Back in 2007 I had a project at a grocery distribution center located in Stockton, California. A new conveyor system was installed that featured a forty-four lane sorter, four of which were pre-sort lanes, fed by a nine lane merge. I designed five control panels that contained the processors (Allen-Bradley CompactLogix & ControlLogix), IO, and motor starters.
The challenging aspect (see below video) of the project involved feeding the cartons onto the sorter. The gap between cartons had to be controlled such that a 9" minimum was achieved (due to sharp divert angles on the sorter) along with a normal gap value of 13.5" which was set according to the required production rate. The conveyor used to control the gap (a.k.a. gapper) was, mechanically speaking, 15 feet long split between 4.5 feet of reduced speed vs. 10.5 feet of full speed belt. The electrical aspect of the gapper consisted of an Allen-Bradley servo with corresponding drive which was controlled from an Allen-Bradley ControlLogix PLC via their fiber network (i.e. SERCOS). Other than that, there was one photoeye across the belt.
Watch the video here:
Cartons entered the gapper back to back, were slightly separated on the gapper due to the mechanical speed change, passed by the photoeye, were quickly slowed down by the servo, and finally exited the gapper with the proper gap. Fortunately, the sorter ran at a slow 350 FPM but, unfortunately, I had to write the code to achieve the gap control all from scratch.
I wrote code (using structured text for the equations and ladder for the motion commands) that, at first, recorded the carton lengths and the gap between each carton. Second and most difficult, calculated what belt speed and how much time would be needed to increase the gap to 13.5" all the while limiting the deceleration so that a carton would not tumble off the belt. Third, issued the motion command to the servo once the leading carton exited the belt. Fourth, returned the belt speed back to its full speed unless, of course, additional cartons required gapping.
The code had to execute very quickly because at 350 FPM or 70 IPS the belt would travel 1 inch in 14.3 ms. After much head scratching and tweaking, I was able to get the code to execute anywhere from 4 to 11 ms, most often at the lower end. However, I would have had a problem if the sorter ran at 600 FPM as some of my other projects did. At 600 FPM the belt travels at 1 inch in 8.3 ms. I had already dedicated a ControlLogix processor to execute the gapper code so I was doing the best I could with the fastest AB processor available at the time. All the gapper controls that I have seen were either controlled from a PC (e.g. Intelligrated) or a special purpose circuit board (e.g. Dematic) and, therefore, had a code crunching horsepower advantage over my PLC. Be that as it may, I am confident that AB will develop (they may already have) a faster processor that could easily handle a belt speed of 600 FPM and higher. Either that or I can further tweak my code to handle the increased speed. If it can be done with a PC, I can figure out how to do it with a PLC.
I had many questions during the project and several people and companies were instrumental in the success. First is the team at D.L. Neu & Associates who designed the conveyor system and were a pleasure to work with. Next, Scott Martin of McNaughton-McKay was the Motion Guru for my local AB distributor and, among other things, reminded me of Newton's accelerated motion equations that I had forgotten from back in my college physics classes. Finally, I was given a huge jump start in understanding the control requirements of a gapper from those who do it routinely - Intelligrated (conveyor manufacturer).
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