Do I need a PLC?

I am designing an attachment for a Skid Steer loader that requires being able to control a hydraulic cyclinder electronicaly. I need to be able to infinitely control the stroke with either a key pad or reostat type switch. Example: If I type "3" in the key pad or set the reostat to "3" and push the "start" button the ram will extend 3 inches and return to the starting point. The loader has 12V power and the attachment will be used in weather extreams and subjected to bumping and jarring.

Is a PLC the right control and if so can someone point me in the right direction?

Thanks!

Presuming that you have a transducer on the cylinder, (like the one in the link below), a PLC would be a good choice.

If your application needs are limited to just setting the desired stroke, you can likely do this with a AutomationDirect D0-05DR-D, or a D0-06DR-D, with a small HMI like AD No. EA1-S3ML.

You will also need an analog card to read the cylinder position: AD No. F0-04AD-1

Link for cylinder w/feedback gizmo..

http://www.parker.com/EAD/displayCatalog.asp?menu_parkerid=46&menu_gid=9&menu_divid=31&catid=2365708&catdesc=WAVE%20SCALE%20LINEAR%20DISPLACEMENT%20TRANSDUCERS&subcatid=2319748&viewtype=1&sMode=Details

As Mike illuded to, you definately need a feedback device. There are several ways to go here. I'm assuming since you are working on a skid steer that ruggedness will be a primary concern. So, mounting the feedback inside the cylinder is a good option.

You may or may not need a PLC but a PLC will probably make your job easer. You could probably get by with a preset counter that has a relay output when you get to the specified count. That paried with an incremental feedback device and a relay or two could do the control.

One thing you haven't mentioned is how accurate you need to position the cylinder. Basic "bang - bang" valves will get you close. If you need accurate control without overshoot then you'll need a motion controller.

How close to 3" do you need to be when you extend? If your flow is constant you may be able to use an adjustable timer for the stroke out.

Even though this could be done with some clever design and hardwiring of a selector switch, resistors, and well featured servo amp/actuator combination, the I/O described is pretty clearly within a PLC's ability.

What caught my eye is the 'extreme weather' and 'bumping and jarring' part. So, the final design should consider that, but also (since the device won't be right outside your shop), long-term (non-battery) storage of the program in flash memory, etc. The 12VDC will come on and go off I presume...and most PLC batteries are not made for long times of no power.

Bfrear,

Is this a pushoff or a grab attachment? Or are you adjusting fork spread etc...??

Thanks to everyone for your replies! First, I had planned to use a Position Sensing Tierod Cylinder like the one Prince Hydraulics has. It incorporates a LRT (Linear Resistive Transducer). Secondly to explain the machine better, the cylinder is attached to a rack gear that drives a spur gear that is connected via a common axel to a steel disk. This disk is what I want to control (turning inches of stroke into dergees of turn. I need to be able to control it in 1 degree increments. And to complicate matters more, I need to be able to reset the the calibration of it via the HMI, preferably a reostat.

It is a standard skid steer quick attachment and would rely on the loaders power source. Also the PLC would be well protected in the attacments steel case.

Thanks again!

If an incoder can be mounter to the disk would be a better way of finding it's position.

Rich,

A "skid steer" is another term for a Bobcat type loader. You can have all sorts of attachments such as a simple bucket, a chisel point, lift forks, grappler, excavator bucket etc... Whatever the attachment, it would be powered by hydraulics. Normally position feedback on a rotating table would be provided by an encoder on the table, however in this case, considering the application, it would be more reliable and simpler to get feed back from the cylinder since the cylinder will come with the feedback device, and/or be pre-configured to accept a feedback device.

BFrear, IMO the analog position feedback should be a current (not voltage) device. Earlier I reccomended the AD DL05. The DL05 is small and capable but does not have long term retenitive memory, or battery backup. If you want the operator to input positiion data that would be retained through and overnight power cycle, then the DL06 would be the better choice.

With the Dl05 you can create pre-sets in your program, then the operator can choose one of the presets, or input a position. But the operator will not be able to create "long term" presets.

On the other hand, with the DL06, the operator will be able to create and save pre-sets that he/she could call up the next morning.

Additionally, afer some re-consideration you probably should use an operator panel with a numeric keypad, such as the AD EA-MG-BZ2.

To better explain there is no reostat involved. The operator keys in, (or selects), the desired position. When activated the unit would rotate to that position and the transducer on the cylinder would feedback to the analog card. When the analog card reads the matching value, the unit would stop.

Good luck with it..

Thanks Mike