Cleanrooms require a high level of security, with limited access to authorized personnel only. To help prevent contamination, it is important that a cleanroom’s sterile environment be protected by a dependable access control system. Available advanced interlock controls can work with any access control systems that may be in use in controlled environments.
Today’s programmable logic controllers (PLCs) easily manage the operation of door locks, automatic door openers, air dampers, and indicator lights. These pre-programmed controllers allow facility operators to manage from two to over 100 doors in any combination, and allow only one door at a time, in a designated area, to be accessed.
Interlock controls can operate access to cleanrooms of any cleanliness standard. Depending upon the product manufacturing standards and cleanroom construction, the interlock controller might interface with electronic magnetic locks (maglocks) and strikes, card access systems, biometric readers, air and HVAC controls, traffic lights, alarm sounders, automatic door openers, inflatable air-tight door seals, operator safety mechanisms, building fire alarm system, and emergency shutdown systems.
Cleanroom door control
Simple two-door interlocks can be controlled with a single door or lock sensor for the status of each door and a relay for operation of each door locking device. Traditionally, these small two-door interlocks are controlled with standard relay logic. These systems can have both doors normally unlocked; both doors normally locked; or one door locked with the other door unlocked. Many environmental airlocks operate with one or both doors unlocked, but these configurations always run the risk of two doors being opened simultaneously. To prevent these violations, both doors need to be locked with either a card access system or a request-for-access device such as a push button.
Multiple interlocks can be operated from a single controller to sequence locks, traffic lights, exhaust fans and airtight seals. A typical PLC controller for 8-door system is shown. All Photos: Dortronics Systems Inc.
PLC-based control
As the number of controlled doors in the system is increased, the risk of door violations expands. Only a PLC-based interlock controller can prevent simultaneous door openings when more than two doors are in a single area or a door is shared between two controlled rooms. Some cleanrooms may require special timing functions and/or special logic sequences to maintain the correct pressure level for a certain operation. In addition, some systems force personnel to adhere to a specific traffic pattern to ensure cleanliness or safety of the operation. While a PLC interlock does offer this flexibility, the learning curve to program these products can be overwhelming. A factory, pre-programmed system can save time and money.
Door control hardware
Most cleanroom controls incorporate electric locking devices that limit access to maintain a sterile barrier while allowing controlled entry and exit. Electric strikes can be used with mechanical door hardware, but these will only restrict traffic from one direction. Maglocks are generally preferred as they are easily installed on new or retrofit applications. Maglocks are also available in various styles and finishes, including stainless steel. Specify maglocks with totally encapsulated electronics to allow sterilization and wash-down of the cleanroom. Manual door closers or automatic door operators will ensure that the accessed door is closed and the airlock is resealed after each passage.
Local code compliance
While most systems use electric locks and/or automatic door operators to maintain the integrity of the airlock, some locations may forbid or seriously limit the use of electric locks on pedestrian doors. Some cleanroom projects use no locking hardware on the doors, only traffic lights to indicate when doors should be accessed. These systems should also incorporate an alarm sounder to indicate if the door is opened in violation.
Hi-intensity LEDs and audible alarms can provide user feedback to indicate when the door into the cleanroom area may be accessed.
Where needed, the National Fire Protection Association- and Underwriters Laboratories-listed delayed egress controls can be incorporated into these interlocks to satisfy local building codes while ensuring the appropriate operation of the cleanroom doors. Emergency override controls can also be designed into the interlock to allow a means to immediately unlock the doors of a specified area. These can be supplied with alarm sounders to discourage appropriate activation.
HVAC interface
Pressurized cleanrooms require air-tight seals on doors and frames plus special HVAC timing to maintain clean conditions. The door seals must be maintained for proper operation. Automatic door operators or manual door closers must be specified and periodically adjusted for the correct spring tension to ensure proper closing and re-sealing of the door after access. While the door logic will inhibit another door from being opened until the accessed door is re-secured, a timing delay may be required to allow for a re-pressurization of the cleanroom. This time delay period should be coordinated with the HVAC contractor and this requirement specified as a feature of the interlock controller.
Automatic door operators
Airlocks incorporating automatic doors are frequently used when materials and equipment must be moved between cleanroom areas. Automatic doors can also be utilized in sterile areas with infrared “touch-less” sensors. These environmentally sealed controls are used to signal the door operator to open the doors without personnel touching any surfaces. The automatic door operator can also be used to control traffic flow direction by using request-for-access devices on only one side of the doorway. Typically the interlock controller would signal the operator when the door may be opened, but sometimes built-in time delays are needed to allow the door to unlock before the automatic operator attempts to open the cleanroom door. The door open timer can be adjusted, independent of the interlock control, to close after a preset time.
An industrial overhead door operator may be used with the door interlock controller by inhibiting the “open” button control when the door should not be accessed. Typically the “close” and “stop” button controls are not inhibited by the interlock controller. For safety reasons, infrared and safety edge devices should remain fully functional. These will automatically reverse the door travel back to the open position.
Additional considerations
Interlocked doors in the mantrap areas can be equipped with LED traffic light for user feedback. Indicators for normally unlocked doors will always follow the door lock status (green when unlocked and red when the door is locked to prevent access). Go/No-Go lights indicate when normally locked doors are inhibited or are available for access. The interlock control specified should have this capability.
Maglocks, used on both manual and automatically operated swing doors, are inherently fail-safe. These door locks will automatically unlock in case of a power failure to allow unhindered egress in an emergency. The interlock controllers should be supplied in a National Electrical Manufacturers Association (NEMA) enclosure with a built-in 12/24 VDC power supply. If high security or other special concerns are present, an uninterruptable power supply with a battery back-up can be supplied to operate the maglocks and lights during a power outage.
Single or double doors can be secured with maglocks and electric strikes for new construction or cleanroom retrofits.
Conclusion
Much thought and preplanning should go into the operation specification. The design engineer should be well versed in the local code requirements and select an interlock product from a recognized, established manufacturer familiar with cleanroom operations. When possible, contact the interlock manufacturer when writing the system description to specify the most current products. Factory programmed and tested PLC-based interlock controllers allow greater flexibility of operations while simplifying the specifying of appropriate devices. Having a single source for the controls and locking hardware also assures compatibility.
Bryan Sanderford has worked for Dortronics since 1999, including time as a manufacturer’s sales representative. For the 10 years prior, he operated his own access control dealerships in Texas. Contact: bryan@dortronics.com
This article appeared in the November/December 2012 issue of Controlled Environments.
