Troubleshooting: When the Obvious Answer Seems Elusive

Though many engineers are familiar with the tried-and-true steps to troubleshooting an automated system, the fastest route to an answer can often go unchecked. How an experience troubleshooting the impact of a power supply on a light curtain reminded one engineer of this often-overlooked solution.

I first met Tina Hull, product safety engineer with Omron Automation & Safety, when she served on an Ethernet Safety panel discussion at The Automation Conference 2014. More recently she shared her experience helping an Omron customer troubleshoot a machine problem in which the machine would only run when the light curtain beams were broken.

If that sounds unusual to you, don't be alarmed. It is unusual. In fact, it is the opposite of how a safety light curtain is designed to operate. Hull even wondered how “it was even possible” for the machine to operate under such a condition.

She described the machine as “part of a modular packaging system consisting of a delta robot with safe motion, allowing it to have a direct-to-the-safety central processing unit (CPU).” According to Hull, the machine also had a vision system and conveyor with a safety system consisting of an emergency stop and two light curtains monitored by a safety PLC.

Hull says the initial thought was that the problem could be quickly solved over the phone. However, that was not to be the case. Here’s how it all unfolded.

The first step taken was to ensure the light curtains were properly connected—a basic troubleshooting scenario. “Most light curtains in Canada, the Americas and Europe are PNP,” said Hull. “This provides a path to +24v DC, which prevents a malfunction due to an earth fault, more commonly a short caused by a wire whisker touching the frame. Machines from other countries may come with NPN light curtains.” She added that suppliers might have different ways to indicate if the light curtain is PNP or NPN. In this case, the part number was F3SJ-B0225P25-D — the P indicating it was PNP.

To verify that the proper connections were made, Hull looked at wiring issues since “most safety-related issues are caused by wiring,” she said. A wiring diagram was sent to the customer for verification. It turned out that the wiring was correct, but Hull shared three key wiring checks that should be verified when troubleshooting connections:

  • Verify the communication connections between the emitter and receiver are made;
  • Check the 0v and24v DC connections; and
  • Make sure any switch being used is rated for micro loads.

Now that the wiring connections had been verified to not be a problem, the next step was to align the light curtains. “The customer even set up a webcam so we could see the blue LED light come on to verify it was correct,” Hull said. “Watching the LEDs is an easy way to see if the beams are being broken,” she said. “But remember to use a test rod or some other disposable device thick enough to break a beam when you do this, not your hand! You can also test the continuity with a multimeter.”

Through all of these tests, everything appeared to be set up correctly, but the machine would still only operate when the beams were broken. At this point, Omron sent out an application engineer to investigate.

After arriving, the engineer discovered that, when he tested the continuity between the power supply and the light curtain, the 24v line was connected to one power supply, while the 0v line was connected to another power supply.

“Most people’s first thought would be to jumper the 24v connections between the two power supplies, but you do not want to do this,” Hull said. “The power supplies used for safety and motion should always be separate.”

Hull noted that safety devices and controllers need a 24v DC SELV (separated extra low voltage) power supply to maintain the integrity of the safety of the machine and help eliminate false tripping due to inrush currents if other devices on the same supply are switched on.

“On the motion side, when the relay coil on the brake is actuated, it causes a negative current spike,” she said. “Diodes are built into many high quality relays and controllers to reduce the spike. While part of the remaining spike can be absorbed by a good quality power supply, a portion of the spike still resonates and causes noise in the I/O signal. High noise can cause unpredictable results, such as the signal being ‘on’ even if the input says the signal is ‘off’.

After hours of troubleshooting, Hull said she realized a lot of time could have been saved if we had “just read the light curtain data sheet.” The data sheet notes that, for a PNP output, the load should be connected between the output and 0v line.

“Connecting the load between the output and +24v line results in dangerous condition because the operation mode is reversed to ‘on’ when light is interrupted,” Hull said.

Following this data sheet discovery, the customer traced the wires and separated them so the light curtain and the rest of the safety were on one power supply. This allowed the light curtain to work correctly.

Based on the lessons Hull learned from this troubleshooting experience, she advises following these four steps to prevent this problem from happening to you:

  • Use separate power supplies for servo brakes and I/O to avoid shorting;
  • Use a separate power supply rated for SELV for safety devices to avoid false tripping;
  • Select a good quality power supply that is properly rated for the devices; and
  • Read the data sheets!
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