1756-HYD02 to be used with a saw.

widelto

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Jul 2005
Location
Barranquilla, Colombia
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HI:

After three years my customer has changed his mind and he finally decided to use a motion control system instead of conventional analog system.

Please take a look at : http://www.plctalk.net/qanda/showthread.php?t=97080&highlight=1756-hyd02

I never quoted , I am no expert in hydraulics so I asked him about the following info:
Hydraulic pump model, pressure, accumulator if they have one, cylinder dimensions, he says he has a servo control valve . I am expecting for servo valve datasheet.

I would like to hear your suggestions about this new system and what other info I would need in order to fulfill this new job.

Is there any literature that I should be aware of about this new design.

Regards,

William
 
widelto, use the M02AS with a SSI rod if you can. The SSI rod provides better resolution but after that the HYD02 and M02AS are the same.

If you are only going to replace controls, you should only expect an improvement in accuracy but this may be limited by the valve.
Provide a link to the valve's pdf.
Is there an accumulator?

I just wrote this on a Chinese servo hydraulic forum. This will give you an idea of questions to ask.

I need to know:
1. mass of the load.
2. the orientation of the hydraulic cylinder
3. the distance to move the load
4. the time to move out
5. time between moving
6. the time to move back in
7. the time to wait between cycles.
8. velocity and acceleration limits.

BTW, I am a moderator for a Chinese hydraulic servo control forum. I must be the only foreign moderator in China and I got drafted.
 

The first video is of one of our customers in China.
The second pdf has some good equations and some bad ones that don't apply to servo hydraulics.

A common misconception is that "flow makes it go". That is terribly wrong. Force makes things move. Flow only equalizes energy ( elevation or pressure). Therefore flow is the result of motion, not the other way around.
The equation about stroke speed and time on page 2-12E are wrong. Very wrong. Most text make the same mistake because they copy what has been written, and wrong, before.

Here is a good conversation about the VCCM equation on my forum
https://forum.deltamotion.com/viewtopic.php?f=18&t=497
Notice that Norm calculates a v_dumb = Q/A for comparison. A Chinese engineer verifies the VCCM equation at the end.

You should always ask yourself, how many hydraulic servo systems has this person installed, designed and done tech support for? Most people that write books have a bunch of foot notes that references the source of their material.
The source is not research or experience.

I have a design guide that isn't too technical but will help people design hydraulic servo systems
http://deltamotion.com/other/designguide/register.php
 
Peter:
Thanks, for the observations and info to get from my customer.
Ok I will ask for both quotations HYD02 and M02AS.
I remember you once mentioned a company that assembles cylinders with the rod inside. They were located in california.
I registered on your web page in order to print the conversation you mentioned.
I also ask for SSI rod.
This new system is going to replace existing one, nowadays the use a valve with two on/off solenoids, they expect to set the saw a little closer to the pipe to be cut and gain some seconds that will represent a lot of pipes in the long run, obviously they also expect to increase line speed.
As soon as I receive all the info from my customer I will pass it to you.
Thanks in advanced.
 
widelto, cylinders with "gun drilled" rods are common in the US and Canada. All the major manufacturers will supply them.
The feedback should be a SSI Temposonic or Balluff rod for the M02AS.
A speed increase will not automagically happen. Oil should be supplied at a constant pressure. That means an accumulator should be added to the system. Pumps are not fast enough to respond to flow demands on their own.
 
Peter:
Finally the customer decided to send me info requested.
It took a long time due to budget problems.

This is the info you requested:

1. mass of the load. aprox - 4 ton
2. the orientation of the hydraulic cylinder, angle - 30° Downward
3. the distance to move the load, (cylinder stroke) 16 "
4. the time to move out, 2 secs
5. time between moving, 5 secs
6. the time to move back in, 2 secs
7. the time to wait between cycles. tiempo entre ciclos. 15 secs
8. velocity and acceleration limits. 123 mm/s max no accel info

They have this proportional valve Rexroth 4WRKE 16 E1 125 L 3X 6E G24 E K31 A1 D3 M and they want to use it.
I do not know if that valve is suitable.

Valve web page: https://www.boschrexroth.com/en/us/...to-hydraulics/proportional-valves/4wrke/index

This is the data I got from cylinder dimensions.

