Cell Design, PLC Integration and Feeding
Layout and reach studies, fixtures, feeders and conveyors, the PLC handshake, fieldbus links and throughput balancing.
Layout and the reach study
A robot cell is a layout problem first. Put the robot where it can reach every pick, place, service and rest point with the tool in the right orientation, with the people kept out of its range. Do the reach study in simulation (see Robotics Tools) with the real tool, fixtures and cables before buying: it is far cheaper to move a fixture in a model than in steel.
Worked example: the footprint of a guarded cell
- A robot with a 750 mm reach, a 150 mm tool and a 500 mm clearance to the fence (see Safety: Standards, Safeguarding and Cobots) needs a working radius of 750 + 150 + 500 = 1 400 mm.
- A full circle covers π × 1.40² = 6.2 m²; the square fence that encloses it is 2.8 × 2.8 m = 7.8 m². Limiting the robot’s travel to 270° with software axis limits (and hardware stops where required) reduces the swept area to 4.6 m², and lets a wall or a machine close one side.
- Floor space is the budget: a cell that is 15 % smaller can often be fitted without moving the line.
- Keep all work in the robot’s good zone (30 to 85 % of reach, see Robot Types, Specifications and Selection) and well away from singularities (see Coordinate Frames and Kinematics).
- Put the robot on a pedestal or the ceiling if that gives better reach over the work; check that the maker allows that mounting.
- Plan the cables and hoses as part of the layout: a dress pack that must loop around the arm limits the wrist rotation.
- Leave access for loading, tool changes, maintenance and a person to see the whole cell from the control point.
- Think about the next product: a cell that can take a slightly larger part, a second tool or a second robot is worth more than a perfect fit.
Foundation and services
A robot pulls on its base. Even at rest an extended arm produces a moment, and moving loads add a larger, changing one. The base plate, the pedestal and the floor must carry it. Take the manufacturer’s foundation loads (forces and moments for static and dynamic cases) and design for them.
Worked example: the base moment
- A 6 kg load at 0.75 m and an arm of 150 kg with its center of gravity 0.35 m out give a static moment M = (6 × 0.75 + 150 × 0.35) × 9.81 = 559 N·m.
- A dynamic factor of 2 for acceleration and stopping gives about 1 118 N·m. With 4 anchor bolts on a 320 mm square, the tension in the most loaded pair is about M ÷ (2 × 0.16 m × 2) = 1 747 N per bolt, before the effect of preload and the robot’s own weight. This is a small number for the bolts; the question is whether the floor (a thin slab, a mezzanine) can take the cyclic load without vibrating.
- Vibration through the floor upsets the robot, nearby machines and any vision system: a heavy pedestal or a grouted plate helps.
| Service | Typical need | Notes |
|---|---|---|
| Electrical supply | Three-phase or single-phase for the controller; 1 to 10 kVA depending on the robot | A clean supply and a good earth; see the manual for the fuse and cable |
| Compressed air | 5 to 6 bar dry, filtered, for the tool | Gripper and vacuum consumption (below); a pressure switch on the supply |
| Network and fieldbus | Ethernet to the PLC, to vision and to the plant network | Segment the robot network (see Industrial Communication) |
| Safety circuit | Emergency stops, gates, light curtains, to the controller’s safety inputs | Designed to the required performance level (see Safety: Standards, Safeguarding and Cobots) |
| Cooling and environment | Controller airflow; clean, within the temperature rating | Do not block the vents; add a filter in dusty cells |
Worked example: gripper air consumption
A pneumatic gripper cylinder of 20 mm bore and 10 mm stroke displaces 3.1 cm³ per stroke. Both strokes (open and close) at 7 bar absolute and 25 cycles a minute use 3.1 × 2 × 7 × 25 ÷ 1 000 = 1.1 L/min of free air, small. A vacuum ejector that runs all the time uses far more (typically 20 to 100 L/min): use an air-saving valve (see End-of-Arm Tooling and Grippers).
Fixtures and locating parts
The robot can only place a part as accurately as it is picked up, and it can only pick a part up as accurately as the part is presented. Fixtures, nests and locating features turn a part’s uncertain position into a known one. The usual principle is 3-2-1 locating (a plane, a line and a point: six constraints for six degrees of freedom; see GD&T for datum features), and clamping against the locators, not the other way round.
Worked example: two locating pins with clearance
- Two pins 200 mm apart locate a part. Each pin has 0.05 mm total clearance in its hole, so the part can sit up to ±0.025 mm off center.
