Running a Robot Cell: Monitoring, Faults, and Upkeep
Load and monitor a robot cell, read I/O and fault messages, recover safely from a stop, and keep up routine robot maintenance.
Table of contents
Most time around a robot is spent running it, not programming it. A good cell operator keeps parts flowing, notices trouble early, works out why the robot stopped, and knows which problems to hand off. Those skills cut downtime more than anything else. They also keep you out of the moments when people get hurt, because fault recovery is when people are tempted to step inside. Lock out the robot and all associated energy before you go inside for any reason.
What You Will Be Able to Do
- Load a cell correctly and keep your hands out while the robot is active.
- Spot the early signs of trouble during production.
- Use the I/O monitor to tell whether the problem is the robot or something outside it.
- Respond to common faults, and recover from a stop in the right order.
- Explain what routine maintenance protects, including grease and encoder batteries.
Loading and Unloading Parts
Follow the loading procedure for your cell. It usually looks like this:
- Wait for the signal that it is safe to load, such as a light, or the arm at its home position.
- Check the part. Reject anything bent, dirty, or wrong for the job.
- Place it in the fixture or on the conveyor in the correct orientation.
- Make sure it is fully seated. A part sitting high can cause a crash.
- Step clear of the load station, and signal the robot if your cell needs that.
Use only the load station or window that was designed for you. Never reach into the cell while the robot is active. If a part slips out of place, wait for the cycle to stop safely and use the recovery steps below.
What to Watch During Production
Most trouble gives a warning before it stops the cell. Each of these is worth a note and a word with your lead.
| Sign | Likely cause | What to do |
|---|---|---|
| Dropped parts | Worn gripper pads, low air pressure, part variation | Check the air gauge, then report it |
| Missed positions | Shifted fixture, bent tool, encoder problem | Stop and report it. Do not keep running |
| Grinding, clicking, or a high whine | Worn bearings, belts, or gears, or contact with a fixture | Stop and report it |
| Bad welds, thin adhesive, scratches | Worn tip, blocked nozzle, tool out of position | Stop and report it |
| Cycle time creeping up | Slow sensor, waits on another machine, low air | Note it, and tell your lead |
| Oil or grease at a joint | A failing seal | Report it the same shift |
Keep a simple record of faults and when they occur. One fault is an event. The same fault five times in a shift is a pattern, and that pattern is what a technician needs to know.
Reading I/O
I/O stands for input and output. These are the signals a robot uses to talk to everything around it. A robot cannot see a clamp. It only knows that a clamp is closed because a signal says so.
| Type | Direction | Examples |
|---|---|---|
| Digital input | Into the robot, on or off | Part present, clamp closed, machine ready |
| Digital output | Out from the robot, on or off | Open gripper, cycle complete, start weld |
| Analog | Either way, a range of values | Weld current, adhesive flow, spray fan width |
Analog signals usually run from 0 to 10 volts or 4 to 20 milliamps. Many modern cells also pass signals over industrial networks such as EtherNet/IP, PROFINET, or EtherCAT. In many cells a PLC, the computer that runs the line, tells the robot which job to run, and the robot reports back when it is done.
Diagnosing a Waiting for Input Message
A robot that stops with a "Waiting for input" message has usually not broken. It has reached a line in its program that says to wait for a signal from an outside sensor or the PLC, and the signal has not come. This is one of the most common stoppages you will see.
- Read the message and note which input the robot wants, for example input 12, "clamp closed".
- Open the I/O monitor on the pendant or the operator screen. This only displays signals and is safe to view.
- See whether input 12 is on or off.
- From outside the cell, look at the real device. Is the clamp closed? Does the sensor's light show it is detecting?
- Compare the two.
| Clamp | Sensor light | Robot's input | What it tells you |
|---|---|---|---|
| Open | Off | Off | The clamp never closed. Check air or the clamp step |
| Closed | Off | Off | The sensor is out of place or has failed |
| Closed | On | Off | Wiring, PLC, or network problem. Call a technician |
In this way a two-minute check tells a technician where to start.
Warning: Never force or simulate an I/O signal unless you are trained and authorized. A forced signal tells the robot something false, and it may move as if a clamp is closed or a door is open.
Vision Systems
Many robots use cameras to find parts, read codes, or check quality. If a vision step fails, look for simple causes first: a dirty lens, changed lighting, or a part out of position. Only trained people should change vision settings.
Common Faults
| Fault | What it means | First response |
|---|---|---|
| Collision detection | Too much force at one or more joints, usually from hitting something | Do not just restart. Check the tool, part, and fixture for damage |
| Waiting for input or I/O timeout | An expected signal never came | Use the I/O steps above |
| Overtravel (joint limit) | The arm tried to go past its range | A trained person jogs it back in manual mode |
| Servo error | A motor, drive, or encoder problem | Report it. Power cycle only if your procedure allows it |
| E-stop pressed | Someone or something opened the stop circuit | Find out why before you reset |
| Gate or safeguard open | A protective stop fired | Confirm the cell is clear, close it, and reset from outside |
| Low battery warning | A backup battery is getting weak | Report it the same shift |
| Mastering or position lost | The robot cannot trust its position | Stop. A technician must remaster it |
A collision fault can also come from wrong payload data or a part that is heavier than expected, and not only a crash. Either way, find out why before running again. A crash can bend a tool, which will throw off every point after it.
