Configurable safety relay protects DNA research

When refurbishing vital flood-damaged research equipment at the Wellcome Trust Sanger Institute engineers looked for a way to safeguard against a repeat occurrence.

Although the Wellcome Trust Sanger Institute is involved in one of mankind's greatest ever research projects, namely analysing and understanding genomes, it is not immune to one of mankind's oldest enemies: flooding.

Two years ago the building was flooded due to severe weather conditions that caused a lake in the landscaped grounds to overflow.

One of the specialist pieces of equipment that was damaged was a robotic storage and handling system for samples of DNA strings.

When the equipment was refurbished, it was retrofitted with a Pilz PNOZmulti software-configurable safety controller, together with Pilz PSEN switches on the access gates.

Originally the equipment had been fitted with an alternative type of safety controller, though this had been let down by continual nuisance tripping from the gate switches.

Moreover, by the time the refurbishment was required, the original type of safety system was no longer available or supported.

Following some research to establish what other options might be open to him, the engineer at the Sanger Institute called in Pilz to discuss the PNOZmulti.

The PNOZmulti is a unique, modular, software-configurable safety controller that is ideally suited to medium-complexity machinery, especially where future modifications are anticipated.

For the Sanger project, Pilz recommended a PNOZmulti system that would take about one-third of the cabinet space required by conventional electromechanical relays, with the ability to incorporate zoned control and diagnostics far more easily.

Because the Sanger engineer was busy with refurbishing the rest of the equipment, he asked Pilz about the supply and installation of a turnkey system.

At this point Pilz turned to Eltek Systems, a system integrator based in Oundle, with which Pilz has worked on numerous previous occasions.

John Smith, Managing Director, says: "We had not used the PNOZmulti before, but it all looked very simple when the configuration software was demonstrated to us.

When we subsequently came to do the project, we found that it really was as straightforward as it had seemed, with drag-and-drop icons and point-to-point connections.

Pilz offered to come and help us with the first configuration, but we found that the software was so easy to use that we did not need any support".

The safety system consists of a Pilz PNOZmulti safety controller with eight Pilz PSEN 2.1p11 noncontact gate switches having a sensing range of 6mm, one reset switch and one keyswitch for selecting "user" or "engineer" operating mode.

Making up the PNOZmulti system are a PNOZm1p base unit, three PNOZmo4p modules with four volt-free outputs each, a PNOZmi1p with eight inputs and a PNOZmc1p communications module that has 16 semiconductor outputs that are used to signal which doors are open and which zones are isolated.

Four zones are defined within the safety controller configuration for the track robot, the three re-arrayers (Cartesian robots), the carousel and the refrigerated storage library.

This fridge has the capacity to store 1,290,240 samples, which are held in microtitre plates stacked in eight-sided carousels.

If a scientist has new DNA string samples to be stored, these are manually loaded into a carousel via the input/output door.

When this access door is opened, the safety controller inhibits the carousel from moving, but permits all other zones to continue functioning.

When the door is closed, the track robot can move the plates from the first carousel to others inside the fridge.

If a scientist needs to retrieve DNA strings, the request is made at one of three workstations.

In front of each workstation, behind an interlocked gate, is a Cartesian robot that takes samples from the library plates and places them into user plates for the scientist.

The supervisory computer instructs the track robot to retrieve the appropriate library plates from the fridge and place them in nests so that the Cartesian robot can transfer samples to the user plates.

When the picking process is finished, the scientist opens the access gate to collect the user plates.

As soon as the gate is opened, the safety controller inhibits movement of this Cartesian robot (though it permits the other two to continue working), and the track robot is also inhibited.

However, the carousel and the fridge continue to operate.

When large numbers of samples are requested, the system can operate in such a way that the user plates are stacked into the carousel, from where they can be collected via the input/output access door.

Depending on the quantity of samples and, therefore, the time required, this might be done overnight or over a weekend.

To achieve the safety system zoning using conventional relay logic would have been extremely complex, yet it was easy to program using the PNOZmulti configuration software.

Other functions that were easier to achieve with the configurable controller were an automatic reset on the carousel door (previously the scientists had to close the door and walk to the other end of the equipment to press the reset button), and comprehensive diagnostics that show which inputs have caused the equipment to be halted and which zones have been inhibited.

In addition to the indicator lamps on the front of the control cabinet, the LEDs on the PNOZmulti give more detailed diagnostics.

During commissioning and troubleshooting, the LEDs on the PSEN gate switches can also be used to see at a glance which switches are not closed.

Martin Horrod, the Senior Electronics Development Engineer for Team 52 (Robotics and Automation Development) at the Sanger Institute, comments: "The previous safety system suffered so badly from nuisance trips that it was seriously impeding the scientists' access to the genomes stored in the library.

But now the Pilz PNOZmulti system is in place we can run the equipment 24 hours per day.

Pilz and Eltek worked together extremely well to deliver and install a safety system that has given us improved functionality and greater versatility.

I would certainly consider using a PNOZmulti on other applications calling for more than a few safety relays".

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