"bang bang" effect on tractor hydraulics.

[BertZegers]

Tap picture to enlarge.

This drawing is done with Easy Cad. My first Cad program bought in the 1980ies. Works great with Windows 7! Just for fun.

This is a drawing of the setup I am planning. The machine laser receiver (Spectra CR600 is mounted height adjustable on the box blade) it has red flashing LED's for up and down indication. I use light sensor modules with contacts for PLC input. The PLC provides timers, so the valve stays open till the laser receiver stops flashing. I have a 1 second delay programmed, and can adjust when needed.
The PLC is "old new stock" left over from my business I quit 10 years ago.
I have to buy the hydraulics though.

I am thinking of using a hydraulic top link to tilt the box blade for height adjustment, just inches.
I am planning to use this for rough leveling. "waves" are not a problem. This setup is considered "farm" use.

I hope this explains.

 

[MHarryE]

I have had problems with voltage spikes from this type of valve damaging the controller. We were suffering a very high rate of controller failures on a system similar to yours, all failures symptomatic of over-voltage. System voltage was 24 volts, controller rated to handle 36 volt peaks. I got one of our electrical engineers to connect a scope and ran through multiple cycles. We recorded peaks up to 84 volts when coils were de-energized. It was the result of mechanical engineers designing an electro-hydraulic control system without getting electrical involved. I also remember having a pump problem with one of our designs when we changed to a very fast response electro-hydraulic control. The pressure spike when the valve was switched diverting flow from return to tank into the cylinder circuit was enough to deflect the gear pump gears causing them to bite into the housing and snapping the pump shaft. As I recall measurements showed a pressure spike well beyond the relief valve was created due to the rapid response, but all within manufacturer's limits. You are looking at the right things in investigating before building, but often you don't know for sure until you build. Spikes I mentioned are way too fast to see on a gauge.

 

[old tractor collector]

I found this thread on the net with the same question.

I have been there and done that and yes the valve will destroy your hydraulics.

I purchased an older construction/ AG leveling system a few years back with the idea of having an automated 6' pull type box blade scraper. The system came with the receiver, control box and cables. I purchased a Bang Bang valve like you describe from northern and added a 1500 psi relief valve.

First off running a laser is nothing like pushing on a toggle switch. The laser rotates at 600 - 1200 RPM every time that laser strikes that receiver and it is either above or below grade it will open that valve for an extremely short period of time. It will do this 600 to 1200 times per minute. The valve will start making that bang bang noise, hoses will start jumping that is the best way I can describe running these older style valves and controllers. The newer systems use a portioned valve system which opens the valve based on how much correction it needs.

When I first set up my system I used a toggle valve and everything seemed fine. I used a flow control valve to control how much oil was heading to that solenoid valve. However as soon as I connected the laser everything was different it was jumpy which took some fine tuning with the flow control. However after about 1/2 hour of grading the 5000 psi rated hose coming out of the pump couldn't take the pulsing from the valve operating and the hose came apart giving a nice shower of oil. This was despite having an oversized relief valve.

After doing a little more research it seems most of the laser systems that were using the bang bang or on off style of valves had an accumulator teed into the inlet of the valve to act as a shock absorber to take up that water hammer.
What type or size of accumulator good question.

 

[BukitCase]

Voltage spikes are easily controlled with a fairly herky clamping diode across the solenoid coil - I would typically use a 2.5 amp 1000 PIV rated diode in fairly small solenoids, one big enough to move a hydraulic spool would probably take a higher amp rating.

Pressure spikes - I'd think the accumulator might be the way to go - another possibility might be needle valve restrictors combined with a lower pressure relief valve and maybe an inch larger diameter cylinders to compensate for the lower sub-system pressure.

The first part I'm sure of, the second part I may be talking out my *** since I've never built a system like this before - I guess we're still waiting on more of the "tee shirt" guys to chime in - already a couple experiences, one first poster. Welcome to TBN old tractor collector :thumbsup: ...Steve

These guys
Models, Level Best, laser graders, tractor mounted, skid steer, compact tool carrier
use gauge wheel control on their models, which I may do to my Gannon even for manual control - I find that to get very good results without the gauge wheels, I need to go pretty slow and watch like a hawk or I end up with a bunch of sine waves due to the short coupling...

