Completely OT

[bishbosh]

Mods - if I am taking the proverbial then kick this into touch.

I am going to fabricate scale hydro steering for my 1/5 buggy. I have the mechanics worked out using a master cylinder with the piston controlled via a lead screw and electric motor. However, what i need is some clever electrickery to control the electric motor so it behaves as a servo would when the transmitter stick is manipulated - in particular, I meed it to return to centre when the stick is centred.

Is anyone out there an electrickery whiz? Might be better to take to PM to avoid upsetting the management though!

Thanks in advance.

[landroversforever]

Need X-Eng-Si on this Charles!

[MECCANO]

I would start by looking at the elecronics of servo control board. What you need is this with a more beefy motor control side. (theres probably stuff for this sort of thing in the robotics side)

If you have taken one apart it is simply a POT that operates with the gears to tell how far the motor has moved away from the centered position. i beleive the pot simply modulates the centered pulse width to bias the motor back to the centre

Essentially ( if the servo board was beefy enough you could put the pot on flying leads mounting it so that its centered with your steering at the acting ram end on the axle( or how ever you plan to do it), and simply desolder the existing servo motor and attach your new motor. I'm sure your hydrualic motor is alot bigger than a regular servo and will probably fry the board in seconds.

Hope that makes some sense.

[MECCANO]

This Might help to explain what i'm on about. Its got pictures and everything

http://homepages.which.net/~paul.hills/Circuits/PowerServo/PowerServo.html

[landroversforever]

Could you run a standard servo pot working a relay of some sort?

[bishbosh]

Thanks guys. What I need to learn is if a pot from a servo can cope with multiple revolutions of the output shaft as I need the screw drive shaft to rotate around 40 times from lock to lock (assuming a 1mm thread pitch) In a servo, the shaft only turns about 180 degrees. So the pot needs to know how many turns to go back to get to centre from anywhere across the steering range.

Not really my area of expertise (understatement!! ) so really need a grown up for this one!

EDIT: Just discovered that multi turn pots do exist. Now to understand how to use them!!

Edited April 29, 2011 by bishbosh

[Hybrid_From_Hell]

Mods - if I am taking the proverbial then kick this into touch.

I am going to fabricate scale hydro steering for my 1/5 buggy. I have the mechanics worked out using a master cylinder with the piston controlled via a lead screw and electric motor. However, what i need is some clever electrickery to control the electric motor so it behaves as a servo would when the transmitter stick is manipulated - in particular, I meed it to return to centre when the stick is centred.

Is anyone out there an electrickery whiz? Might be better to take to PM to avoid upsetting the management though!

Thanks in advance.

^^

Nope Bish, don't think that at all, its fabrication and on a buggy, so its 1/5th OK with me

Nige

[Pete Attryde]

Bish,

Don't know if you have seen this :- http://www.rccrawler.com/forum/showthread.php?t=204085&highlight=hydro+steer

He seems to have used the 2 syringe principal. one operated by a servo and the other moving the steering.

On you original plan, I suspect you will need to buffer the out put of the standard servo control board using a couple (or so) FET's to allow for the extra current.

Wiki on FET's

Pete

[bishbosh]

Not looked at that one but have seen others with hydro. My challenge is that at 1/5 scale a servo driven system is going to need a very expensive and powerful servo to drive it. I also don't like the rapid response of a servo system on a scale truck - it just looks wrong when the steering flicks back and forth in a fraction of a second. Besides, it has been done that way before so let's be a bit different!

With a screw driven system you can generate significant linear forces with very little torque. The downside is, to get enough travel you have to have multiple revolutions of the screw shaft. If the shaft is connected to a motor that is capable of 1,000's or rpm then I should be able to dial in a speed that looks scale whilst still being responsive enough to be drivable. I will also get the capability to generate large linear forces to push the fluid around the system and overcome the inevitable frictional losses in a home built system.

Being realistic, my electronics skills are non existent so I will need to commission someone to build me a bespoke circuit. So, who likes beer and can do electrickery???

My cylinders will be based on this design - master and slave will be the same capacity to keep stroke length manageable so I will rely on generating enough force from the screw drive to turn the wheels

.

[mickeyw]

I'd say you need to be looking at small stepper motors driving a leadscrew of whatever pitch you wish, for the level of control you want.

Such things are used in expensive car headlights for the self levelling mechanism, and of course aren't cheap. They're also used in modern gas boilers for regulating the gas flow.

Programming the interface would be quite a complex task too. Are you good friends with any electronics geeks?

[MECCANO]

I'd say you need to be looking at small stepper motors driving a leadscrew of whatever pitch you wish, for the level of control you want.

