Relays Instead of PLC in High Speed BarCode Application for Pharma???

jdcochran

Member
Join Date
Jun 2004
Location
Atlanta GA
Posts
5
OK, for my first post, I need a sanity check. For justifiable logistical reasons, I proposed to a client that I might be able to save them lots of FDA validation effort$ on a bar-code scanning operation to augment an existing line. How? By staying out of their existing PLC logic and adding an ancillary device/system that was not subject to extensive code validation and revision controls.

I proposed to accomplish this with a simple prox switch, simple relays, and some relay counters.

Now that I did it, I'm fearing that the line's high-speed indexing rate may byte me in the ASCII. :D

In essence, a small carton is scanned for pharmacode and compared with an expected result. If a bar code anomaly occurs, the product is rejected. Normally, it would be simple to immediately activate a reject function (in this case, a solenoid is activated). However, the reject action does not occur until later down the line, say 10 cycles later.

At any rate (no pun intended), I sketched out some prelim relay logic code. I have a few other logic decisions to make in the system... but ended up with a prox switch (for trigger), 2 counter relays, and 6 standard relays.

Questions:

1) what is the maximum rate at which todays off-the-shelf industrial relays (24VDC coil) typically respond?

2) in a PLC program, I must concerm myself with scanning sequence. In the hard-wired world, I must be concerned about everything happening at once. Any gotchas I need to consider before implementing a high-speed hard-wired relay system?

3) my line speed is somewhere between 400-600 clicks/min. Is this doable with discrete components? I'll even consider solid-state devices. Any suggestions on high-speed counters relays that could stand up to this speed? Discrete relays that can take the abuse?

I have a vision system as a contingency (has internal logic capability and runs at 3600/min rate). However, I'd like to present both options, the former being less expensive than the ladder.

[subtle PLC humor intended in previous statement]

Any assistance will be greatly appreciated and returned in kind.

-- John
 
Last edited:
I'm going to suggest that your two counters, and 6 relays sockets, etc.. are going to cost far more than a small PLC like a Automation Direct DL05 (about $100.00), that has more relays, timers, and counters than I care to count. A simple display (for your counts) like a DV1000 could be plugged directly into port 2 of the PLC. The DL05-DR has a couple of hi-speed inputs also.

The big advantage is that it will be more flexable for the usual necessary adjustments.
 
elevmike said:
I'm going to suggest that your two counters, and 6 relays sockets, etc.. are going to cost far more than a small PLC like a Automation Direct DL05 (about $100.00), that has more relays, timers, and counters than I care to count. A simple display (for your counts) like a DV1000 could be plugged directly into port 2 of the PLC. The DL05-DR has a couple of hi-speed inputs also.

The big advantage is that it will be more flexable for the usual necessary adjustments.

Mike -- thanks for the response. The validation considerations are driving the decision to attempt this with a non-programmable solution. Any device that has a programming port is subject to an order of magnitute more complex and expensive set of validation documentaition, testing, revision controls, etc.

Thanks Again,

John
 
By 400-600 clicks per minute I assume you mean number of decisions.
600/60=10 per second. Depending on a number of factors mechanical relays will begin to fail between 1 and 10 million cycles. Add in 6 relays and the probability for failure increases significantly. I think the mechanical relay solution will very quickly have reliability issues.

If programming is out, look into designing a circuit board with old fashioned shift registers, counters, and mosfet output driver.

See if the requirements for validation and revision control can be relaxed if you use something like a PIC or Basic stamp with write once capability
 
Project ended up with a PLC anyway. A-B MicroLogix. Designed, fabricated, tested, shipped and commissioned in 3 weeks.

Thanks all for the help.

-- JDC
 
When relay speed is a concern then I would look to SSR (solid state relay). The cycle time is as fast as a LED.

Rod (the CNC dude)
 

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