Piston / Bore Diameter 4.75 in
Rod Diameter 2.5 in
Stroke 16 in
Pressure 1500 psi
Oil Flow 26.6218 gpm

Bore Side Rod Side

Area 17.7205 12.8118 in2

Volume 283.5287 204.9889 in3

Force 26580.8190 19217.7113 lbf

Time 2.7662 2 sec

Velocity 5.7839 8 in/s

Outflow 19.2473 36.8217 gpm

Ratio 1.3831

I have some pictures but they are too big, please let me zip them I want you to see them.
 
Wow! I wish more people would provide specifications like that. It is Thanksgiving here so I will get to in on Friday.

However, I did look at that valve. I hope your customer hasn't bought it yet. If the valve does not have a Bode plot, the valve should not be used for hydraulic servo control.
In short, the valve is garbage. You would waste a lot of time tuning the system only to find it really can't be tuned.

I have software that will calculate the correct cylinder and valve size. We will see how well it matches the data provided.
 
The numbers are not making sense.

how can you move 16in*25.4mm/in in 2 seconds when the speed limit is 123mm/s.

I use two formulas to get good approximations of the velocity and accelerations required.

max_vel=1.5*move_dist/move_time
max_acc=4.5*move_dist/move_time^2

To start with I assume the motion profile will be ramping up for 1/3 of the time, at constant velocity 1/3 of the time and ramping down 1/3 of the time.
This results in an acceleration distance of 1/4 the move_dist, a constant velocity distance of 1/2 the move_dist and a decel dist of 1/4 the move_dist.
moving 16in*25.4mm/in in 2 second results in a max speed of 304.8mm/s


I am a moderator on a Chinese forum. A person there just posted his problems with using a 4WRKE valve.

BTW, I do my calculations using MKS units. I assumed your 4 tons is about the same as 4000 Kg.
 
Peter:
As I mentioned before this control is only for the saw, the flying cutoff is being driven by a 1756-M02AE module with a servo motor for the carriage, this part is working properly. The saw is using a bang bang valve nowadays.
If we use this new design I am totally sure that we are not going to use full stroke of cylinder, I want to take advantage of servo hydraulic control to position saw closer to the pipe to be cut in order to diminish time, I would say 1 inch away from pipe so I will only move the saw 1 inch plus wall thickness of pipe. When we change pipe diameter the saw is going to be positioned at same distance from pipe, the idea is to travel almost the same distance regardless of pipe diameter.
Yes, 4 tons means 4000 kg.
Can you please provide me the link for that forum in china.
 
Peter:
As I mentioned before this control is only for the saw, the flying cutoff is being driven by a 1756-M02AE module with a servo motor for the carriage, this part is working properly. The saw is using a bang bang valve nowadays.
I am confused. Isn't all the data for the saw?
You didn't explain the speed limit of 123mm/s. Is that the line speed?

If we use this new design I am totally sure that we are not going to use full stroke of cylinder, I want to take advantage of servo hydraulic control to position saw closer to the pipe to be cut in order to diminish time, I would say 1 inch away from pipe so I will only move the saw 1 inch plus wall thickness of pipe. When we change pipe diameter the saw is going to be positioned at same distance from pipe, the idea is to travel almost the same distance regardless of pipe diameter.
Yes, 4 tons means 4000 kg.
Can you please provide me the link for that forum in china.

http://www.iyeya.cn/thread-63637-1-1.html

If hydraulic cylinder is for the saw, is precise positioning required? I know the M02AE position should be precise.

BTW, I have canned programs for flying cut off saws or shears.
http://deltamotion.com/peter/Videos/FlyingShear/SimpleFlyingShear.mp4
With a RMC you can test your program at your desk.

I still don't like the valve.
 
Last edited:
The valve mentioned has overlap. These are generally good for use with a joystick or basic velocity control. Because of the overlap, it's hard to do accurate positioning with them. This series is also pretty slow.

The workhorse 2 stage positioning valve in the Rexroth line is the from the 4WRLE series. The spool code should be V or V1 and the code for the characteristic curve should be L for linear.

You also mentioned 30 degree orientation of the cylinder. You need to consider gravity and what happens when there is an e-stop or loss of power. You should install blocking valves at the cylinder that close on e-stop to prevent motion. Avoid the temptation to use a counterbalance valve which will make the control harder.
 

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