estop used to unlatch motor

[et8sec 0]

WHERE IN A PROGRAM WOULD YOU PUT AN ESTOP TO JUST TURN OFF OR UNLATCH THE MOTOR CONTACTS SAY FOR EXAMPLE THE PHOTO EYE WENT BAD AND THE MOTOR/PUMP KEPT RUNNING SO YOU HAVE TO HIT THE ESTOP IN ORDER TO STOP THE MOTOR/PUMP .... and sorry about the caps i just saw it was on here is my code i`m trying to write and estop and a man/auto switch is kicking my butt SCISSOR LIFTS.RSS

[Ken Roach 194]

That's sort of a trick question: you cannot use a general-purpose logic controller as a safety device. Your emergency stop circuit must interrupt the power to the pump motor and solenoid valve directly, not through any action of your logic program. That being said, you generally want to use an auxiliary contact from the E-Stop circuit to unlatch the logic that starts the motor. That's a very ordinary "motor seal-in" circuit. Is this for a real machine, or is this an experiment or learning lesson ? I wouldn't use Latch and Unlatch instructions, but rather a branch with the Motor output bit as a seal-in. If the "Up Switch" is pressed, while the Photoeye is not made and the E-Stop is not tripped, the Motor will run until the Photoeye is made or the E-Stop is pressed.

[et8sec 0]

yes this is for a real machine i am trying to leave the original buttons/etc there and add a man/auto switch when in auto and the machine is pushed on the lift a prox sensor will detect it and drop the safety clams , then when the operator pushes the up button (once) the motor and pump run until the wheel blocks the photo eye turning off pump and motor, once that is done a 2-5 (not sure yet) timer will run then engage safety pins so the lift can not drift down. the reverse is down button in pushed (once) safety pins retract delay timer when timer is done only pump solenoid is engaged and lift comes down. when down another prox sensor will turn off solenoid and disengage clamp and turn and a timer will turn off first prox sensor for 3min while the operator has time to pull the machine off the lift and reset....... so if i don`t use latch commands how to i turn off pump and motor in the up cycle once the photo eye is blocked?

[et8sec 0]

also the photo eye is being used as when down it see the reflector and can turn on the pump/motor and stays true until it is blocked turning off pump/motor........... this is for a machine i run every day and have talked my boss into letting me automate it for safety and less down time( me holding the button up/down to run the lift.) it has been about 15 years since i have messed with plc's

[et8sec 0]

oh i forgot about the original estop yes i am using an estop switch but just as my man/auto switch don`t i have to list it in the program?

[et8sec 0]

can anyone that has looked at this help me any? i am not a plc programer i only want to automate a machine i run every day to save me some steps, i think i have explained the steps needed to write the program i am just having problems thank you

[Ken Roach 194]

I liked it better when I thought it was a homework assignment ! Your description of the machine includes three different mechanisms or circuits referred to as "safety". If I give you advice on how to automate this machine, and somebody gets hurt, we're both in legal jeopardy. Because I have a professional license, my obligation to give good advice (or none at all) is different from a random Internet forum member. Machines that can hurt people have a whole pile of legal and practical requirements, starting with OSHA and the NFPA here in the States, and extending to things like the Machinery Directive in Europe. You absolutely have to start with a risk assessment before you even begin to think about programming a PLC to control the machine. It sounds like a neat challenge, and other Forum members might be able to give you advice on typical latch and sequence instructions and program elements. But I have to bow out.

[JeffKiper 9]

I have to agree with Ken. Anything that says "safety" is red flagged. Not always a stay away from it red flag , but always a CYA. My 2 cents worth on the safety side of things. If you are using any safety rated device to protect the operator, environment, or equipment ( in that order usually for me) then you automatically have to be in safety rated components, assembled and installed accordingly. The risk assessment that you and your team will do, will point you in the direction of what Performance Level the safety system needs to be. Please don't think we are just being harda$$es and wont help you. We are trying to help you. Remember OSHA has a definition of competent person http://www.osha.gov/SLTC/competentperson/index.html I am not saying you are not competent. I am saying that without knowing your background and not fully understanding the scope of the system I will not be able to help in the design. The legal side of things get very scary. I am a one man show. So if you got advice that I gave and misunderstood my intent. You can come back on me both professional and personally. That would jeopardizes my wife and kids. Enough said on that. Useless rambling to follow. The standard PLC has 1 processor to evaluate the state of the device. In a safety system you at least 2 processors and often multiple processors. Think if a single channel stop pb button. 2 wires to the pb 1 supply 1 return. Look at what a shorted between the 2 will look like to a standard plc. The plc will think the pb is in a normal state when in fact it is bypassed and will not open the circuit when pressed. Now use a 4 wire estop with safety I/O. The safety I/O can detect the short between the wire when setup accordingly. In my experience a simple sloppy installation job can kill any installation. A wire not stripped and terminated neatly can bypass a safety switch easily. Today as things get smaller and smaller terminals get closer and closer. Years ago we had room in switch enclosure. I have spent $5,000.00 replacing a weight scale and a loadcell because I had a wire stripped and a fray wire jumped over from 120 to the control board. The magic smoke was released.

[Shiner 2]

Jeff and Ken, I have followed your postings for a while and you bring up a good point that I have warned my customers about on a regular basis. I am curious though, have either of you gone through a certified SIL assessment training? Is there such training/certification? I have briefly looked without good results online. We come across alot of instances where a customer wants some sort of safety system integrated into their machine. We have gone as far as to build the controls for it if they design and rate it, but we will not install/modify/put 2 cents in on anything like that for the reasons you mentioned. We have them sign off that they did the assessment and designed the control to that end. It would be nice to offer that in our bag of tricks.

[JeffKiper 9]

I don't know of a certification program program. I always have my customer be part of the risk assessment. They know the machine better than we do any day. Start looking at PL (Performance Level ) instead if Categories. Side note I have seen a safety PLC put in that was no safer than a standard PLC. The installation and program makes all the difference. There are several ways to program them. Old school terminology being used. Take for example if you need a CAT B safety circuit on a PowerFlex 70 drive. You can use a standard contractors and kill the power. If you need CAT 3 you have to use the safe off board with a controller for EDM and use 2 independent outputs to control the enable and safe-off. I got to see where a guy was using a Compact GaurdLogix for this, he used standard I/O for this. Obviously this isn't rated for a CAT 3. He thought because it was a safety controller that made the I/O was rated as well. NOT TRUE.

[Shiner 2]

A chain is only as strong as its' weakest link...........

[JeffKiper 9]

You are correct. Yesterday I was at a customers shop and they had an E-Stop on an HMI. The alarm banner covered the E-Stop up and they couldn't stop the machine. I guess things only get better for us.