Relay Logic to Ladder questions

STL???

Member
Join Date
Sep 2005
Location
UK
Posts
879
Hi All,
This may seem like a simple question, but as i have a mechanical background,things like following always seem to trip me up!

Ok - We have an old Hydraulic press which is being refurbished to use on a new project. The electrical control currently is Relay Logic, the usual problematic relay issues have swayed my boss to replace this with PLC controls (Siemens S7).

Currently the parts are loaded, the operator presses the start button momentarily and providing the Lightguards(Sick) are clear - The cycle runs.

First Question:
Where can i find or is there a calculation for the distance for the lightguard position with relation to the moving ram? I want to check this as it just looks to close!

Second Question:
The start button has a N/O & N/C contact which is interlocked using relays(Not safety type) i.e one makes/one breaks to monitor correct operation.

I dont like the idea of a one handed start even though the work area is protected by Lightguards, However that feature must stay according to my boss! How would you update this PLC wise?

I was thinking of using just a single N/O contact to a PLC Input,and then monitoring the rising edge to protect against sticking contacts, Plus a timed supervision if exceeded fires a Alarm Message (Contact Error - Start Button etc).

Hope my ramblings make sense, and thanks for any advice offered

STL
 
STI has a good site with some excellent information.

http://www.sti.com/safety/index.htm

Down on the bottom is a document that describes how to calculate safe distances.

One handed starts can be safe with a light curtain provided they are placed at a safe distance and integrated in with the motion devices correctly. A safety assessment should be done before your specific questions can be answered.
 
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The assesment should include time to stop and time for hand to reach danger.

The two hand start is only essential I believe without a lightguard.
 
Hi Guys,
Thanks for the replys - Interesting stuff, Who would normally carry out the assessment? Obviously an instant stop would be preferable, but what would be an acceptable stop time?
 
what would be an acceptable stop time?
Any time, as long as it is shorter than the time needed for the hand to reach danger (as PeterW pointed out). IOW: it depends on the mechanical setup.

Kind regards,
 
STL??? said:
Hi Guys,
Thanks for the replys - Interesting stuff, Who would normally carry out the assessment? Obviously an instant stop would be preferable, but what would be an acceptable stop time?

You can actually perform the assessment and there are some guides at different websites (A-B off-hand) that can help.

The farther away the light curtain is placed the longer the acceptable stop time can be. If your light curtain is already installed then you'll need to work STI's calculation in reverse.

Say, your STI light curtain is 15" away from the hazardous motion then subtract off the "Depth Penetration Factor" from 15" (say 1.5") and then divide by 63"/sec (operator movement). Your max stop time would be 0.214 seconds, basically all of your propagation delays in the electrical and fluid power circuits (if they exist) added up.
 
Thanks jvdcande,

what would be an acceptable stop time?

Dumb question! i wonder sometimes o_O

Reading the posts through again, I think i have an acceptable solution utilising the dedicated Lightguard safety relay, However ill do the math and then seek expert assistance to verify.

Thanks again.
 
You dont say how big the presses are or what sort they are ?

Just done a press job and used the Pilz Press PSS System, combined with a Touch Screen and an SLC....

Nice, Clean, Complies with all UK HSE Regs

Depending on the press you may find that a light guard is not sufficient.

I cant remember the two regs we had to read up on but i have the EN references at the office somewhere.
 
STL??? said:
Dumb question! i wonder sometimes o_O
When I started out being a trainer 13 years agoo(didn't realize till now it's already that long) I was told that there is no such thing as a dumb question, only dumb answers. But time and time again I too wonder. Your question was nevertheless not seen as a dumb question, I know regulations can be so confusing you start to loose grip. We are all proffesionals when it comes to the technical aspect of a problem, but we all have to deal with matters that are not that clear to us technicians.

I'm simply glad we could all help you find a good solution, that's what this site is all about.

Kind regards,
 
504bloke said:
You dont say how big the presses are or what sort they are ?

Just done a press job and used the Pilz Press PSS System, combined with a Touch Screen and an SLC....

Nice, Clean, Complies with all UK HSE Regs

Depending on the press you may find that a light guard is not sufficient.

I cant remember the two regs we had to read up on but i have the EN references at the office somewhere.

Prior to my emigration to Oz about 18 months ago I did some upgrades to some 25T power presses that were shipped in from the USA that were dangerous and poorly controlled (to say the least).
This also involved the end user CE marking the machine.
I used a Siemens press controller PLC and a Balluff cam box (to detect TDC). In order for the power presses to gain a CE mark the system was then vetted by a notified body (This is a requirement for Power Presses).
I would be very careful in using a lightguard system on this type of machine.
The standard for Power Presses is EN962 which the Pilz and Siemens controllers comply with.
 
Last edited:
Thanks for input,

Specification:
Pressing Power 1250kN
Pull back Power 141kN
Pressure 260 bar
Ram width 1200 mm
Ram depth 1100 mm

Speed:
Rapid advance 180mm/s
working advance 10mm/s
Rapid retract 160mm/s

Supply voltage 415v
Control voltage 24v

At one time the load/unload was via robot, i was told today.

Safety wise there is the following relays:
Pilz PNOZ X2.1 - Emergency stop system/perimeter guarding
Pilz PNK press overrun relay
Sick lightguards controlled via Sick LCUX control relay

The machine was supplied with the lightguard setup and single
button start from new.

The machine is approx 10 years old and is unreliable at best,Hence ditching the current relay setup in favour of a PLC.

Unfortunatley it has had 10 years worth of messing,with the responsible people departed from the company and no notes of the work undertaken - i.e Removal of the Robot system.

Regards
STL
 
Hi Guys,

The standard for Power Presses is EN962 which the Pilz and Siemens controllers comply with

are you sure?

EN962:1997Title of StandardTransportable gas cylinders. Valve protection caps and valve guards for industrial and medical gas cylinders. Design, construction and tests


504 Bloke
If you could dig out the EN references, i'd greatly appreciate it.


Thanks STL
 
There were two in particular that a nice lady at the HSE head office pointed us to, ill look them up on monday for you as i was out all today at a 5min job!

From the Pilz website the PSS covers Presses for

Press applications:
EN 692, EN 693, EN 12622

See here
 
I created a small app to calculate safe distances. Let me know (via email) if you are interested and I'll email it to you.

I believe this calculation is from ANSI/RIA 15.06 1999

I used this app to calculate the length of a light curtain needed to protect an operator.

The calculation is:

DS = 63 inches per second (IPS) X (TS + TC + TR) + DPF DPF = 2 inchWhere:

DS = minimum safe distance TS = stopping time of device TC = worst stopping time of control system TR = response time of safeguarding device including interface DPF = maximum travel distance toward a hazard once someone has entered the field
Email: [email protected]
 

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