 

[CJONE]

Use a soft shift control valve. This is the first thing I will do if there is a problem with either pressure surges or broken steel lines/fittings with electrically shifted valves. CJ

 

[boggen]

a solenoid valve = on/off or rather fully open or fully closed for most part.

a proportion valve = more of a linear actuator. were you can "adjust" how much the valve open/closes and you can hold the valve open/close at different doings.... kinda like how you might open a faucet on a kitchen sink, one moment you want it off, then turn it on to fill say a water bottle then as bottle getting full you more than likely slowly shut the faucet off. this is more of a proportion valve.

a solenoid valve = come to a complete stop in a vehicle by using E-brake / emergency brake plus regular brakes to close a valve. and then when light turns green shifting to neutral for a moment, and raise the RPM's high, and then shifting into first gear. maybe leaving some black marks on the pavement or rather opening the valve. vehicles do not last long when abused, more so with a teenager behind the wheel.

a proportion valve = a dimmer switch
a solenoid valve = a generic on/off light switch

there are different duty cycles for a solenoid. from say a starter solenoid that only needs to work for a few seconds. and then has plenty of time to cool down. vs say a proportion valve. that is more geared to hold the solenoid in different areas continuously and in that is not effect as much from heat build up of being used.

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due to fluid nature of not being compressible. and dealing with water hammering effects. if valve is constantly being adjusted, it might be worth while for an accumulator just before the valve. heck with a garden hose and a generic nozzle on it. i can get the garden hose to have a water hammering effect. just by opening nozzle wide open, then suddenly letting it close. and watching the garden hose shake and rattle and roll of all things. at moment i have garden hose looped across top of fence. vs coiled up. and i can see the fence wire shaking. more so when air is in the garden hose and waiting for the air to get removed.

on general duty with the old allis chalmers CA 1954ish tractor to the industrial ford TLB 555c when operating hydraulics i can see hoses expand and contract and shake, rattle and roll. there is a couple spots on the old TLB that i double check the metal hydraulic lines, due to i notice a couple possible "wear spots" either from just general vibration or perhaps water hammer effect. and using / abusing the machine rather hard.

 

[CJONE]

A simple way to stop the quick response or hammering in a open system other than a soft shift is to put a manual pressure comp flow control in front of the control valve so it sees a reduced flow that is adjustable to the need of the application. Proportional valves are expensive and not dirt tolerant in any way. I have never seen them for open systems but I would think they are out there but with the drivers and extra stuff to make them last I don't know how practical they would be. I also use a 3 micron filter in front of all the proportional valve I install to keep them from sticking and in a open system that would be a problem in short order with filters plugging off. Anyway just thinking out loud. CJ

 

[muddstopper]

We are talking about putting this thing on the back of a farm tractor arent we? I did a little reading on the PID controllers, way over my head, but corrections 6times a second seems to be very extreme for tractor work. One thing we use for leveling railroad tracks are inclinometers. Heres a link to one, P Series Inclinometer?Position Sensors?Single Axis Tilt Sensors (Inclinometers)?Dual Axis Tilt Sensors (Inclinometers)?Measurement Specialties, similar to what we use for leveling track. They are pretty accurate, 1/32in in 56 1/2inches. Of course the adjustments are electric linear actuators attached to a shadow board the cuts the light off between a light projector and receiver. Plazer Tampers dont even use a light and projector setup, they use a wire. An inclinometer mounted on your blade sending a correction signal back to you controler probably wont be correcting at the rate of 6 times per sec, but it should correct faster than your tractor can pull dirt.

 

[oldnslo]

Set up properly a laser level system can hold accuracy of 1/4" over 1000 ft but this is also not your typical ag system nor used on any Illinois roads.... I would uspect that for a simple ag system you would have to take the average of x number of readings and adjust accordingly. Yes soft shift valves will help and some models are also tunable for shift response time.

For the super accurate systems, Accumulators are required to provide the instataneous flow & pressure required for repeatable movements over time and also to provide some shock suppression on the pressure side of the valve.

Agree with cjone, proportional valves are not very dirt tolerant and would only be considered on the high accuracy high cost systems.

 

[old tractor collector]

Portional hydraulic valves have been the standard on laser earth moving equipment for what the last 20 years.

And yes the Portional time or bang bang valve gets 6 pulses a second.


The early laser earth moving equipment such as motor graders and scrapers that were using laser control with PT or bang bang valves all had dampeners on them mounted right ahead of the valve.



Those who don't think Portional valves are used in laser earth leveling need to read pg 33

http://www.grouser.com/pdfs/belly_blades/Control Box Manual.pdf

Here is another brand with Portional valving
MEI: Laser-Tech Model 304 Machine Control System

Portional hydraulic valves are also the standard on all the large farm tractors so dust hasn't stopped them. Most of the 125+ tractors use something like Rexroth Portional valves.

As far as a 1/4" error in 1000 feet in ag would be to great of an error in that short of a distance. That would get you to the unemployment line quick.

Most ag systems now use GPS to take in the curvature of the earth as water doesn't follow a flat plane.

Actually ag requires more accuracy than a road. A road or parking lot will require a minimum of a 1% grade to get water to flow to a drain or off the road. When you talk to the ag levelers they are calling a steep slope .2% many of them are running .003% grade and 0 side slope some are running 0 in all directions in rice. The flatter the slope the tighter you need to hold grade to get water to flow and prevent potholing.