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JayB

Which PLC would work best for industial mobile equipment

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Hello, this is my first post. I hope you all will be able to point me in the right direction. I am in the process of building a piece of machinery that will be exposed to the elements, will create vibration due to the diesel engine powering the machine and the shock produced by the machines processes. The machine will be moved from site to site, so I would like for the controller and circuitry to be contained on the machine. The voltage for the controller, inputs, and outputs will need to be 12 VDC. Estimated operating temp range will be -20 to 100 F. I should be able to control the machine with 15 inputs and 15 outputs. So, my question is, which PLC would work the best for my application? Also, this is my first time using a PLC so ease of learning and user friendliness is important. Any suggestions will be greatly appreciated.

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Welcome, JayB. For your application, and since you are scouring the field for potential solutions, I would start by building your spec (which you have a good start on). 1. Size limitations 2. I/O type and number (discrete, analog, motion, etc.) 3. User interface (program ports, operator interface screen, etc.) 4. What logic will the PLC need to perform (on/off discrete, motion control, signal conversion, etc.) 5. What is the expected life cycle (durability) and cost you are shooting for? If you want "last a few years" and "cheap", that is a far cry from "military grade" and "budget is open". When looking for a control system, one needs to have most of these questions answered up front. For total system cost, another thing to look at is if you need training and also special software/cables. Allen-Bradley, for instance, is solid equipment, but software and cables will be much, much more than most of their competitors. Another bit of advice is to consider what your competition or other machine builders use. We are a machine OEM and we typically offer 3+ control system options that have different price ranges and capabilities. We tend to offer what our customers already use, so keep that in mind (i.e., most of our customers use Allen-Bradley, Siemens, GE-Fanuc, Omron, etc., which locks us into what control system to offer up). That leads to the last question... 6. What control systems does your customer prefer

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These might interest you. http://www.controlmicrosystems.com/

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12 VDC is very uncommon. Most PLC's and such are designed to run on 24 VDC. This really isn't a problem though. You can easily get a DC-DC converter to get to 24 VDC. Most PLC's are designed to handle the vibration, and that temperature range is easily within the range of most PLC's. So as it stands, I don't think any of your requirements are anything tough.

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Actually, wouldn't you say this would be a good practice for isolation even if you were using a 12VDC PLC?

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Some random thoughts I. Your choice of cabinetry and your design of cabling and wiring will do a lot for you in the "hardening" category. II . I'd suggest IP67 as a minimum and maybe IP68. III. I'd oversize the cabinet beyond what you need to allow for extra heat dissapation. IV. The extra size will also help you to make all components field servicable. V. I'd also suggest your cables and conduits enter the cabinet bottom only and have "drip loops". VI. If dry pressurized air is availble on your machine you might want 1 to 5 psi above ambient in your PLc enclosure. VII. You will also want to look at your sensors for "hardness".

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BobLFoot speaks to the environment quite a bit. Since this is what I would consider "normal" for an operating environment (my 11 years of service are all in mining and heavy industrial areas), I have some points of agreement and disagreement here. Remember that I'm a plant engineer. I have to LIVE WITH every piece of equipment that I design and build. I WILL probably end up involved in troubleshooting or repairing it down the road. Some of my ideas about equipment design add cost (and some reduce cost) but where I come from, down time is incredibly expensive, often hundreds of dollars per minute. All of my designs have slowly evolved into working towards minimizing downtime any way possible by reducing troubleshooting and repair time (quick changeouts of components) as well as beefing up the designs to make them last as long as possible. The general attitude to take is how to make it possible to change out a component in under 5 minutes and how to make it last for 5-10 years or more once you've made the change. Many of my "ideas" will cost more money. Some actually reduce costs. But when you are selling into the intended market, your customers will quickly recognize who sells "junk" and who sells stuff that "holds up" better over time. Actually the correct device if this is your goal is a "buffer". AB and Sola both recently started selling these for 24 VDC systems to allow you to "ride through" large power draws from devices with large inductive coils (solenoids). Just as in the typical industrial world, it is always a good idea to isolate your heavy power devices from your electronics. A DC power supply can be used for this application because of the nature of the device. Whether AC-DC or DC-DC, switching power supplies have some sort of front end that charges a capacitor and a charge pump on the back end that uses pulse width modulation to control the output voltage by rapidly switching the connection to the capacitor "tank" on and off, followed by an inductor or capacitor to smooth out the ripples. The "tank" is normally sized to a minimum but can be oversized to allow for large surges or "super sized" and labelled a "buffer". The other advantage is if you have a source of RF in your system, it adds an extra filter to the incoming power, although this feature is more or less accidental than intentional. A simple DC choke can go much further towards protecting against spark plug noise (but you said this is diesel where this is not a factor), CB radios, inductive switching transients, and such hooked into vehicle electronics though and costs <10% of what the power supply will cost. It does add some buffering but nothing close to what a power supply can achieve. The consideration here is that you already have power supplies as such on board most of your devices. Even for 24 VDC "nominal" devices, a typical sensor spec for DC power is often "10-30 VDC", quite a wide range. The reason is that the sensor has it's own DC-DC converter or other power supply circuit on board. It can easily use almost anything for a power source. These devices will come with the absolute smallest size capacitor possible to do the job however because they aren't concerned with reserve capacity. So in summary, Advantages of power supplies: 1. Conversion if the voltage levels aren't ideal. 2. RF/transient filtering. 3. Buffering. Disadvantages: 1. Extra cost. 2. Duplication of devices (DC-DC converters already present on board most devices). 3. It's a general device, not designed for specific problems (buffering, RF filtering) if you don't need voltage level shifting. 4. Power losses and heat generation (80%+ efficiency = 10-20% losses as heat). 5. Restricts power draw. In a high resistance/battery depletion scenario, direct connection is better. 6. Adds an extra set of connections and potential points (and devices) for failures. So I'm of the school of thought that with vehicle electronics, if it's just a filtering problem, I'm comfortable with a big fat DC choke if the electronics is already more or less designed to work in a vehicular or industrial (noisy) power environment. Adding a power supply would not necessarily add anything additional except heat and costs.

