Design Question

I have used plctalk.net as a resource for several years now--but this is my first post. I'm really just looking for some design ideas for the next project that I've been assigned.

In our shipping department we have a transfer car that runs along rails. I work in a steel mill, and the transfer car carries up to 20 ton steel bundles from one bay to another. The transfer car is powered by a 10hp AB 1336 VFD, fed via channel cable on a retractable reel. The distance from one end of the rails to the other is about 240 ft.

Our dilemma... we need to automate this transfer car. My partner and I have the general design in mind, but we need some better ideas as how to measure the position of the car on the rails. The stop position of the car will depend on the size of the product they're running, the number of cars on the tracks (between one and four) and the position in the bay where they want to weld on product labels.

We're throwing around the idea of using an AMS200 laser distance sensor--connected to our SLC5 Plc via RS232 or profibus. The problem is this sensor is 4000 bucks and we're not sure we can get the capital approved. Short of using a series of photo sensors along the distance of the track, does anyone have another suggestion on how we might track this position? We've thought of using a wireless rotary encoder on the tail reel...

Thanks in advance.

Walker

What is your budget for entire project?

In the past on some car body skid conveyors we use a series of puck proxes for decel and in position at each station you need to stop. much more reliable than photoeyes but you need to have a good way of flagging each prox. Also you won't need an encoder or any type of distance sensor.

Thanks for the replies... I don't have a total budget yet. This project will be based on ROI analysis. My best guess is that my budget will not exceed 4000 dollars--hence, the AMS200 distance sensor is out. The SLC5 rack is already in place... this is a re-design. The original process used a west and east analog timer--operators could adjust the timers to get the car to stop in the desired spot. The problem is that the stop position will vary by up to 25 feet (we'll call this the stop zone) in one of three locations along the 240 foot railway. We are not using (currently) a dynamic braking system--the car coasts to a stop. However, the coast distance varies based on the number of cars they are using (one to four) and the amount of weight on the car (from 5000 to 20000 lbs).

I can use a 4-20 input in RSLogix 500 to give the operators an off-set adjustment on the stop zones, but my primary concern is figuring out a rugged field device that will give me the position of the car in these stop zones.

Thanks again, in advance.

Walker

In the past I used latched limit switches to signal where my car was located, basically it is a limit switch that is touched by the car while traveling forward and remain in the latch position, the limit switch is unlatch when the car is going home, so even if you loose power you will always know where your car is located, this limit switches are to be conected to the plc.
I recall they where produced by a company named namco.

I've done something like that back in the low budget days.
I just used a sensor and flags and counted my way down the track.
Actually I counted teeth of a rack. I don't know the resolution you need.
That is if you have I/O, or the profibus running in the reel.
Call it a low budget encoder.
I work for a steel mill also.
Capitals seem to only get questioned at 500k or more.

I formally worked for a packaging giant in Melbourne. They have a large Pentek system there. There are 4 trolley cars all up servicing 150 odd conveyors. Pentek's solution was a shaft encoder and a reference P/E mounted halfway along the run. At each end there were also prox sensors as over run protection. The cars control was quite intricate as it preformed measurement functions on the product been transferred, but this went back to an onboard Omron PLC, through a 110V 2 wire comm bus and up to the overhead buss bars.

Wireless can be achieved cheaply. IP (serial to IP if needed $230) at the car end, 2 X $50 wireless router's and then off ya go into the network. A ML1400 would suit well for this one.
IMO HTH

Thanks for the input... I'm completing the re-design today. I've decided to create a distance reference using a prox/sprocket design on one of the idler wheels on the drive trolley car. As an added bonus, I'm replacing 90 feet of guard railing with light curtains.

Thanks again,

Walker

wdreynolds1 said:

I'm replacing 90 feet of guard railing with light curtains.

Click to expand...

I'm impressed. 90 feet of E stop light curtain to replace a guard rail and the other "goodies" for less than $4,000.

Remember the guard rail (if high enough) will keep people out.
The light curtain will stop the systyem as people approach but once they are through the cutain then the system can be reset and re-started.
Regards Alan Case

Alan,
You're exactly right. I'm not replacing all of the guard railing with light curtains (we avoid calling them e-stop curtains for legal reasons). There is over 450 ft of guard railing around the entire the rail system. I'm replacing three 30ft sections with light curtains in lieu of gate/prox set-ups. The material handlers need to access the cars to perform rigging work on the steel bundles. The previous system used swinging gates/prox and a push-button reset for the protection, I'm replacing those gates (because they continually become damaged by traveling bundles, falling material etc.) with light curtains and push-button resets. We keep everything for this design in stock so I don't have to fill out a capital request for anything other than 300 + ft of R/P conduit.

All the best,

Walker