Can I still get it to work using that hall effect sensor or should I find another one that switches high when it turns on?
 
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mneary

New Member
Ahhh....

120mA solenoids, draw about the same as automotive relays. :p You could probably do without the relays. Shorten the time delay with the pot you suggest; the time is roughly R1 times C1. Put 100k ohms in series with the pot so it doesn't put 12V directly on the gate.

This circuit could probably accept its input from the Hall Effect sensor across C1. Whenever the magnet reaches the sensor, its output goes low and shorts C1, turning off the solenoid. Put 1kΩ in series with the Hall Effect sensor to limit the discharge current.

This requires very clever placement of the the sensors since ordinary placement as limit switches is the wrong polarity. Or you could try to find the opposite polarity switches, or an inverting gate, or transistor inverter.
 
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