PLCs/PICs (computer interfacing and controlling)

[g8rpi]

Hi Joe,
Welcome to the group. My issue with PLCs is that you have to buy one for each project. The LOGO unit costs about £100 ($150) with the display about the same again. A PIC chip Costs less than £2 ($3) with a bare display about the same. The basic LOGO starter kit costs £260 ($400). it only has 8 inputs (up to 4 Analog) and 4 outputs. For that you could have a PicKit2 debug Express, full PicBasic Pro, Display and £50($75) change. I've not looked at it in depth but the LOGO units functions seem limited. PICs can easily be linked to PC's running VB or C applications using the serial port on the PIC and a serial to USB lead (if the PC does not have a serial port). The $3 16F887 PIC that comes with the PicKit2 debug express has 33 inputs or outputs (including 8 x 10 bit analog in, 2 x 10 bit PWM analog out). It can drive small relays directly and only needs a 5V supply to run. A demo board is $25. its the same one that comes with the PK2 debug express. They are cheap enough to use for even basic functions such as simple timers, though a smaller 8 pin PIC costing less than $1 may be a better choice for simple tasks.
Regards,
Robert.

[joe0545]

Hi Robert,
You are quite right what you say about the PIC chips being v cheap, but one would also need the necessary electronics skills and resources to be able to make printed circuits or ( strip board yuck ) and the soldering etc, to be able to put them to use or as you also correctly say, buy the PicKit

Btw I have used the same plc for many different projects, I just download the configuration I want to use connect the I/O and hit run,

I just thought I would mention the LOGO as a ready to mount connect and configure option for those who don’t want to get too deep into electronics, and ladder logic is dead easy to pick up

Either way anyone who does not have much programming experience would have a bit of a learning curve to deal with,

Not in any way suggesting PLC is better or more suitable, just that It may be an option for some people

Take care

Joe

[g8rpi]

Hi Joe,
Sorry for the slow reply. I don't disagree with what you at all, as you say PLC's are an option. I've just found that PICs are a lower cost, more versatile solution. I have the advantage of being handy with a soldering iron which gives me an advantage. I started this thread with the view of finding if there is any interest in a readymade PIC based controller designed for use with turbines. It could be ready programmed for standard applications with the user just entering calibration factors, limits etc. those who can program can write their own in PicBasic or C. Standard interface circuits for turbine applications would be included. If anyone is interested, drop a note in this thread.
Regards,
Robert.

[ernie wrenn]

That seems to be what I need. Stupid simple... I can get Richard to program it up.

Ernie

[jettoymaker]

Hi all,

The problems for John and myself is that our field of expertise isn't electronics and a ready made controller , plus peripherals would greatly assist and facilitate our design work by freeing us up to do what we do know best. If we could all come to some consensus as to what should be included and in what form of controller, then we could all benefit from each others specialist field...with fewer delays in forward momentum in the overcoming of the many other difficulties faced. I believe Robert had kicked things of in this discussion by trying to determine if a ready made unit would be a better way to go and I would have to, in my case, lean heavily towards purchasing a ready made controller...however in my case it would have to be affordable as both John and I have been on pensions for some time and project funding is at bare minimums.

Any thoughts??

[ernie wrenn]

Understand the money end, I am 62.

Everyone need to kick in a list on what you want.

Ability to read egts (2)
RPM % . This is easier than a true rpm tach due to way to many vairables in turbines
fuel pressure
oil pressure
fuel pressure

Add to the list as necessary. If several of us band together we may be able to go factory direct for pricing. I have years of haggling... might be able to get them involved.

Light sponser ship with product would be nice. We could ALL run their decals.

Ernie

[g8rpi]

Hi Ernie,
I was actually trying to see if there was interest in a cased unit / assembled circuit board / bare PCB (depending on capability of user) that I, and perhaps other group members could design. I can certainly design the electronics and circuit board. Adding two EGT's and a tachogenerator input would increase the cost of a PLC solution quite a lot (two expansion modules). For a custom solution the cost is minimal. EGT's are the most expensive, needing a special interface. For a custom unit there would be 1 channel fitted and space on the board to fit the extra chip if two were needed. The other interfaces are fairly easy with a PIC and couple of cheap chips (comparators amd op-amps). I have a design that worked well with my GTP-30 so I know it works. If other group members can help, with machining boxes or PC display programming for instance, we could have a good solution. post your interface needs to this thread please.

Robert G8RPI.

[ernie wrenn]

I have complete a complete machine shop compltete with CNC and waterjet. Several of the other members can do programing.

I am just on the lower limbs of the dumb tree when it comes to PLC/PCB etc..

My whole life has been industrial HVAC, machine shop, mfg'ing firearms for the military and other non-intelegent operations.
ernie

Richard not only does programming, he also builds computers for any needs.

[jettoymaker]

Hi Ernie,

I was would agree with those types of controls being part of the base programming, but we would also need additional controls to include starter ramp-up, over speed detection etc. in a package that would be expandable or flexible enough to install other requirements as needed or assumed.

