Continuous Forward/Reverse Motion

[mechenix]

Awesome, thanks. I had built something very similar earlier minus the interlocking feature. However, I thought that it wouldn't work when I was messing around with the demo software. I guess if the limit switches are pushbutton, that type of logic should work fine for continuous motion.

It looks like the micrologix may not be what I need, as it has only 1 DC output. I believe I'll need 2 DC out, 1 for each relay (the fwd and rvrse). I've been looking around on automationdirect.com, maybe I'll need one of the expansions.

 

[Lancie1]

It looks like the micrologix may not be what I need, as it has only 1 DC output.

What? When did that happen? The MicroLogix 1000 User manual, Chaper 2 Wiring Your Controller, Page 2-20 plainly shows 8 relay-type digital outputs, with each output able to be powered with a wide range of DC or AC voltage. See the attached copy of that page. Above each output voltage terminal, there is a label that says: "VAC / VDC". That means that eiter type voltage can be used for this PLC internal output relays. Besides that, I have wired up this model of the MicroLogix 1000, and know that its output relays will operate from 12 VDC, 24 VDC, 120 VAC, or even 240 VAC.

I think you have some studying to do before you have to teach this stuff.

I guess if the limit switches are pushbutton, that type of logic should work fine for continuous motion.

No, No! Limit switches are different from pushbuttons. A limit switch should be "made" or "operated" when its design condition is met. A Forward Limit Switch should be "made" or "operated" when the device is at the forward position, and not operated or OFF at all other points. A Reverse Limit Switch should only be operated when the device is at the reverse position. A limit switch is a maintained-contact device at both its operated position and all non-operated positions. The contacts may be NO or NC at operated or non-operated position.

Now, if the device powered by your motor is at the Forward position, then a closed contact on the Forward Limit switch can be used to start the Reverse motor and drive the device in reverse. But as soon as it starts, it will move off of the Forward Limit switch, causing the closed contact to open and the motor to stop not long after starting in reverse. For that reason, you must have a reverse seal-in contact to keep the motor running in reverse until the Reverse Limit switch is operated to STOP the Reverse Motor. The same condition applies for the forward direction.

The motor interlock contacts are to prevent short circuits in case some student (or instructor) accidently somehow tries to energize both at the same time. As someone pointed out, for real Fwd-Rev motors, probably a short time delay befor switching directions is a smart idea, to give the typical motor starter contacts time to switch before applying opposite power. The PLC can switch outputs very fast (specially if they are transistor-type), but a motor starter contactor takes a little bit longer to switch.

In contrast, a pushbutton is usually a momentary-contact device because it is only operated while a finger is pressing it in. There are a few rare pushbuttons that are maintained-contact. Selector switches are usually maintained-contact devices because when you turn the switch to a postion, the contact is maintained in an on or off state until the switch is turned to a different position.

 

[mechenix]

Okay, I see now from that figure that the necessary outputs are available. I was looking at the summary on A-2 and it made it seem like what I needed was not included.

I understand that the limit switches are contact switches. I'll keep looking at the demo software and make sure I understand how it works.

Luckily, I will never be teaching this to anyone. I am very inexperienced with this stuff.

 

[Lancie1]

Here is the same program converted to run on the LogixPro Simulator software.

Step 1: Off Mode, parked in the Reverse position
Step 2: Start Pushbutton was pressed.
Step 3: Motor running forward, and Reverse LS has gone off.
Step 4: Forward LS tripped, Motor Forward stopped, Motor Reverse on.
Step 5: Forward LS off, Motor Reverse still running.
Step 6: Reverse LS tripped, Motor Reverse stopped, Motor Forward on.
Step 7: Stop Pushbutton has been pressed, Motor Forward goes off.

 

[Lancie1]

Here are the last 2 steps.

Step 6: Reverse LS tripped, Motor Reverse stopped, Motor Forward on.
Step 7: Stop Pushbutton has been pressed, Motor Forward goes off.

 

[mechenix]

Wow, nvm. I was just being an idiot earlier. I don't need any analog inputs/ outputs.
Also, I finally realized that the inputs/outputs are electrically isolated.

And yes, the logic ladder makes complete sense. The only thing is the user will have to manually close one of the switches once to get it to move initially.

Alright, I think I finally got it, thank you for your patience. Also, don't worried, the future of engineering is safe. I'm much better in my other classes, haha.

 

[Lancie1]

The only thing is the user will have to manually close one of the switches once to get it to move initially.

The only button you need to operate to make this program start initially is the Start Pushbutton. The 2 limit switches would normally be operated or tripped by the device itself (but can be simulated with manually-operated knife switches).

It is a safety thing. There are very few machines which should be allowed to start completely automatic, in my lowly opinion. Say it was a buzz saw, running, and the power went off for 15 minutes. Bubba decides to take a break while the power is off, but leaves the machine in RUN mode. Now if it could restart automatically as you wanted, suppose Joe the maintenance man decides to take advantage of the outage to do some clean-up around the saw. Then the power comes back on, and good old Joe's right arm gets cut off.

Others think differently, that it is okay to have a completely auto-start mode if there are proper safety equipment, safety training, and so on. Who can ever be certain that there is enough safety training and safety devices for any one machine? Safety assessments often miss small items that turn out to be very dangerous.

 

[Viks]

I understand that the OP is only trying to do this theoritically and Lancie1's code does exactly as what the OP requested. Now as the OP has this going, the OP could now try adding a homing sequence to take this to another level. A homing sequence would be required in a real world application to home the equipment after a power outage, a mechanical failure or even an estop situation. The code (provided by Lancie1 in pdf fromat) does not include this and will not work if either one of the limit switches are not made.
No offence to Lancie1 but I just felt like pointing this out for future references.
Cheers,

 

[Lancie1]

Viks, A homing routine would be a nice addition for extra student credit. Thanks for your suggestion.