pick and place

hey all, I'm new to the boards and relatively new to PLC programming in itself. I have been working on a pick and place program consisting of a horizontal/vertical mechanism which is used to transfer work pieces from one place to another and it seems to be coming together, well i hope anyway. I have no software to test the program just working out of text books!

There is one requirement that is causing me bother, there must be safety interlocks on the clamp of the picker machine as it undergoes a process while also having a power failure recovery system.

Are these requirement just another input and output i have to include in the system, or is there specific instructions for these types of commands?

The failsafe is hardware. The picker clamp must fail in a clamped position, so it doesn't drop the part if power fails. If you use air or hydraulics, you must also protect from loss of these mediums.

Using screw drives with mechanical brakes is one way to do it.

not sure wut software ur using by I have found that the sim they have at www.thelearningpit.com to come very in handy. You have 15 days for a free trial and it is $40 which is pretty cheap if you ask me. could give that a try when u think ur ready.

unfortuneatley i dont have any software at my disposal. I was wondering however if anyone new of a shareware program that can be used to view say ladders created by melsoft Gx devellopper??

The program i'm working on at the moment is as attached

ballymeeney said:

unfortuneatley i dont have any software at my disposal. I was wondering however if anyone new of a shareware program that can be used to view say ladders created by melsoft Gx devellopper??

The program i'm working on at the moment is as attached

Click to expand...

Not having the software will be an impossible hurdle to overcome in any project.

Whoever is funding this project will need to fund the proper software and liscence as well.

i'm sorry to say no one is funding the project. I completed a certificate in cad cam engineering a few years back. This is just a side hobby,as instrumentation was a subject that was not covered in my certificate. I'm just trying to improve my knowledge for my own benefit.

"There is one requirement that is causing me bother, there must be safety interlocks on the clamp of the picker machine as it undergoes a process while also having a power failure recovery system."

Most processes are designed in a way that depends on the various energy mediums (electrical, pneumatic, hydraulic) always being available. And then, of course, if one or more of those is lost... there could be hell to pay!

Instead of assuming that all of the necessary energy sources will always be available... design your system for failure! That is, if any one of those sources is lost, have the affected devices default to the fail-safe condition.

In the case of your clamp, design it so that it has to be driven to open. If the particular energy source fails, the clamp should default to the closed condition. When you want to close the clamp under normal conditions, simply remove the "cause to open". You can use any of several methods to control the speed of opening and closing.

For your vertical movement, install a brake that defaults to the closed (engaged) position. Before moving vertically, open (disengage) the brake.

If the PLC goes through a power-cycle, then, it would be a good idea to use retentive memory to keep track of what was happening when the failure occurred. Hopefully you have a sensor at the clamp that indicates the presence of a part.
If not, then, when the PLC comes back up, have your code designed to assume that the clamp is holding a part. Then have the program go through the steps necessary to handle the "assumed" part.

The idea is... design your system under the assumption that one, or more, of the energy sources will fail at any time.

thank you for your helpful information. I think i may have been going along the wrong path in my own origional program. Whats confusing me is how i get the y1 output to come on as off so to speak so that the clamp will be engaged as detailed in the diagram?

i'm lost!

well this is really starting to bug me, but in the mean time encouraging me to find answers.

I have enclosed my idea of how the program should look, i'm still lost as to where the power failure recovery system would be placed in the system.

Any suggestions would be greatly appreciated or if you want to tell me to give up all together,jasus i wouldnt blame you, I could be completely wrong.

Thanks again for your help,

Start set latch Mo to zero point
--[x16]----------------------[set Mo]

Mo turns on y0 downward motion
--[mo]------------------------(y0)


Yo engages the clamp in unclamped position Y1
--[y0]-------------------------(y1)

lower limit x1 stops down movement y0 - y1 off and piece begins to clamp over 1.7 secs

--[x1]---[/]y0--[/]y1-----------(T0 k1.7 secs)

timer times out and arm begins up movement

--[t0]----------------------------(y2)

x2 stops up movement y2 and right movement starts y3

--[x2]--[/]y2----------------------(y3)

same again and arm down begins

--[x3]--[/]y3----------------------(y0)

stopss and opens over 1.5 sec
--[x1]--[/]y0--[y1]-----------------(t1 1.5secs)

timer 1 times out and proceeds with y2 up movement

--[t1]-----------------------------(y2)


x2 stops up y2 and starts left moving y4

--[x2]--[/]y2-----------------------(y4)

if photo electric switch is onreset to beginning

-[x5]------------------------------[rst Mo]

----------------------------------[end]

In the first place... the design spec is confusing.

The spec calls for the Clamp to "...clamps the work-piece onto the table..."

Actually, the design spec is quite poor. At the very least it is incomplete.