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In response to the actual question though...the answer is almost any of them. The key consideration in the 12 VDC/vehicular/outdoor environment is just that...the environment. I'm not aware yet of any PLC that comes with IP 68 "M12" or "M8" style connectors on the face of it with nothing else which would be the ideal situation (a sealed box with no need for an enclosure). That being said, it really depends on your application. If you want to go by brand name only, then I have 3 recommendations. If you are looking for simple on-board machine controls as a replacement for several relays, I have purchased or worked on many machines in the past with Omron PLC's. They are relatively inexpensive and popular, just something that I've never had to fool with beyond working "with" them (no programming). They have a pretty good presence everywhere. Software is also fairly cheap. In the U.S., hands down the favorite is Allen Bradley. They have close to 85% market penetration. Two product lines I'd steer you towards are the CompactLogix series and the Micrologix series. The others are either legacy (PLC-5, SLC) or probably much more power than you really need (ControlLogix). You can get a Micrologix for under $500 and certain models have free programming software. AB also has a starter kit for CompactLogix for around $2000 list. CompactLogix is extremely powerful and it is effectively a cut down version of AB's most powerful controller series (ControlLogix). The Micrologix is simple to use (I have several) but drastically limits your distributed IO capabilities and lacks a lot of higher level functions. I would consider Micrologix to be a direct competitor to Omron. In Europe, the market penetration is not as good but the favorite is still Siemens, which also has a relatively decent U.S. presence as well. I have never had any experience at all with their equipment. That being said, I've heard that certain members of the S7 line are comparable to the CompactLogix although the price is a bit better. If you are dealing with a European market, I would definitely consider these guys as well. I'd like to say a lot more to promote Siemens because I really do feel that they have a very competitive product. It's just that I haven't felt like I get the same level of support from them in the U.S. and I truly have NO experience with programming their stuff at all. But I'm not about to knock them because outside of the support issue, I feel they have a very high quality product and good quality programming systems. In terms of ease of programming, that depends. If you are coming from an electrical background, then keep in mind that with more powerful controllers comes additional setup and steeper learning curves. With a "smart relay" (see below) there probably won't be anything except bit instructions (examine if on/off, latch/unlatch, momentary output, timer) and a few others. These will be the simplest to program. The "typical" PLC's (Micrologix to name a particular one) will be next. You will have to have some knowledge of the PLC memory structure and chances are that symbolic naming capabilities ("Piece_Counter" instead of "N7:0") will be minimal. If you step up to a PAC level controller (CompactLogix as an example), the symbolic naming is really handy but there is a LOT more detail in setting up IO for instance. The learning curve goes up dramatically. If you are coming from a programming (PC) or process oriented background, then just the opposite applies. The higher level controllers will have languages such as structured text (very similar to typical PC languages) and process flow diagrams (very similar to a signal flow diagram or P&ID). These higher level languages are much easier to grasp than ladder logic if you don't have the electrical background to begin with. The caveat is that there is still that nasty setup learning curve but if you are going to be learning a lot of new things anyways, then at least when you get to the actual programming work at least there will be a stronger comfort level there. The price is of course that you are paying for a much more powerful controller so it will be more expensive but offset by development time. I do have a list of PLC's that I'm not really in favor of. Telemechanique/Square D/Modicon is definitely not my favorite. They change their product lines a lot and have real problems with backwards compatibility issues. They just don't have the longevity of everyone else. The vendors tend to be the local electrical supply shops that really don't understand PLC's at all so the level of support is not very good either in many cases. That being said, I love Modbus for distributed IO because every single major PLC vendor supports it. It has been around since the 1970's and in spite of many attempts to dethrone it, it continues to live on and all kinds of strange devices out there might not support anything else but they almost always come with Modbus. Just stay away form their PLC's. CoDeSys PLC's are interesting because it's the one system that is multivendor. There is a single language and programming environment but there are multiple vendors. The company sells the firmware to vendors such as companies making HMI's, or pneumatic or hydraulic valves. The language superficially resembles what everyone else sells but has a lot of strange quirks that turn off most guys I've talked to. I