One of my machines currently has a M02AE controlling a hydraulic servo. It was done this way because the M02AE is kind of a buy one get one free. We already were using a electric servo on one channel so it was just easy to add the hydraulic servo to the other channel.

Now I may change the hydraulic servo control to a HYD02. The main reason I'm looking at this is I may change the linear encoder to a linear transducer. Aside from some tuning issues I have never had any real problems with using the M02AE with a hydraulic servo.

Just wondering if anyone had any experience with the HYD02 and had any good things to say about it, warnings, or just general differences between the 1756-M02AE and the 1756-HYD02

Have not used the M02AE or the HYD02 but I did have a chance to use a 1747 family HYD servo module one time. Found it to be occaisionally subject to oscillation around the set point and a little obtouse to program and tune. Hopefully these where resolved in the 1756 family cards.

Thanks for the tip. I don't think this module is as popular as the motion module so I'm going to give Rockwell a call today and see what they think. I'll let you know how it goes

I don't have a lot of experience with the HYD except for some in house testing. However, I wrote a lot of the firmware code for it. It is basically the same code as the M02AE and M02AS with just a few mods to make it more hydraulic friendly. The idea is that all modules should be nearly interchangeable except for the obvious difference in I/O.

All the modules have a standard PID where all gains work on the error. This maximizes bandwidth.
All the modules have velocity and accelerations feed forwards. Now this next part I am fuzzy about because it has been so long. The HYD may not have the speed control loop because hydraulic systems do not run in torque control mode. The HYD compensated for this by adding a derivative term but I think all the modules have derivative terms just to make them the same. The HYD may have the speed control loop but it normally isn't used just to keep all the modules the same. The key is that they are pretty much the same except for necessary differences due to the feedback device.

Something you may want to consider is the 1756-M02AS. It has a SSI interface that allows it to do both rotary and linear applications or even mix rotary and linear actuators on the same module. The SSI module is the most flexible of the three modules.

I haven't seen the M02AS module. I will read up on it today. Thanks

The M02AE requires a full turn of the encoder for tuning but we do not actually have enough travel for a full turn so we manually tune them. Do you know the requirements for auto tune on the M02AS and a linear transducer?

I began reading about the M02AS but I am a little confused on exactly what the SSI (Synchronous Serial Interface) is.

The one application I worked with where we had SSI Encoders they were called was an extrusion press. We had a linear displacement encoder which gave us a 4 - 20 ma signal for the PLC to know distance, but also a DB9 connector module could be inserted to calibrate and set things like resolution and steps over length etc. Don't know that this is what he refers to but sond like it.

The company that developed the SSI specification is Stegmann in Germany,
http://www.stegmann.com/
It basically is a way of synchronously transmitting a 24 bit position from the encoder to the motion controller using only 6 wires. Power, common, clock+, clock- data+ and data-. The motion controller sends our 24 clocks and it received 24 data bits presenting the position. or angle. SSI encoders may use binary or gray scale and anywhere from 10 to 32 bits. I believe the M02AS supports the full range.

Temposonic ( MTS ) and Balluff both have MDT rods that can provide resolutions down to 1 micron.

Note, you may not need this resolution for positioning but what most people don't realize is that this resolution is necessary for the derivative gain which is basically a gain on the error between the target and actual velocities. Most of the 'noise' people complain about when using the derivative gain is actually due to quantizing errors.

Bob is describing a MDT rod which may or may not have an SSI interface. From Bob's description it is an analog MDT where the serial port is just used to do what Bob said. It is NOT used to return position to the motion controller.

An SSI interface is a RS422 type of interface. It is differential and point to point normally. The motion controller sends out 24 clock pulses and gets back 24 bits of position data from the sensor. It is that simple. The data can be either binary or gray code. I see know advantage to gray code when using SSI. Also the number of bits can be configured. We have two 13 bit single turn encoders that we use for testing. Here, we also have a mulitple turn encoder that can be configured to use 13 bits for angle ( 8192 ) and 11 bits for the turn count ( 2048 ). The good part is that these are absolute so no homing is required.