- It can also rotate: the two pins can be off in opposite directions by up to 0.05 mm over 200 mm, an angle of 0.25 milliradians. At a feature 300 mm from the pins that adds 0.08 mm: the total position uncertainty at the feature is ±0.100 mm.
- Use a diamond (relieved) pin for the second locator so the part is not over-constrained, and see the fits in Fits and Tolerances for the clearance. This error enters the placement error budget (see Vision, Calibration and Accuracy).
- Make the nest forgiving to the robot: generous lead-in chamfers, so a part that is a millimeter off still drops into place.
- Sense the part: a presence sensor in the nest confirms the part before the robot goes for it, and that it is seated before the next step (see Sensors and Field Devices).
- Keep the fixture in a calibrated frame (see Coordinate Frames and Kinematics), mounted on dowels so that it can be removed and replaced without re-teaching.
- Separate the loading side from the robot side where operators load parts: a two-station or rotary table lets the operator load while the robot works.
Feeding: conveyors, feeders and trays
| Method | Parts presented | Good for | Watch out for |
|---|---|---|---|
| Fixed position (nest, tray, pallet) | Orderly, in known places | Most precise and simplest; trays loaded by people or machines | Loading is the bottleneck; tray tolerance |
| Conveyor with vision | Random on a belt | High rates; mixed parts | Tracking, lighting, the pick window (see Vision, Calibration and Accuracy) |
| Vibratory bowl and linear feeder | Orientated and singulated | Small identical parts | Noise; changeover per part; jams |
| Flexible feeder | Spread on a plate for a camera to find | Small parts, many types | Throughput per feeder |
| Bin picking | Random in a bin | Removing the need for orderly parts | Success rate; collisions; 3D vision (see Vision, Calibration and Accuracy) |
| Magazines and stacks | Flat parts or sheets in a stack | Sheet metal, cartons, blanks | Separating the top part (double picks) |
Worked example: a buffer for a feeder stoppage
A robot with a 2.4 s cycle consumes 25 parts a minute. If the feeder can be stopped for up to 30 s (a jam clearance or a refill), the robot needs a buffer of 30 ÷ 2.4 = 13 parts to keep working without a pause. A small buffer between a bursty machine and the robot raises utilization more than a faster robot.
The PLC handshake
In most cells the PLC runs the cell and tells the robot what to do; the robot runs motion. They must agree on who does what, when. The robust way is a handshake: each request is acknowledged, so neither side assumes. Do it with a few well-named signals, preferably over a fieldbus.
| Direction | Signal | Meaning |
|---|---|---|
| PLC → robot | Program number (binary, with strobe) | Which routine or product recipe to run |
| PLC → robot | Start (pulse or level) | Run the selected program now; keep until Busy appears |
| PLC → robot | Reset, stop, hold | Clear faults, stop motion at a safe point |
| PLC → robot | Part present, station clear | Permissions, so the robot does not enter an occupied station |
| Robot → PLC | Ready | Powered, referenced, no fault, at home or a safe point |
| Robot → PLC | Busy / running | A program is executing |
| Robot → PLC | Done | The routine ended normally; the PLC may continue |
| Robot → PLC | Fault and a code | Something stopped the robot, with a number the PLC can show |
| Robot → PLC | In position / area flags | The robot is in a zone where the PLC may act (a door may open) |
Worked example: what a handshake costs
- Each signal change goes through a PLC scan (about 10 ms) and a bus update (about 4 ms), so each hop costs 14 ms. A four-step handshake (Start, Busy, Done, release) costs about 64 ms per cycle: 2.7 % of a 2.4 s cycle.
- Overlap the steps and the cost falls: the PLC sends the next Start as soon as it sees Done, and the robot starts the next move while the PLC is still releasing the previous one. That saves about 28 ms, to 36 ms.
- Pass the recipe over a number: 5 binary bits select up to 31 programs (0 means none). For more, send the number over a fieldbus register (see Industrial Communication).
- Level signals, with acknowledgment, are more robust than pulses: a missed pulse is a lost command.
- Define what every signal does on a fault or an emergency stop so that the PLC and the robot restart in a known state.
- Put a watchdog on the link: a heartbeat bit that toggles; if it stops, both sides go to a safe state.
- Keep safety signals out of the handshake: they belong to the safety circuit (see Safety: Standards, Safeguarding and Cobots), not the PLC program.
