Safety in Industrial Robot Cells

Why is robot cell safety treated separately?

In automotive supplier plants in Bursa, spot and MIG welding robots, press feeding and palletising robots are now common in small businesses too. A robot follows its programmed path at high speed and without warning; a moment when it is stationary may mean it is waiting or waiting for a signal. Robot cell safety therefore relies not on a rule that “the operator should not approach the robot” but on an arrangement that physically prevents approach and brings the robot to a safe state at the moments when approach is necessary.

The basis of this arrangement:

• Annex I, Art. 2.8 of the Regulation on Health and Safety Requirements for the Use of Work Equipment: where there is a risk of mechanical contact with moving parts, equipment is fitted with guards or protective devices that prevent access to the danger zone or stop movement before the zone is reached. Guards must not be easy to remove or render ineffective (Art. 2.8.1).
• Art. 2.13 of the same annex: maintenance work is carried out with the equipment switched off, its energy supply disconnected and all movement stopped.
• The Machinery Safety Regulation (2023/1230/EU): the regulation that will apply to new machinery placed on the market from 20 January 2027; the previous regulation based on 2006/42/EC will be repealed on that date. The safety design, declaration of conformity and operating instructions for a cell built with the robot, fixtures, conveyors and guards should be requested from the integrator.
• In practice, the TS EN ISO 10218 series (safety of robots and robot systems) and the standards on safety distances and interlocking devices are used as references.

Robot fencing: drawing the cell boundary correctly

The position of the fence is determined by taking into account the furthest point the robot can reach: the arm itself, the tool at its end (gripper, welding torch) and the largest part it carries. This area can be reduced with safe axis and zone limits in the robot controller; however, the limit must be designed and validated as a safety function.

Points to check in robot fencing design:

Light curtains and area scanners used at part loading stations must detect a person entering and stop the robot. If the muting arrangements used to let parts through are set up so that they cannot distinguish between a person and a part, the cell is effectively left open. We explained the general selection of guard types in our article on machine guards.

Door interlocking: how should a robot cell door work?

The cell access door is the deliberate opening in the fence; if it is defeated, all the protection of the fence becomes meaningless. With a correct door interlocking arrangement:

1. The robot comes to a safe stop when the door opens. The interlock switch is connected to the robot’s safety circuit; connecting it only to a PLC input to give a “warning” is not enough.
2. The door is kept locked if the stopping time is longer than the access time. If the robot cannot stop before a person reaches the danger zone after the door opens, an interlocked door with guard locking is used; the door opens only after the robot has stopped.
3. The robot does not start by itself when the door closes. Restarting is done from outside the cell, from a point with a view inside the cell, with a deliberate reset.
4. A person inside cannot be locked in from outside. Everyone entering the cell attaches their personal padlock to the door lock hasp or uses a personal key system. This way the door cannot be closed and the robot started while someone is inside.
5. The interlock switch cannot easily be bypassed. Coded or locked switch types are needed, spare actuators must not be left hanging next to the door, and bypass attempts must be made a subject of inspection.

Robot teach mode: rules for working inside

Entering the cell is sometimes unavoidable for programming, point correction and test movements. Robot teach mode is designed for this work and can be used safely under these conditions:

• Mode selection: Switching from automatic to teach mode is done with a key-operated selector; the key stays with the person entering the cell.
• Reduced speed: In manual mode, the robot moves at reduced speed. The long-applied value in ISO 10218-1 is 250 mm/s at the tool tip; check the value and setting in your robot’s user manual.
• Three-position enabling switch: The switch on the teach pendant allows movement only in the middle position. When released, or squeezed fully in panic, the robot stops.
• Single-point control: In teach mode, the robot can be moved only from the pendant in the person’s hand; remote start, automatic signals from the line PLC and other control points are disabled.
• Position: The person stands at a point where they will not be caught between the robot and the fence, a column or a fixture; the escape route is kept clear.
• Other equipment in the cell: Other moving equipment inside the cell, such as conveyors, rotary tables, presses or welding guns, is also brought to a safe state.
• Observer: For risky teach work, a second person with access to the emergency stop button is present outside the cell.

Teach mode is used only by people who have been trained and authorised by the employer. The list of authorised persons and the training given should be kept on record.

Entering the cell in maintenance mode

Teach mode keeps the robot energised; maintenance, however, is done without energy. For work such as gearbox replacement, cable harness repair and gripper and welding gun maintenance:

1. The energy supply to the robot and all equipment in the cell is cut; electrical, pneumatic and hydraulic sources are locked and tagged. For the steps, see our article on LOTO lockout tagout.
2. Gravitational energy is considered: vertical axes and arms that could fall when the brake is released are secured with mechanical supports.
3. The part in the gripper is lowered: a vacuum or pneumatic gripper may drop the part when air is cut off.
4. It is verified that energy has really been cut by trying to move the robot from the pendant.
5. After maintenance, the cell is not switched to automatic mode until it has been checked that guards have been refitted, interlock switches work and the emergency stop has been tested.

Our article on maintenance technician risks gives guidance on typical accidents during fault intervention.

In-cell rescue scenario

One of the most serious scenarios in a robot cell is trapping: a person is caught between the robot arm and a fixture or the fence. What to do in this case must be written down and rehearsed in advance; otherwise a movement made in panic can worsen the injury.

Robot manufacturers provide a brake release arrangement so that axes can be moved without drive power in an emergency. Write the location of this arrangement, how it is used and which axes carry a risk of falling into the cell’s emergency instructions, and post them next to the cell. For first aid steps in crush injuries, see our article on first aid for amputation and crush injuries.

We address the risk assessment of robot cells and the on-site application of door, teach and maintenance rules within our risk assessment and OHS specialist services. For other topics in the sector, see our automotive supplier industry page.

Frequently Asked Questions

How high should a robot fence be?

There is no single fixed value. The height is determined together with the fence’s distance from the robot’s reach and the height of the danger point; the aim is that the danger zone cannot be reached by reaching over the fence. This calculation should be made by the integrator according to the standards on safety distances.

Is it safe to enter the cell in robot teach mode?

It can be done by a trained and authorised person if reduced speed, a three-position enabling switch, single-point control and stopping of the other equipment in the cell are ensured. Maintenance work, however, is done not in teach mode but with energy isolated and locked out.

Is it enough if the robot stops when the door opens?

It may be enough if the robot’s stopping time is shorter than the time it takes a person to reach the danger zone from the door. If not, the door must be kept locked until the robot stops. In addition, the robot must not start by itself when the door closes, and a person inside must not be lockable from outside.

How is the arm moved in a robot accident?

A trained person moves the arm in teach mode at reduced speed in the direction opposite to the trapping. If there is no power, the brake release arrangement provided by the manufacturer is used; since vertical axes may fall, the arm is supported beforehand.