How-To: Program a Cobot for Safe Pick and Place Tasks

Prerequisites and Preparation

Before touching the controller, gather the physical and digital components. You need the cobot, the end effector such as a parallel gripper or vacuum cup, and the safety equipment. Check the load rating of the arm against the weight of the heaviest object in the bin. If the load exceeds the rated capacity, the torque sensors will trip, and the program will fail.

Confirm the power supply. A standard 220-240 V three-phase connection is typical, but verify the voltage matches the nameplate. If the site uses 400 V, you may need a step-down transformer. Ensure the emergency stop circuit is wired correctly. A missing e-stop connection is a common cause of startup failures.

Load the latest firmware. Older firmware versions often have bugs in the safety envelope logic. The update process usually takes ten to fifteen minutes. Do not interrupt the power during this step.

Step 1: Define the Tool Center Point

The tool center point, or TCP, is the reference point at the tip of the end effector. Without an accurate TCP, the gripper will miss the target or dig into the pallet.

Use a calibration tool if your cobot supports it. This is a small sphere or pin attached to the TCP. Move the arm to a single position, press the TCP button, then move to five other positions and press it again. The controller calculates the offset.

If you do not have a calibration tool, use the manual method. Place the gripper on a flat surface, such as a steel plate. Move the arm along the X axis. Press the TCP button. Repeat for Y and Z. This method is less accurate but works for simple tasks.

Record the TCP coordinates in the job documentation. If you replace the gripper, you must recalculate the TCP. A mismatch here causes the gripper to hit the box instead of the air above it.

Step 2: Set the Safety Envelope

The safety envelope is the virtual boundary around the cobot. If the arm crosses this line, it slows down or stops. This is the primary safety layer for collaborative tasks.

Open the safety settings menu. Select the “Cobot Envelope” option. You will see a 3D box. Adjust the X, Y, and Z limits. Keep the box tight around the work area. A box that is too large forces the arm to slow down constantly, which hurts cycle time.

Define the force threshold. Most cobots allow you to set a contact force limit in Newtons. For light tasks, 10 to 20 N is typical. For heavier loads, you may need up to 50 N. Do not set this too low. The arm will stop every time it bumps into the conveyor belt.

Test the envelope. Have a colleague stand near the arm. Wave an arm through the boundary. The cobot should slow down immediately. If it does not, check the sensor calibration.

Step 3: Program the Pick Cycle

Start with a simple linear path. Use the jog mode to move the arm to the “Home” position. This should be a neutral spot where the arm hangs vertically.

Move to the “Pick” position. Lower the gripper to the object. Open the fingers. Close the fingers. Lift the object to the “Place” position. Open the fingers. Return to Home.

Use the teach pendant to record these points. Label each point clearly. Do not use generic names like “Point 1.” Use “Home,” “Pick,” “Place.” This helps you debug later.

Add a delay after closing the gripper. A delay of 200 to 500 milliseconds ensures the grip is secure before lifting. If you lift too fast, the object falls. If you wait too long, you waste cycle time.

Step 4: Add Force Sensing Logic

Force sensing allows the cobot to detect contact and adjust its behavior. This is critical for safe human robot collaboration.

Connect the force sensor to the I/O port. Use a shielded cable to avoid electrical noise. The sensor sends a signal to the controller when the force exceeds the threshold.

Program a branch logic. If the force sensor is triggered, stop the arm and sound an alarm. If not, continue to the next step. This prevents the gripper from crushing a fragile box or hitting a worker.

Test the sensor with a known weight. Place a 5 kg weight on the gripper. The sensor should trigger if the threshold is set to 30 N. Adjust the threshold until the sensor reacts correctly.

Step 5: Integrate with the Production Line

The cobot does not work in isolation. It must communicate with the conveyor, the PLC, and the vision system.

Use an Ethernet connection for the PLC. Most cobots support a standard TCP/IP protocol. Map the I/O points. Pin 1 is usually the “Start” signal. Pin 2 is “Stop.” Pin 3 is “Cycle Complete.”

Synchronize the cycle. The PLC sends the “Start” signal when the conveyor moves the box into position. The cobot picks the box. The PLC detects the “Cycle Complete” signal and advances the next box.

If the vision system is used, it sends the X and Y coordinates of the box to the cobot. The cobot updates the Pick position based on these values. This handles small variations in box placement.

Step 6: Run the Cycle at Reduced Speed

Do not run the full-speed program immediately. Set the speed to 25% of the maximum. Run the cycle five times.

Watch the arm closely. Check for any collisions with the conveyor rails. Listen for unusual noises in the joints. The servos should move smoothly. Any grinding or jerking indicates a mechanical issue.

Check the cycle time. At 25% speed, it will be four times longer than normal. Note this time. You will use it to calculate the full-speed cycle time later.

If the arm misses the target, go back to Step 1. Recalibrate the TCP. Do not try to fix it in the code. A bad TCP causes every subsequent step to fail.

Step 7: Full-Speed Verification

Increase the speed to 100%. Run the cycle ten times.

Monitor the force sensor readings. The peak force should be below the threshold. If it exceeds it, slow down the arm or adjust the path.

Check the alarm log. Any faults should be zero. If there are faults, note the code and the time. Common codes include “TCP mismatch,” “I/O timeout,” and “Safety envelope violation.”

Measure the cycle time. Record it in the job documentation. This is the baseline for your production plan.

Common Mistakes

The most common mistake is skipping the TCP calibration. The arm moves, but the gripper hits the box. This damages the end effector and wastes time. Always recalibrate after swapping the gripper.

Another mistake is setting the safety envelope too wide. The arm slows down constantly. This reduces throughput. Tighten the envelope to match the work area.

A third mistake is ignoring the I/O wiring. A loose connection to the PLC causes random stops. Check the terminals with a multimeter. A 5% voltage drop is acceptable. A 20% drop is not.

Final Verification Checklist

Before releasing the cobot to production, complete this checklist:

Check Item	Status	Notes
TCP Calibrated	Yes	Gripper aligns with target
Safety Envelope Tested	Yes	Arm slows at boundary
Force Sensor Active	Yes	Triggers at 30 N
PLC Communication	Yes	No I/O errors
Full-Speed Run	Yes	10 cycles, no faults
Cycle Time Recorded	Yes	4.2 seconds

Sign the sheet. Date it. Store it with the job documentation. This record protects your team if a failure occurs later.

Conclusion

Programming a cobot for pick and place is a structured process. You define the points, set the safety limits, and test the cycle. The key is to verify each step before moving to the next. A small error in the TCP or a loose I/O wire will cause major downtime later. Take the time to check the basics. A reliable cobot cell depends on it.