like the concept but there are enough quirks in the popular systems. I wasn't really a fan of it when I downloaded a copy of the programming software and tried it out either. That being said, it usually runs on an ARM or similar processor so the PLC's themselves tend to be very fast. It seems that every pneumatic, hydraulic, and HMI vendor also has their own "integrated" PLC platform, outside of the above mentioned CoDeSys system. You can save a ton of money with these. They all tend to be very inexpensive. I haven't had much experience with many of them but what they all seem to lack is that they didn't put much money into the software side of things so the software always tends to be clunky or lacking in the ease of use category, or flexibility, so I've categorically stayed away from them. For example, many of them have very fixed memory models. I've seen one vendor that had fixed "rungs" (hard to describe). The programming software will always drive you up the wall, and there are always a lot of "unknowns" and undocumented things when deal with it. Finally, there are several other "sort of PLC's" that I feel are worth mentioning that can work great in particular applications. Unlike the above ones, these are ones that I would feel very comfortable using. I haven't had an application come up yet where I used one but I have at least done due diligence in each of the categories multiple times for various projects. Another category is one I'll just call "smart relays". These are replacements for "Eagle Timers" and such. They do great for what they are intended for...replacing timer relays and such. Some of them are programmable directly from the relay itself (no software). Since almost all my equipment always called for a real PLC though or just a simple timer, I've never actually used them in practice. Another category are safety PLC's. In theory, I love the idea. You can use distributed controls and implement all the safety relay functions in a real PLC. In practice they have a very, very steep price differential. I've always done the cost/benefit analysis and come out in favor of a real safety relay module instead. If I had to to a larger burner safety system again though (these can be very complex), I would have rather used a safety PLC. If you are doing motion control, then there are two ways to go. You might buy a real motion controller that has some aspects of a PLC (Galil for instance), or you might buy a PLC with motion control built into it and forgo the extra box (only an amplifier/encoder interface is needed). That's one of the target markets for PACs (programmable automation controllers). Examples are the upper end of the Siemens S7 series, the AB ControlLogix series, and some of the GE Fanuc PAC's. Finally, there are two categories of what I will call "non-PLC's". If you have to have a PC anyways, there are some "soft PLC's" out there. Remember though that a Windows blue screen of death on a hardware device controlling moving equipment is probably considered a very bad thing...hence the reason I've never trusted them no matter what the vendor says if it involves Windows. That being said, I wouldn't hesitate to jump into an embedded system instead (having built a few of these myself). You won't be able to get your typical industrial electrician to troubleshoot it (hence the reason for PLC popularity) but if it's a "black box" to your customer anyways, that doesn't matter. Second, there are distributed controls systems. An example is the National Instruments system or Foundation Fieldbus. These are generally meant for process controls. Instead of ladder diagrams, you usually use process flow diagrams. The "program" is fully distributed into the hardware. There isn't a "controller" as such in general other than perhaps a place to store the program. It is really similar to the way the old DCS systems worked. I started in the process industry so I'm kind of partial to at least giving these things consideration. But just as with off-brand PLC's, you've got to give serious consideration to longevity and support. That being said, NI and Opto-22 have been around for years, and Foundation Fieldbus enjoys at least a fairly large group of enthusiastic vendors. If you are working in a process (chemical) industry, this sort of control should be fairly well accepted. Edited by paulengr

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Thank you all for your input. You have definitely given me alot to think about and several avenues to pursue. Again, I very much appreciate the responses.

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Jay, You may want to consider our products in that we have done both 12 VDC and extended temperature range controllers for what we call mobile applications. The primary issue will be on the below zero temperature rating. 100 F is only 38 C -20 F is -29 C Industrial components temperature range is -40 C to +85 C 12 VDC control power is not an issue. If you require up to 16 in / 12 out, we offer the Smart-PAK PLUS and SK1600-RIC as the best solutions. For the 15 / 15, the SK1600-RIC and Elite-2000 would be your best options. Information can be viewed through our web site - www.entertron.com If you have any questions, you can email us from our web site, complete our contact form or call directly. All information is available on our web site. Hope this helps. God Bless,

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