John would agree that it will have to be idiot simple in that it have the ability to be at a "plug and play" entry level, with possible USB port or equivalent for programming adjustments or tweaks. AMT makes a FADEC for hobby jets that have included most of the basics we appear to also need in a larger engine, although we learn new things all the time.

I hope we aren't asking too much in this regard, but as we don't have a grasp in this field as to can or can't, I thought it might be helpful to establish just what is doable and what isn't.

Andrew

[g8rpi]

Hi Andrew, sorry for the late reply I've been a bit busy. Overspeed, overtemperature and low oil pressure protection / shutdown are easy and would be implemented as standard. The user would just have to set the limit values. I'm not sure what you mean by starter ramp-up. Most starters I'e seen do not have speed control. Some have a series resistor to limit inital torque that is then shorted out. A relay output to short a series resistor is not a problem. One thing that the unit I'm propsing is NOT is a FADEC. While a PIC could be used as a FADEC, it would have to be engine specific. 'the proposed unit would basically be a start sequencer, protection unit and instrument / PC interface. The "instruments" would be an alphanumeric LCD display or/and PC running either a terminal program or dedicated application. The PC application is not difficult but is not my "thnig" so I'll leave that to others. The PIC would output an ASCII (text) string with the current parameter readings and status. The unit would be set up using a simple program running on it using a few switches to select and set options, limits etc. For more advanced users an in-circuit programmer such as the low cost (<$30) PicKit2 could be used. Just about anything is possible. My idea of the basic unit is a single unit with a two line by 16 character LCD display.
Robert.

[Richard OConnell]

The way I was looking to do it orgiginally was with a PC and a simple I/O interfacing card like a parallel port or something. The parallel card can not, however, determine a sweep in current like a tachometer would produce. I really want some hardware that serves as an interfacing gateway, rather than a standalone part. I learned how to interface to the parallel cards and use them to turn on and off LEDs, so those could easily be replaced by relays. I am experienced in programming, so it really wouldnt be a problem.

I think a lot of these cards you guys are talking about carry the programming onboard, where I am looking to kepp the programming on the computer itself in one easy to debug location.

[jettoymaker]

Hi Robert,

Thanks for the response, it appears to be the sort of start we would like to see in a controller. The "ramp up" I was referring to is the requirement of the starter to be moved slowly enough into engagement with, in our case, a drive dog, and once in proper engagement, allowed to fully accelerate to self sustain. John also feels that in the initial part of our development, that a controllable time interval, up to 30-40 seconds, should be included to control motor speed from a couple of rpm for alignment and engagement and then a proportionally gradual increase in rpm...to allow for variables and allow for operator intervention if necessary.

Once a full series of occurrences/parameters has been established for a motor/engine combination start-up, then the controller should be able to be adjustable for the time to self sustain interval..as the 10/98 unit has been proven to be able to reach self sustain from zero in around 5 seconds or thereabouts.

We, and most likely anyone wishing to have electric start would need the control unit to give a very slow first rotate, so I was wondering if the series resistor you mentioned would be a single step usage compatible with the starter sequencing required?

Sorry my ignorance but most of this is not my thing

Regards, Andrew

[ernie wrenn]

The starter controller can be a Curtis motor controller from a elec golf cart. The unit is controlled by a variable resister pedel and will give 1 to 100 % with full torque and handle up to 400 amps. Units are available in 12/24/36 or 48 volts and are available used from E-bay for approx $50/100 ea.

I have used several on starter projects and they are bullet proof. This is the only controller that worked... I now there are several other mfg'ers (elec forklift etc) but the Curtis unit is a 100% duty cycleand you can be sure I have tried to kill them.

They also work great for electric powered car projects.

Ernie

[g8rpi]

Hi Andrew. The same board would be able to fullfill your requirement as well. It would send an ASCII string containing EGT, RPM, oil pressure switch status etc, and receive commands to activate relay drive outputs, starter speed (see below) control etc. Essential monitoring and shutdown can be built in so you don't loose protection if the PC has a problem. The interface would be serial RS232, either real or via a USB adaptor (FTDI chip). You could also program it yourself in C or PicBasic Pro. This would be more suited to turbine control than a "generic" I/O card. In fact you would probablly need at least two standard cards.
Ernie thanks for the info on speed control. This is not normal on aircraft, but I fully understand and think it's a good idea. The PIC has a hardware PWM (pulse width modulation) output that cand drive a high current MOSFET bank or produce a low level control voltage for a golf cart / wheelchair module. At $50-100 it's not worth building your own. By the way my background is as an Avionics engineer / designer who has had a few years out designing robotics for biotech applications.

Robert.

[g8rpi]

I just did a quick check and have confirmed that the PIC can drive the standard Curtis controller 5K potentiometer input. Didn't see any for $50 though ;-)
Robert.