Machine upgrade using Micrologix 1200

[Buckaroo 0]

Greetings all, I have a machine that is in basically 2 parts but work together. The base machine is controlled via a Micrologix 1200 catalog # 1762-L24BWA. The other piece uses an output from the PLC to initiate a cycle that performs a number of functions such as positoning, crimping, and sealing. These functions on the secondary machine are controlled by cams mounted on a shaft that actuate air switches that in turn operate the various air cylinders and such. Boss says to me one day: "Gee it sure would be nice if we had a way to adjust the settings on this system easier and with much finer control. Why don't you in your spare time come up with something to make the machine better." So it takes me awhile but I am now in the process of forming a gameplan as to how to do this and what needs to be ordered. My PLC experience compared to the members here would seem limited but this is a project I would like to do to expand my knowledge and show the higher ups what I can do. So with that said I am looking not for a handout but a gentle nudge in the right direction from time to time as I get confused. The 1200 has the High Speed Counter and my first thoughts are to use this to develop a positioning system where the operator can insert an on and off parameter for the various functions. They would do this from the Panelview that is already part of the machine. I also have an Allen Bradley encoder catalog # 845H-SJHZ24CMY2C that is a spare that I can use. My thoughts are to use the encoder on the HSC inputs. Does this seem like the direction to go in or is there something else that might be better/easier? Any ideas or suggestions are greatly appreciated. If more info is needed let me know. Thanks

[OkiePC 14]

Yes, you can use a high speed counter to create a PLC version of your cam shaft. You will also want to plan how to connect outputs to the devices which will most likely be upgraded from air switches to solenoid valves so that your PLC can control them. I don't have any experience with the high speed counters in the ML line, but I do know that they are very flexible by design, which means that getting them configured correctly for you application may require a little studying of the manuals in order to determine how to use it for your specific needs. There are quite a few posts here and and plctalk.net regarding micrologix high speed counters. Be sure to read those that apply to your controller, since there are differences between each of the micrologix controllers in how they're implemented. Paul

[Buckaroo 0]

Thank you for the reply. If I am reading things correctly then I should be able to connect the encoder A, B, Z signals to the Micrologix inputs 0-2. As for configuring the HSC would I then pick mode 6 or 7 and what are the advantages to each one. Or am I off base and need a different mode? As far as the outputs go, I am not that far but I envision a bank of solenoids properly protected of course. Again thanks for all the help. Tom

[Ken Roach 194]

If I understand that part number correctly the encoder will need some kind of signal converter to work with the MicroLogix 1200. The MicroLogix has only 24 VDC inputs, but that encoder 845H-SJHZ24CMY2C has a 5V DC differential line-driver output. If that 4 was a 6, the encoder would have a 24V DC output. Mode 2 is the most common way to configure the HSC on the MicroLogix to work with a Programmable LImit Switch function. This is a Micrologix feature that is specifically intended to provide a programmable alternative to physical cam switches. Read up on the Programmable Limit Switch function of the MicroLogix highspeed counter in the Instruction Set Reference manual and the Quick Start documents on the Knowledgebase in Answer ID# 19961 and Answer ID# 21891.

[Buckaroo 0]

Ken, Thanks for the reply. I guess I should have looked more into the specs for the encoder to see if it would work. Thanks for the links. I will check them out. Regarding your reference to the programmable limit switch function. I will have to read up on that and get back with any more questions. Tom

[KidPLC 2]

Hi Buckaroo, I have done a similar project many years ago using the HSC counter, I have dug the old code out to refresh my memory. There are a couple of things you might want to consider or at least keep in the back of your mind. 1. The Micro Logix high speed counter inputs are rated at 20KHz which is pretty good however this may not be suitable if your rated encoder counts is high and your machine speed is high. eg your 500 ppr encoder can spin at 2400rpm and the micro will handle it.....any higher and the high speed counter will miss pulses. 2. Don't forget to go into your I/O configuration -> Advanced Config and adjust the input filter to allow for a suitable input frequency. 3. Note that mode 6 and 7 state they are quadrature this usually means that the A and B channel provide direction and the number of pulses is multipied by 4 which gives greater resolution. In the AB micro the A and B channel still provide direction but there is no multiplication of counts. 4. Since the encoder is incremental and not abolute you will have to home the cam on power up or else the system will not know where it is.....you may have already considered this.....if not let me know I have a couple of suggestions of how you might handle it. 5. I did not use the PLS function in my project instead I monitored the HSC accumulator in the program, I set up trigger windows using a GRT followed by a LES instruction and lived with the inacuracy due to the scan of the plc....it was not a concern in my application. 6. One thing you might want to consider is the use of an encoder that has a count value equal to 360 or divisable by 360 so that you can relate the rotation to degrees. I had a little play with the PLS data file this afternoon using the HSC in mode 00 and mode 04 it seems reasonable however I did not look too deeply into how it manages counting down there where a couple of issues with HIP and LOP values being the same....it cause the plc to fault basically. Will your machine travel in reverse ???? Cheers

[Buckaroo 0]

Sorry its been one of those weeks already. I am still trying to find time to read up on all this stuff and keep up with production. The machine will not be going in reverse. I suppose since the encoder I have will not work, I will spec out a new one. Besides having to rehome a machine with an incremental encoder, is there anything else advantage or disadvantage wise with an incremental versus absolute encoder? In other words which would be better or put another way when do you use one over the other? Regarding the PLS function. If I understand right and if not please point me in the right direction. The HSC inputs read the encoder pulses and depending on what I have in the HIP and LOP parameters then I can manipulate the various outputs. For example the encoder has 500 ppr and I want to turn on an output I could program say 100 and 200 in the HIP and LOP. Now if this is the case would it be more advantageous to use an encoder with 360 count for ease of changing the setpoints since it would also be degrees of rotation? Or would it be better to use one with a higher resolution and then use some scale function so the operator can still input a change in degree format? As you can see I am trying to get as much information before I commit to any one approach. Thanks for any and all help you are willing to provide. Tom

[KidPLC 2]

Hi Buckaroo, So your machine will not be driven in reverse....great, one less thing....but can your operators push it in reverse during a machine stoppage? for example to clear it? As for absolute vrs incremental you really have no option as your Micro will only deal with an incremental encoder. If you feel that homing on start up is going to be an issue you could use a third party device that accepts an absolute encoder signal Omron used to make a nice unit H8PS or PR not sure if they make it now. You could feed the signals from this into your plc and go from there. Only problem is that the cam settings will be in the OMRON and you will have to access that every time you want to modify them. Maybe this isn't as user friendly as you would like. As for resolution you really need to evaluate what machine motion is traversed during one revolution of the encoder then split that up into the number of pulses you intend to use eg 360. If you require more adjustment than the machine advances in one encoder pulse then you need to increase resolution until you have satisfactory adjustment.....I hope I made sense then?? Going for a 720, 1080, or 1440 pulse encoder will give you greater resolution and you can scale the setpoints then load them to the PLS file ready for execution whilst still working in degrees. (I will test this some time this week but I reckon there is no reason why it won't work) Warning though make sure you don't increase the number of pulses to the point where the Micro can't count them (see post #6) If you are not hung up on degrees then any count will do.....I just think degrees is the most logical....but that is only my opinion. From what I can make out HIP is the setpoint when counting up and LOP is the setpoint when counting down. Edited 13 Aug 2008 by KidPLC