Be that as it may, the spec DOES call for the Clamp to be driven OPEN when Y1 goes ON. The Clamp DOES go CLOSED when the "cause to Open" is removed. That part is good.

This indicates that the name of the output should be... "OPEN CLAMP".

However, the Clamp is controlled by a single-action valve which requires an air-supply to drive the Clamp to either position. That is, when the valve is ON, the Clamp is supposed to be driven to the OPEN position, and when the valve is OFF, the Clamp is supposed to be driven to the CLOSED position - either case, BY THE AIR-SUPPLY.

However, again... if the air-supply is not available (failed)... then the Clamp will NOT be driven to either position in any case. When the air-supply fails, the Clamp will tend to remain at, or near, wherever it is at the time - without pressure!

Even though the valve might be OFF, which should keep the Clamp CLOSED on the part, if there is no air pressure, then the part might simply fall under its own weight.

As long as the air-supply is available, if the PLC drops dead, any part that is currently held will continue to be held. That is almost "fail-safe".

To be truely "fail-safe" the Clamp needs to be "spring-loaded" to CLOSE.

Without that, there is NOTHING to guarantee that the part will not be dropped.

Are you looking to do this the right way, making modifications to the spec as necessary... or are you looking to simply meet the poorly designed spec as it is?

RE: Terry woods

I think primarily for my own benefit I'm looking meet the poorly designed spec as it is. I'm new to this game and would like to have a soloution to match the spec provided for reference to improve my learning.

How close is the effort i displayed to a some what fesable ladder logic program that would work??

The ladder you posted is barely discernable as to its presence. That is, I can barely see that something is there.

I can see it on the screen, but I can't read it... nor can I read a printed copy.

If you insist on meeting that spec, as poorly designed, poorly described, and contradictory as it is, I think you need to step back and look at the process from a purely "human" point of view.

The Arm is at home, without a part in-hand, waiting to begin a cycle.

When a "part is ready to be picked up", go for the part until you are at the part.

When you are "at the part", grab the part until you have the part.

When you "have the part", lift the part until the part is lifted.

When the "part is lifted", transport the part until the part is transported.

When the "part is transported", and if the "right-table is empty", deposit the part.

When the "part is deposited", release the part.

When the "part is released", return to the transport position.

When "at the transport position", return to home.

When "at home", wait for the next part.

Each of the quoted conditions are subject to physical (sensor) conditions and possibly timed conditions.

So... what say you now?

Layman answers or fill in the blanks

The Arm is at home, without a part in-hand, waiting to begin a cycle.
At this point the "ARM" should have a sensor showing it to be in "home" position.
When a "part is ready to be picked up", go for the part until you are at the part.
There will need to be conditions that tell you the part is ready for pickup i.e. a sensor to show part in place.
When you are "at the part", grab the part until you have the part.
Sensor(s) will let you know you are in position to grab the part i.e. if the "ARM" goes straight down from "home" there may be a sensor to show it is extended. There could also be a sensor to show it in position.
When you "have the part", lift the part until the part is lifted.
It depends on the system but when you tell it to clamp another sensor tells the plc it has the part.
When the "part is lifted", transport the part until the part is transported.

When the "part is transported", and if the "right-table is empty", deposit the part.
Depends on system but for simplicity lets say you have it clamped then you tell it to go "home" then tell it to move above "place" position then move down to "Place".
When the "part is deposited", release the part.

When the "part is released", return to the transport position.

When "at the transport position", return to home.

When "at home", wait for the next part
As mentioned you will need to use some type of sensors to tell you what position the part, arm, clamp etc is in and condition the rungs accordingly.

Ballymeeney,

Why dont you try the PLC simulator that Phil provided on this site? There's a link to it right under the header at the top of this page.

Terry Woods said:

The ladder you posted is barely discernable as to its presence. That is, I can barely see that something is there.

I can see it on the screen, but I can't read it... nor can I read a printed copy.

If you insist on meeting that spec, as poorly designed, poorly described, and contradictory as it is, I think you need to step back and look at the process from a purely "human" point of view.

The Arm is at home, without a part in-hand, waiting to begin a cycle.

When a "part is ready to be picked up", go for the part until you are at the part.

When you are "at the part", grab the part until you have the part.

When you "have the part", lift the part until the part is lifted.

When the "part is lifted", transport the part until the part is transported.

When the "part is transported", and if the "right-table is empty", deposit the part.

When the "part is deposited", release the part.

When the "part is released", return to the transport position.

When "at the transport position", return to home.

When "at home", wait for the next part.

Each of the quoted conditions are subject to physical (sensor) conditions and possibly timed conditions.

So... what say you now?

Click to expand...

sorry about the image, i am limited by the upload size of the image. I will try an upload the attempt again, but by the sounds of it I think i'm going on the wrong track.

How would the sequence your described be displayed in ladder logic format? like would there be a need to for so many lines of programming compared to my effort??