Ok so just as another option instead of adding the M02AS say I replace the existing M02AE with the M02AS. Would I be able to hook an encoder to it in the same manor as the M02AE through the SSI?

Currently Channel 0 is an Electric Servo Motor and Channel 1 is the Hydraulic Servo

i bieleve you can acomplish this realatively easy. here at the brick plant i am using 2 ssi mods on the one i have absolute encoder and hyd servo connected and the other i am using only absolutes i will see if i cant get the setup file loaded here a little later

Yes, as long as you can change the electic servo motor to use a SSI encoder. We often mix and match linear and rotary applications.

The electric motor would not need homing.

It doesn't appear our setup will work with the SSI. But I have spare slots so that's not a big deal. So now I just need to make a decision between the HYD02 and the M02AS. Time to do some more reading

Just going off of list price the HYD02 is about $600 more than the M02AS but from what I understand I would have to buy the SSI additionally to the HYD02.

Now I guess the big question is how much more hydraulic friendly is the of the HYD02. I'm happy with the M02AE besides the auto tuning problem but if I'm going to make the investment in this module and the new linear transducer setup, then $600 isn't going to break me if it will the better setup

Certainly sounds interesting there TW, I have in the past used REXROTH 2 x HNC100 in a closed loop config with SSI transducers to control four axis.. (Skywalk - a moving platform on top a building 270 meters (885 feet) above ground)...sorry i don';t have any exp with the clogix on this one...

Well this is definately not a skywalk or anything precision here.

But thinking about it a little more and assuming the HYD02 and the M02AE are pretty much the same besides the feedback hookup and the hydraulic friendlyness, I think the HYD02 will be the way to go.

I don't have a problem with the current M02AE or the servo valve that it is connected to. The only problem I have is the encoder was the wrong product for the application but was quick since I had the extra channel on the M02AE. It is more of a durablilty issue. If I can cram the linear transducer into a cylinder where it won't get damaged I think that would solve everything.

If I can pickup a few perks on the way such as auto tuning and a little bit smoother operation, athough it is very smooth, then that will be great.

Of course I still need to make sure there are no big differences between the two modules. Some of the configuration tabs are a little different but program wise I don't see much. Peter says they are the same code but a little more hydraulic friendly. If that is all then I say they are close enough to the same. Anything else or any other advice?

I'll admit to a senior moment. Seeing someone mention the MDT Name and the six wire decsription was the type we used. There was a mix of these and analog probes. I still am fairly positive you could connect a laptop to the SSI encoder rod and adjust the usable portion of the span and thereby the ratio of distance per pulse. May be using plain english and not technical term to describe the function.

Bad boy, you need to use the M02AS or HYD02. The control logix allows one to synchoronize axes across the back plane. Hopefully you didn't have to synchronize axes between two HNCs. Of course if you want the finest in hydraulic motion control then use Delta RMCs. Since all the axes are in one box there is very little phase difference between the axes when syncrhonizing. Just a few microseconds of calculation time depending on the number of axes being used.

Yep thats exactly what we did....the stroke of the cylined was around 2.4 meters. Also to make things a little more interintering, because the portioning of the cyliners and the design of the platform, 2 axis were are the rear of the platform and the two front ones were mid way thus creating a natural canter lever whereby the rear of the platform would want to raise and the front so to speak lower.....I posted a pic of the skywalk in the gallery....synchronisation was achieved through the HNC bus, with 1 HNC being assigned the master and the other the slave. Each HNC controlled two axis. The postioning data and at level signals between the HNC were generally achieved through the bus. (Not just through analogue signals as u might be eluding to Peter....)

I am sure that the Delta RMC's are really cool but i did'nt spec the hydraullic control system....You should know that i would be happy to use your products mate especailly with the likes of your self backing them up....

BTW good to see that you are still snooping around.... Peter.

Sleepy Wombats Gallery

[quote name='Sleepy Wombat' date='May 9 2006, 11:05 PM' post='33397'] [quote]
Sleepy Wombats Gallery
[/quote]

Some pretty awesome shots Sleepy. Just one question. Where does the oil go if you spring a hose leak on a platform that high up?

