Brian, You are correct regarding the interlocking device - it needs to be designed and rated for this function, and should incorporate 'direct- drive' or 'positive-drive' type contacts, but this is not a mandatory requirement.
The interlocking device is not required to have direct control of the power to the prime mover. This can be done, but is typically limited to very simple equipment. The interlocking *system* is what must have final control of power to the prime mover - in this case it sounds like you are talking about a pneumatic actuator of some kind. This could be achieved by using a valve in the main pneumatic supply and the control valve for the actuator to provide you with diverse and redundant control of the pneumatic power to the actuator. If it is an electric output, such as a motor starter, then control power to the controller outputs could be switched, and an additional contactor installed at the point of supply to the branch feeder to create a similar condition. If you want to use a programmable controller as part of your safety system, it must meet the reliability requirements required based on the hazards presented by the equipment, so step 1 is a risk assessment. Step two is to apply the relevant parts of IEC 61508 to the design so that you can be confident that you have achieved a high enough SIL (Safety Integrity Level) for the application. There are parts in this standard that relate to the design and the validation. If your equipment is reasonably hazardous - meaning that you have some hazards that will result in severe injuries or possibly a fatality - you will almost certainly end up with a requirement for redundant controllers with sophisticated cross-checking. Typical safety PLC's that meet these requirements often have two or three processors internally for program control, validated function blocks for gate interlock inputs, and redundant I/O. If you plan to market the product in the EU, check to see if it is included in Annex IV of the Machinery Directive. You can download a copy at http://tinyurl.com/y8b592, or read it on line at http:// tinyurl.com/2zxnjs. If it is explicitly included in the Annex IV list, then you will need Notified Body involvement before you market it. If not, you will need to complete the 'internal production controls' requirements and keep a Technical File on hand. Don't forget that there will also be Low Voltage, EMC and possibly other directives to deal with as well. If you want to discuss this more off-line I would be glad to chat with you at your convenience. -- Doug Nix, A.Sc.T. IEEE PSES Kitchener-Waterloo Section, Ontario, Canada [email protected] mobile (519) 729-5704 fax (519) 653-1318 Find me LinkedIn at http://www.linkedin.com/in/dougnix On Jul 20, 2007, at 9:29, Kunde, Brian wrote: > I have limited experience working with industrial machines with moving > parts so I would appreciate any help you can provide. > > To protect an operator from moving parts, my understanding is that I > have to use an approved interlock device (in this case a switch) > and it > has to directly control the device that makes the motion (pneumatic > solenoid). > > But on a future project, we would like to have the interlock switch > talk > to an FPGA (silicon) and have firmware and software controls the > motion > device after many sensor checks are made. These "checks" are quite > complex where many things have to be just right in the right sequence > for the motion to occur. Engineering is saying that the sequence is > too > complex to limit with a simple interlock switch system. > > Is it possible to use silicon chips, firmware, and software in such a > circuit? Can such an approach be used and if so what is involved in > qualifying it? > > Thanks, > The Other Brian > > LECO Corporation Notice: This communication may contain > confidential information intended for the named recipient(s) only. > If you received this by mistake, please destroy it and notify us of > the error. Thank you. > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Jim Bacher: [email protected] > David Heald: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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