Need some suggestions.. please!

Hi everyone,

I am looking for a cheap and accurate product for this simple application - It is a dosing application whereby liquid nitrogen is dosed for specific duration (dose duration time) into bottles traveling on a line.

There is a sensor that senses the bottle presence in advance. After this input is received, we need to wait till the bottle reaches the doser (dose delay time) and then open the nitrogen dosing valve for the period of the dose duration. The user has to have some means to change the dose delay time and dose duration time on some kind of an user interface. As you can see it is a simple application.

But the problem is the both these times ( dose delay and dose duration ) are in milliseconds and have to be accurate (+ 3ms max) I am also looking for a very cheaper solution for it. Some cheaper controllers I enquired have a scan time/ timers in increments of 10ms which will not suit this application.

Can someone recommend a good product that can execute this application and also have some means to enter the dose delay and duration values? Again, I need a cheaper solution for this

Thanks for your time!

2 issues:

1) Usually, (but not always), it is prudent to do a filling operation by weight, volume, pressure or something to sense the actual presents of the material being transfered. Time is not a good indicator for deciding as to the fill level. Your requirement for 3ms indicates your application requires great accuracy, and time is not a good tool to provide an accurate fill.

2) The requirement for your timing accuracy to be within 3ms is out of the capabilities of any plc I know of. Additionally I know of no coil activated valve that can fully open or close in this time frame.

To get around these two issues I would suggest a two stage filling process. The 1st stage would fast fill (higher volume rate) to about 80% or 90% full. The second stage would fill to full at a much slower rate that can be accurately controlled by a 100ms timer.

Mike.

Why not use a timer?

Timers are available which will time in .001 sec. intervals. These timers usually have a LCD display and keys on the front of the unit for setting the time period. Omron model H5CR, amoung others, has this capability.

Check out Giddings & Lewis MMC family. Relatively low cost with very high performance. This is a PLC that supports motion control, but I have used it on a number of very high speed, high performance, PLC ladder applications with 'roll your own' control routines. It's a screamer!

The control supports timed interupt task with 1ms resolution. The timed interupt is a true interupt and is extremely accurate, is not effected by overall ladder scan and CAN be used as a timer by counting scans. So if you needed to set a timer for 72msec for example, you would count 72 scans in your timed interupt task. Your accuracy would be +/- 1msec.

The input response times are 1ms for standard inputs, 50usec for 'fast inputs', and the output switching time is 50usec for resistive loads. I would recommend a solid state relay to drive your actual solenoid load. The basic 2-axis controller also supports 2 encoder channels @ 1Mhz/channel (after quadrature) and also includes 2 analog outputs and 1 analog input. The base unit has approx 20-24 inputs and approx 16 outputs, but you can expand with remote I/O blocks.

They offer a number of low cost display units ranging from simple 1 and 2 line displays all the way up to color touch screens.

http://www.giddings.com

The instruction set of the MMC is identical to the Pic900 family which also includes the Pic90.

Mike,

There is a system delay with the valve (38ms) but this is constant, so what we do is give an open command to the valve 38 ms before its actually supposed to open. So if the delay time (time to wait after the bottle is sensed till the bottle reaches the dosing valve) is 100ms. We give the open command after 62ms.

You are right, time is not a right way to open the dosing valve, but we have a lot of assumptions here - line is moving at the same speed, containers are present, etc.

Siemens, AB gives a scan time of 1-2 ms for this application ,but as I said we need a cheaper solution to this

Vic,

Thanks for your reply. I think hardwiring to this display timer can be one of the solutions.

Preeya

If you use Vic's timer to directly control the valve, and provide an input to the PLC from the timer, the rest of the operation could be done by almost any cheap PLC. Personally my favorate cheap PLC is the DL05 by Automation Direct (about $100.00).

You might want to check out the Giggles & Loudest link Greg provided above. This may be the best way to go, but that may be out of your price range also.

Best of luck with it.

Mike.