Industrial robot applications and safety

Why it matters

Robots earn their place on the shop floor only when the application suits them and the cell is safe. Most robot accidents happen not in automatic running but during teaching, maintenance and fault recovery, when people are inside the cell. Engineers who design or run cells must be able to pick the right application, justify it economically and apply the safety principles and standards correctly.

Key ideas

Typical industrial applications.

• Material handling — pick-and-place, machine tending (loading CNC machines, presses, injection moulding), palletising/depalletising, packaging. The largest share of installed robots.
• Processing operations — spot welding (automotive bodies), arc welding, spray painting and coating, sealing and gluing, grinding, deburring, polishing, laser and water-jet cutting.
• Assembly — screw driving, insertion, electronics placement; SCARA and delta robots dominate small-part assembly.
• Inspection and testing — robot-held cameras, gauges or CMM probes.
• Growing areas — collaborative assembly, logistics AMRs, food handling, medical and laboratory automation.

When a robot fits. Repetitive tasks, hazardous or unpleasant environments (heat, fumes, heavy loads), need for consistent quality, multi-shift operation, and stable product designs or families. Poor fits: very low volumes with frequent changeovers and unstructured parts — unless vision and quick programming are added.

Economic justification. Compare investment (robot, tooling, integration, guarding, training) with annual savings (labour, scrap, rework, throughput) minus running costs (power, maintenance, programming). Common measures: payback period, return on investment, cost per part.

Hazards. Impact and crushing/trapping by the arm or tooling; ejected parts or dropped loads; process hazards (weld arc, laser, paint fumes, electrical, pneumatic stored energy); unexpected start-up; human error during teaching and maintenance.

Safety approach — risk assessment first. Identify hazards for every task (including setup, teaching, cleaning, maintenance), estimate risk, then reduce it in priority order: (1) inherently safe design, (2) safeguarding and protective devices, (3) information, training and procedures (signs, PPE, lock-out/tag-out).

Safeguards.

• Perimeter guarding — fixed fences with interlocked gates: opening the gate stops the robot in automatic mode.
• Presence-sensing devices — light curtains, laser area scanners, pressure-sensitive mats — stop or slow the robot when someone enters.
• Emergency stop buttons on the pendant and around the cell (stop category 0 or 1).
• Teach mode controls — reduced TCP speed (typically limited to 250 mm/s), three-position enabling switch, hold-to-run.
• Soft axis and space limiting — safety-rated software zones that restrict where the robot can go.
• Safety functions are built with redundant, monitored (dual-channel) circuits and safety PLCs, rated by performance level (PL) or SIL.

Collaborative operation (ISO 10218 and ISO/TS 15066) has four modes: safety-rated monitored stop; hand guiding; speed and separation monitoring (robot slows/stops as a person approaches); power and force limiting (contact allowed below body-region force/pressure limits taken from the standard). Collaboration is a property of the whole application, not just the robot.

Standards. ISO 10218-1/-2 (industrial robot and integration safety), ISO/TS 15066 (collaborative), ISO 13849 (safety-related control systems), ISO 13855 (positioning of protective devices — safety distance), ISO 12100 (risk assessment). Numerical limits come from these documents — always take values from the current edition, not from memory.

Formulas

Minimum safety distance (ISO 13855 form): S = K · T + C

• S — minimum distance from the protective device to the hazard (mm); K — approach speed of the body (mm/s, from the standard, e.g. 1600 mm/s for walking approach in many cases); T — total stopping time = device response + control + robot stopping time (s); C — intrusion allowance depending on device resolution (mm, from the standard's tables).

Stopping distance at constant deceleration: d = v² / (2a), stopping time: t = v / a

Payback period = Net investment / Net annual savings

Annual output = (3600 / cycle time) × hours per shift × shifts × working days × availability

Holding torque check: τ = m · g · r (worst case: arm horizontal)

Worked examples

Example 1 (standard). A light curtain guards a robot cell entrance. Light-curtain response time is 0.02 s and the robot's measured stopping time is 0.28 s. Use K = 1600 mm/s and an intrusion allowance C = 850 mm (taken from the standard for this curtain's resolution). Find the minimum mounting distance.

1. T = 0.02 + 0.28 = 0.30 s.
2. S = K·T + C = 1600 × 0.30 + 850 = 480 + 850 = 1330 mm.

Answer: S = 1330 mm (1.33 m). If the curtain cannot be placed that far away, reduce T (slower speeds, better brakes) or use a curtain with finer resolution (smaller C).

Example 2 (GATE level — justification and throughput). A welding cell costs ₹30 lakh and replaces 4 operators (2 per shift, 2 shifts) earning ₹3.6 lakh/year each. Running costs are ₹2.4 lakh/year. The cell has a 6 s cycle, runs 2 shifts of 8 h for 300 days a year at 85 % availability. Find (a) the payback period, (b) the annual output.

1. Labour saved = 4 × 3.6 = ₹14.4 lakh/year.
2. Net saving = 14.4 − 2.4 = ₹12.0 lakh/year.
3. Payback = 30 / 12 = 2.5 years.
4. Output = (3600 / 6) × 8 × 2 × 300 × 0.85 = 600 × 16 × 300 × 0.85 = 2 448 000 parts/year.

Answer: (a) 2.5 years, (b) about 2.45 million parts per year.

Example 3 (stopping). A robot moving at 1.5 m/s brakes at 7.5 m/s². Stopping time t = 1.5/7.5 = 0.20 s; stopping distance d = 1.5²/(2 × 7.5) = 0.15 m.

Common mistakes

• Assessing only automatic operation and forgetting teaching, maintenance and cleaning tasks.
• Mounting light curtains closer than S, so a person reaches the hazard before the robot stops.
• Using the robot's datasheet speed instead of the measured stopping time at the actual payload.
• Assuming a "cobot" is automatically safe — a sharp tool or heavy part can make the application hazardous.
• Treating an emergency stop as a safeguard — it is a complementary measure, not a substitute for guarding.
• Quoting safety numbers from memory instead of the current standard.

For GATE ME

Expect conceptual MCQs on application areas (which robot type suits which job), safeguarding devices, collaborative operation modes and the principles of risk reduction. Simple numericals may involve cycle time and output, payback period, stopping distance or a safety distance with all constants given. Read every given value carefully and keep units consistent (mm vs m, s).

Quick check

1. Name the four collaborative operation modes.
2. Payback for a ₹20 lakh cell saving ₹8 lakh/year net?
3. S = K·T + C with K = 1600 mm/s, T = 0.25 s, C = 850 mm?
4. In risk reduction, what comes first: training or inherently safe design?
5. Which application area has the largest share of installed robots?

Answers: 1. Safety-rated monitored stop, hand guiding, speed and separation monitoring, power and force limiting; 2. 2.5 years; 3. 1250 mm; 4. Inherently safe design; 5. Material handling.