Yes sir, a detailed version would help me tremendously.
Here it is.
Solution Description for the LogixPro Silo Lab Exercise 1 – Continuous Operation
In this exercise, you are asked to "completely design and de-bug a ladder control circuit which will automatically position and fill the boxes which are continuously sequenced along the conveyor." You must also use all the Outputs (Conveyor Motor, Solenoid Valve, Run Mode Indicator, Box Filling Indicator, and the Box Full Indicator) and 4 Inputs (Stop PB, Start PB, Box Proximity Sensor, and Box Level Sensor).
One way to lay out a framework for your program is to simply add a rung for each output, starting with the Conveyor Motor O:2/0, and ending with the Box Full Indicator light O:2/4. Normally this would be a logical and correct method, but in some PLC programs, some rungs must precede others, or the logic will not work exactly right. This is one of those cases, where the Solenoid Valve Output O:2/1 must be placed after or downstream of the Box Full Indicator O:2/4, or spilled material will pile up in the floor and cause a plant shutdown! The reason is that if the Solenoid Valve comes before the Full indicator, the PLC will have to scan another cycle (after the box is full) before the solenoid valve is cut off, allowing some extra material to fall before cut-off. For the solution I posted, I left it with the Outputs in numerical order, so the students would need to figure out what was causing those spills, and also to introduce the concept of using an analog box level indicator using a counter, and later the concept of dribble-mode feeding.
Rung 0: This is output O:2/0 for the Conveyor Motor. It is controlled by a XIC O:2/2 System Run, and a parallel combination of XIO I:1/2 Box Proximity Sensor and XIC O:2/4 Box Full Indicator. This says that the conveyor motor cannot run until the System Run relay (and indicator light) is ON. (It is a good practice to use a RUN relay of some type for nearly all programs.) Also the conveyor will normally run until the box is over the proximity sensor. Once there, the conveyor motor stops. Then the fill is done (see Rung 1), and the conveyor needs to be restarted. We can’t use the box prox sensor to restart the conveyor, because the box can’t move out from under it without other help. What goes ON at the time we need to move the full box? Right, the Box Full Indicator. So use XIC O:2/4 to restart the conveyor motor and keep it going until the I:1/3 Box Prox Sensor is again not made or false, allowing the top parallel branch to take over and keep the conveyor running.
Rung 1: This is output O:2/1 for the Silo Box Fill Solenoid Valve. Again I used the System Run Mode as a master control, followed by a XIC I:1/3 Box Prox Sensor. This sensor must be ON with a box in place, before we open the solenoid to dump material into the box. Now we have it started filling, how are we going to stop it filling? We need something that goes OFF when the box is full. I used a XIO for O:2/4 Box Full Indicator. Using the Box Full Indicator here allows that indicator to be controlled by the Box Level Sensor, but also a parallel back-up analog control constructed from a clock bit and a counter. This allows the box fill to be stopped at any level from 0 to 100% full, if desired (See the paragraph for Rung 4).
Rung 2: This is output O:2/2 for the System Run Mode (also the RUN indicator light). Normally this would be the very first rung for many control programs. I think Bill Simpson maybe only intended this to be a run indicator, but a System Run relay was needed, so this worked for both. I used the classic Stop PB and Start PB with seal-in from the RUN relay. This is such a common useful method that it should be the first thing taught in PLC classes. Instead, I see students on PLCtalk.net using latched outputs for their motors, with numerous latch and unlatch points for the same device, making the program hard to troubleshoot.
Rung 3: Output O:2/3 is the control panel indicator light for "FILL". The box is filling anytime the solenoid valve is ON, so I used a XIC for O:2/1 Solenoid Valve to turn on the FILL light output. Normally control indicators are not cut off when the RUN mode relay goes off, so that the operator can still know the situation even when the system is off.
Rung 4: When the I:1/4 Box Level Sensor goes ON, we want the O:2/4 Box Full Indicator to go ON and stay ON until it is no longer valid or needed. The XIC I:1/4 takes care of turning on the Full indicator at the moment when the box is full AND still under the sensor. But I need the Full indicator to stay On until this box is gone and the next box pops up on the other end of the conveyor. So I must self-seal the Full indicator with a bit from itself, and keep it on until the box has moved far enough that the Box Prox Sensor goes off again. So I used XIC I:1/3 Box Prox Sensor to turn off the Box Full Indicator, so that it is ready for the next box.
As a bonus feature, I used Counter C5:0 to count the amount of material in each box (really it counts the time elapsed during filling), in effect an accurate analog level sensor. This creates a parallel back-up sensor for the Box Full Indicator. A redundant or back-up sensor is often a good idea for critical steps in a process, where the failure of one sensor could result in a great loss. The C5:0 Preset can be set at different levels from 0 (empty) to 93 (full).
Rung 5: This is the optional bonus Box Analog Level Indicator, using the C5:0 Counter that counts pulses from the fastest internal clock bit S:4/0 while the solenoid valve is open. Box is empty when Counter C5:0.ACC = 0, box is full when C5:0.ACC = 93. 50% full is 47, 25% full is 23.
Rung 6: The optional box level counter is reset when the I:1/3 Box Prox Sensor goes off (box has moved downstream of the sensor).