Leaks? why would it leak? No that wasn't oil, it was bird droppings

I have in fact seen a hose blow completely, the conrtol system can generally detect a problem and shut the hydraullic pump off pretty well immediately, although a number fo litres would have already being spilt....opps...so break out the kitty litter and nappies boys.....it time for a clean up...

Just curious if you had to plan for "Secondary containment" of the oil should a leak occur. We had a set of Cold Extrusion presses where I was working and each took 1700 gallons of hyd oil. They where mounted in such a way that a line leak dumping the entire main resevoir was contained within a secondary trench, prevent ground water and city sewer contamination.

Since we are off topic. Great pictures, sleepy. I have a few of my own now. I could have stayed home and looked at yours. :) I was in Sydney for a day in February before going home. I stayed down town across from the train station. I didn't see any platforms like that. I was at the Sydney tower and took pictures in all directions. Sydney reminded be a lot of Vancouver, BC.



Oscillation is caused by tuning or some non-linear function somewhere in the system. A common problem for hydraulic rookies is to use a cheap valve with a dead band. The causes the valve to not respond to command signals until the output gets past the dead band. Then the actuator moves and passes the set point before the output must wind down to the other polarity. This causes a hunting action. The 1746-QS, 1756-HYD02 and 1756-M02AS have a deadband eliminator feature that should be used when using these valves with dead band. The deadband eliminator jumps the control signal past the dead band so the valve responds instantly instead of letting the control output transition through the dead band which takes time.

The moral of the story. Don't use valves with dead band unless you can get by with just using one edge of the valve like in a conveyor application. Positioning and force control application should alway use a 0 lapped or servo cut spool.

The dead band eliminator is one of those hydraulic tweaks I mentioned.

I have posted a few more pic's in the gallery if you guys are interested. Peter, u needed to go onto the Skywalk Tour to get up to the platform. BTW was it a business trip or just a holiday ? Could have caught up for a beer or two or three .... hic..

I went to the top of the tower but didn't want to wait for the Sky Tour. So close.


I was at Alan Case's part of Australia for two weeks. We met and had a few beers. We were both working at a saw mill in Tumbarumba owned by Hyne TImber. There are 10 RMC modules there for a total of about 60 axes. I was there to make sure the modules were tuned, the hydraulicswere right and that the best techniques were used for the motion control.

I was in Sydney only 24 hours before going home. I was going to have a week holiday but I spent more time at the mill than I fiirst thought I would. I didn't have schedule then and didn't think to look for PLC people.

I bet I will be back for tune ups. There is another mill being started up about 4 hours north of Tumbarumba with 9 modules. There are getting to be quite a few between the Canadian and US OEMs. Are there good distributors there?

Being a while since i was involved with a saw illing company, the last one i remember specialised in Hardwood. Interesting industry and safety standards......
60 axis, thats a darn decent project....cool.


Thats always the way, isn't it.

There is an easy way around this: dithering.

Simply purposely add a little random number ("white noise") to your signal. In fact, add or subtract exactly 1/2 of the LSB of your encoder (+/- 0.5) as a uniform random number. For example if you had a 10 bit input, multiply by say 8 and then random add a number in the range of -3, -2, -1, 0, 1, 2, 3.

What this does is to "unquantize" the actual inputs slightly and forces a little noise into the derivative term...just enough to prevent it from locking onto an incorrect value. It gives you the "continuous" signal that you want without having to go nuts on the resolution of the encoder. And if the encoder drifts a little (so that you see slight random noise on the LSB's of the encoder), so much the better...it all helps contribute to helping keep the derivative term in check.

I know about dithering, but I don't see how it helps to compute the actual velocity. I am talking about the feedback resolution. They are two different things.

Dithering is old stuff. If you are using a valve that requires it then you are using the wrong valve. A servo system is contantly updating the valve spool position so the spool doesn't have an opportunity get 'get stuck'
Dithering is for the output to the valve.