Such things are used in expensive car headlights for the self levelling mechanism, and of course aren't cheap. They're also used in modern gas boilers for regulating the gas flow.

Programming the interface would be quite a complex task too. Are you good friends with any electronics geeks?

The problem with using a relay to "step up " the output to handle the larger motor is its an on off switching device. this means you wouldn't be able to slowly regulate the speed. it would be all or nothing and consequently very jerky steering.

You can use a fet to slowly regualte but its quite precise. the control voltage for the fet needs to hover the "jsut switchign on and switching off levels"

[MECCANO]

This should do what you want to do:

http://www.pololu.com/catalog/product/392

It will also allow you to use an encoder as instead of a pot which should be more durable in mud etc...

[landroversforever]

Just a thought, How many turns are you after? I built a robotic arm at work and used some of the large scale winch servos which have 3.5 turns of movement.

These are what I used: http://www.hitecrcd.com/products/analog/boat/hs-785hb.html

[V8 Freak]

Where is Landy Man Luke when you need him... I'm sure this is his field of expertise....

Drop Luke a note Bish..

Neil

[simonr]

I think you may be over-complicating things!

You can buy a board which will drive a BIG motor as a servo - the kind of thing you would use to R/C a full size car - but for 1/5 scale I would just pick a servo with sufficient torque to move your master cylinder / syringe / closed loop pair of pneumatic cylinders filled with oil directly. It will weigh less and likely cost less than using a lead-screw.

Si

[TheRecklessEngineer]

I agree with what Simon said - it does seem to be a little overcomplicated.

If you must use a lead screw however, then why not close the control loop around the whole system - i.e. put the sensor for the servo board on the steering output rather than the motor?

If I was building from scratch, then I'd likely use a small microcontroller.

[simonr]

If I was building from scratch, then I'd likely use a small microcontroller.

And people say I over complicate things! (Me too!)

If you dismantle a servo, you have a motor, a gearbox, a bit of electronics and a potentiometer (like the volume control on your gramophone). The Potentiometer measures the position of the arm on the servo so the electronics can drive the motor to a desired location.

You could take the motor, connect it to a lead screw and mount the potentiometer via a linkage / beldon cable on the axle. The servo will then be controlling the position of the wheels - regardles of how much slop you have in the linkages / hydraulics. This will give the most precise steer control.

Si

[jameslwt]

Bish - have you looked at sail winches (model, obviously)? Multiple revolutions and serious amounts of Torque. Hitec do a cheap one, Atlas do some serious stuff with metal gears etc if preferred. The good ones (in fact most these days) allow speed and No of turns adjustment. I play (when time allows) with Marblehead yachts - up to 8ft masts - and you can easilly rip parts out of the boat if not careful

[FridgeFreezer]

Bish - you have a PM.

[Bowie69]

Hmm, if you geared the motor output down sufficiently, you could use a normal pot.

Alternatively, have you thought about the slider pots? Connect it to the piston of the input cylinder, upgrade the output stage on the servo and bingo?

Plenty here(if the link works):

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=retrieveTfg&Ns=stockPolicy_uk%7c1%7c%7cnew_uk%7c1&Ne=4294922322&Nr=AND%28avl%3auk%2csearchDiscon_uk%3aN%29&N=4294611870+4294920016&multiselectParam=4294611870&selectSubRange=Slide,%20Slidetrack%20and%20Faders#breadCrumb

Just match the value with what you take off.

[bishbosh]

Thanks for all the feedback folks.

I know it might appear over complicated, but I am reliably informed that using a traditional servo with links arrangement (i.e. no hydro) is going to need in excess of 40kgcm of torque which adds up to a pretty expensive servo. Add in the losses of the hydro system and the price goes up still further.

If I can't do it this way i have a back up of a pair of 33kgcm servos(best price point I could find on the bay) driving the master cylinder, but to get 40mm of stroke they will need to be using 20mm horns which leaves me with 33kgcm of force acting on the piston.

Anyhow, this is all theory, I am going to have to make something and actually test it!

[bishbosh]

And so it begins - first cylinder on the lathe turned to final size. Just need to put it on the mill to trim back the "ears" for the inputs and bore it out for the piston:

Progress pic:

[A Twig]

Unfortunately my PC with all my design software on is up the swanny at the moment. I will see if I can break into my old Uni labs and have a play. We did something pretty similar in my degree for a dynamic sculpture that changed shape responding to different inputs. I will have a looksee at what I can dig up. If I do find anything it won't be light reading though!

[Night Train]

Slightly off topic but I saw this and thought of your 1/5 scale truck.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110684272586&category=108783&_trksid=p5197.c0.m619

[landroversforever]

Thats excellent