Recovering From a Stop
Your plant's recovery sheet tells you which faults an operator may clear. Stay inside those limits. The general order is:
- Stay outside the cell. Read the fault message on the pendant or operator screen.
- Note the fault code, the time, the program step, and whether a part is in the gripper.
- Find and fix the cause. Clearing a fault without knowing why it happened only brings it back.
- If anyone must go inside, lock out and tag out the robot and all associated energy first. A part held in a gripper can fall when air or power comes off, so plan for that before you lock out.
- Acknowledge the fault.
- If the arm must be moved, a trained person jogs it to a safe position in manual mode, with the enabling device in hand.
- Confirm that everyone is out, the gate is closed, and the cell is clear.
- Switch to automatic, reset, and restart. After a crash or a jog, have the first cycle run at reduced speed if your procedure allows it.
- Watch the first cycle closely, and check the first part.
- If the same fault comes back, stop clearing it and call your lead or a technician.
Warning: A restart resumes from where the program stopped, and the arm may move in a way you do not expect. Know where it will go before you press start.
Keeping the Robot in Shape
Robots need scheduled care like any machine. This table shows typical work. The maintenance manual for your robot sets the real tasks and intervals.
| When | Typical tasks | Usually done by |
|---|---|---|
| Daily | Look over cables, hoses, and tool. Check joints for leaks. Test E-stops and safeguards | Operator or technician |
| Monthly | Check base, arm, and tool plate bolts. Inspect the cable harness for wear | Technician |
| Quarterly to twice a year | Check backup battery status, and confirm brakes hold with the power off | Technician |
| Yearly, or by run hours | Grease joints, replace encoder batteries, inspect reducers and belts | Technician |
Robot vibration can loosen bolts, so a slipping base or tool plate can throw off a whole program. Lock out for any task that puts a person within reach of the arm.
Joint Grease
The gear reducers in each joint carry heavy loads, and the grease is what protects them. Many schedules call for greasing about once a year or at a set number of run hours, so joints last their full life.
Use only the grease the robot maker specifies. Grease types are not all compatible, and the wrong one can break down and destroy bearings and gear surfaces. Follow the manual's method as well. It usually says to open a drain port and add a set amount, because forcing grease in under pressure can blow out a seal.
Encoder Batteries
On many robots, small backup batteries let each joint's encoder keep track of position while the controller is off. Replace them on schedule, before they die.
If the batteries die and power is lost, the robot loses its position reference. The robot must then be remastered, which can take a technician hours. The program files themselves are normally kept, but every taught point depends on that reference, so no point can be trusted until the robot has been remastered and the program has been checked at low speed.
Replacing these batteries is a technician's job. Many makers require the controller to stay powered during the swap so the position data is kept, which is a special written procedure. A low battery warning is your cue to report it right away.
Some controllers also have a separate battery that holds memory on the main board. That is one more reason to keep a current backup of programs and settings off the robot.
Brakes
Motor brakes hold the arm up when power is off. If a joint drifts or sags after power-down, keep clear of the arm and report it. A failing brake can let a heavy arm drop.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Clearing a fault again and again | A small problem becomes a crash or a broken part |
| Restarting at full speed after a collision | A second crash, because the tool may be bent |
| Ignoring a low battery warning | Lost position, then remastering and lost production |
| Using whatever grease is on the shelf | Damaged gears and bearings in a costly reducer |
| Stepping in to fix a crooked part | The arm moves while you are in reach |
| Having no current program backup | Long downtime if memory is lost |
Check Your Understanding
1. The robot stops with a message that it is waiting for "machine ready" from a CNC lathe. The lathe's door is open and its screen shows an alarm. Is the robot broken?
No. The robot is doing what it should. The lathe never sent the signal, so the alarm on the lathe is the problem to solve.
2. A collision fault stops the robot. The gripper has a fresh scrape, and the fixture looks fine. What should happen before production continues?
Find out what it hit and why. Check the gripper and TCP for damage, have a trained person clear the arm, then run at reduced speed and check the first part.
3. The operator screen shows a low battery warning near the end of your shift. Can it wait until next week?
No. Report it now and put it in your pass-down notes. If the batteries die and power is lost, the robot will need remastering.
4. A technician says the spare grease from another brand of robot is about the same. What is the risk?
Mixing or substituting grease can break down the lubricant and damage gears and bearings. Use the grease the maker specifies.
Key Takeaways
- Load only from the designed station, and never reach in while the robot is active.
- A waiting-for-input stop usually means an outside device or PLC signal is missing, so compare the I/O monitor to the real device.
- Fix the cause before clearing a fault, and call for help when a fault repeats.
- Anyone going inside must lock out the robot and all associated energy first.
- Use the specified grease at the specified interval to protect joints.
- A dead encoder battery plus a power loss means remastering, and taught points cannot be trusted until that is done.