I am in the process of choosing a control system for a chicken litter gasifier. I have been looking at different options. The client wants an hmi. I have been looking at the ctouch and the directlogic205 or similar from automationdirect. Since I have some background in microcontrollers and also vb.net, I have also considered creating my own cpu and controlling standard i/o modules via a modbus/rtu communications module.

I think the plc method would be easier to get up and running but here is the crux of the matter to me. There will be a fairly complex pid style loop. There are numerous thermocouples inside the gasifier. Depending on the temperature of these, the incoming air will need to be adjusted to some as of yet unknown formula. This will be controlled with a frequency drive on a 1/4 fan motor. The material feed may also need to be adjusted or modified from this loop. It will be a hydraulic feed. The gases will be burned in a controlled chamber also. This also will need some sort of controlled loop.

My preference would be to be able to program this in c or the like. However, that is not a must. I see many plc's software (at least directlogic) is unable to do low level programming. Will off-the-shelf PID take care of these requirements? Are there any recommendations on what level of equipment would be needed?

 

The best process control engineer I know is Gregg Knapp. He is also a chemical engineer which brings an additional benefit. His firm has designed and built numerous SCADA systems for me.

The firm is Knapp Engineering Prospect CT
(203) 758-3503

 

There's absolutely no point in re-inventing the wheel here. Microcontrollers & Modbus I/O is not going to do the job you want. Yes, you're going to need a fair amount of PID control, which is hard for a micro controller but very easy in a modern PLC. An HMI is only half of the job. If you go the home built micro controller CPU route you're likely to spend 2 months doing a 2 week job and the next 5 years fixing it when it breaks.

Also, be aware that there's not much difference between a gasifier and a boiler, and you need to be sure that you understand the legal requirements for controlling and operating such a device. If you are using a gas burner to start the gasifier for example, our local regulations say you must use a SIL-2 rated controller.

We prefer ABB's Compact products range but there's plenty of other choices out there. Siemens & Allen Bradley have good solutions as well.

Rob
www[.]lymac.co.nz

 

I agree with you, Rob. I don't wish to create my own setup. There is a little flexibility in micros that you have to give up with a plc though.

I just wanted to bounce this off someone with a little experience with PID before embarking onto what could be a dead end path.

As far as the legal requirements, We will have to check that out. The main thing we had in mind on this project was to avoid any pressured water. We haven't decided what type of ignition there will be. It may be hot air in the bed, and I don't know about the burner yet.

In your opinion, is it better to stay with a plc + hmi setup, or use something like Unitronics combined setup?

One more question about plc & pid - How feasible will it be to have user adjustable temperature set points? For example I am guessing that different locations will require somewhat different settings because of barometric pressure, etc.

 

If you have quite a small plant, then your "HMI" can be as simple as a dedicated off the shelf display unit communicating with the PLC via a Bus of some kind. You dont need a PC except for programming the PLC. We would typically do a small job of less than 100 or so I/O job with an AC800M controller and a separate "Process Panel" display.

Most PLC's are very very good at doing PID control these days and will include all those features like anti-reset windup, output tracking, Setpoint tracking, cascade control, setpoint & output limit checking etc etc that you really don't want to code and test yourself. User entered setpoints are a fundamental part of PID control in a PLC.

Obviously, it's mportant that you choose the right type of PLC. The cheaper and more basic "brick" style of PLC are not going to be as easy to program or as flexible in what you want to do. I'm not familiar with the Unitronics products but from their website it seems they only support ladder logic programming and they may sacrifice some of their PLC functionality to be an HMI. However, the decision is up to you whether it will suffice.

Regards

Rob
www[.]lymac.co.nz

 

Which plc vendors would have software that included structured text? Would there be any for around the $500 software mark?

I may check out the ac800m. I am curious how they line up in the price range. I see their software has structured text, which would be the least trouble to learn from my perspective. I would also like to know if there is some form of interrupt or multitasking allowed within their structured text.

 

Structured Text is not always the best language for what you are trying to do. ie. simple PID control. Function Block Diagram (FBD)is easy to program and understand once you get the hang of it.

ST doesn't allow multitasking & interrupt handling because thats all handled automatically by the PLC operating system. With an IEC61131 based PLC you simply define the execution rate for your various programs, and the OS will interrupt and prioritise your control logic, perform I/O scanning etc as required.

The AC800M is not a $500 PLC but you need to balance that against the cost of your engineering time.

You can contact us through the web address below if you want to know more.

Rob
www[.]lymac.co.nz