- Read the controller side too: see PLC and Logic Control for the PLC program structure and state machines.
Fieldbus or wires?
| Interface | Typical update time | Notes |
|---|---|---|
| PROFINET RT | 1 to 4 ms | Siemens and many robot makers |
| EtherNet/IP | 2 to 10 ms | Rockwell, many robot makers |
| EtherCAT | 0.25 to 1 ms | Motion and fast I/O |
| Modbus TCP | 10 to 50 ms | Simple, widely supported |
| Hardwired I/O | Scan time (1 to 5 ms) | A few signals, safety |
Worked example: 24 signals
Wiring 24 signals between the robot and the PLC as separate hard-wired cores costs perhaps $35 per signal (cable, terminals, I/O channel, wiring time): $840. A fieldbus connection costs about $450 for the interface and cable and carries hundreds of signals, with a diagnostic status as a bonus. Hard-wiring pays only for a handful of signals, and for the safety circuits, which have their own rules.
- Use the robot’s native fieldbus if the PLC speaks it; add a gateway only if not.
- Check the robot controller’s limit on the number of fieldbus signals, and which are reserved for the system.
- Safety over the network (PROFIsafe, CIP Safety, FSoE) is possible with certified devices; keep it separate from the standard data.
Balancing the cell with the line
Worked example: how many robots?
- The line must handle 60 parts a minute. One robot with a 2.4 s cycle does 25.0 a minute at best.
- With an availability of 92 % (see Commissioning, Maintenance and Troubleshooting) the usable rate is 25.0 × 0.92 = 23.0 a minute.
- Robots needed: 60 ÷ 23.0 = 2.61, rounded up to 3 robots. Shortening the cycle to 1.8 s (see Payload, Inertia, Speed and Cycle Time) would give 31.4 a minute per robot, and 2 robots: a cycle improvement may save a whole robot.
- The slowest station sets the line rate. Balance all stations to within about 10 % of one another, and add buffers between them.
Documentation and acceptance
| Document | Contents |
|---|---|
| Layout drawing | Robot, fixtures, fence, doors, services, cable runs; with dimensions and the reach envelope |
| Electrical and pneumatic schematics | Power, I/O lists, the safety circuit, valve manifolds |
| I/O and signal list | Every handshake signal with its address and meaning |
| Programs and data | Robot programs, tool, load and frame data, calibration, backup (see Commissioning, Maintenance and Troubleshooting) |
| Risk assessment and safety validation | Hazards, measures, stopping times, validation tests (see Safety: Standards, Safeguarding and Cobots) |
| Operating and recovery instructions | Start-up, changeover, fault clearing, the list of alarms and what to do |
| Spare parts and maintenance plan | Parts that wear, schedules and who does them |
See System Integration for the panel, commissioning and handover of the whole system.
Related
- Industrial Robotics: the overview of this guide.
- Safety: Standards, Safeguarding and Cobots: guarding and the safety circuit.
- Applications: Palletizing, Welding and Machine Tending: palletizing, welding and machine tending cells.
- PLC and Logic Control, Industrial Communication and System Integration.
FAQ
Should the PLC or the robot control the cell?
Usually the PLC runs the cell sequence and the robot runs motion, linked by a handshake. A robot-led cell is fine for a standalone task, but the PLC is better when the robot is one of several machines.
Fieldbus or hard-wired signals?
A fieldbus for anything beyond a handful of signals: it is cheaper to wire, carries diagnostics and numbers (like recipes), and is standard on current robots. Keep emergency stops and safety gates on the safety circuit.
How do I decide how many robots a line needs?
Divide the demand by the usable rate of one robot (rate × availability) and round up; shortening the cycle can save a whole robot. Check each station against the line rate.
Why does the robot miss parts in the nest?
The nest or fixture has too much clearance, the part varies, or the frame moved. Add lead-in chamfers, sense the part, and re-measure the frame.
References
- ISO 10218-2, Robotics: safety requirements for robot systems and integration; IEC 60204-1 (electrical equipment of machines).
- Robot makers’ installation and foundation manuals; PLCopen and fieldbus (PROFINET, EtherNet/IP) specifications for I/O profiles.
- Groover, M. P., Automation, Production Systems and Computer-Integrated Manufacturing, Pearson (line balancing and feeding).
Last reviewed: October 2026. Costs, update times and loads are typical and illustrative: use the manufacturers’ data and the detailed design.