Robot Types, Specifications and Selection
Articulated, SCARA, delta, cartesian and collaborative robots, work envelopes, payload, reach, repeatability and how to choose.
What an industrial robot is
An industrial robot, by the usual definition, is an automatically controlled, reprogrammable, multi-purpose manipulator with three or more programmable axes, fixed in place or mounted on a mobile platform. It carries a tool (a gripper, a welding torch, a camera) along a path in space, repeatably and tirelessly. What makes it different from a machine tool or a transfer line is flexibility: the same arm can be reprogrammed for a new part or a new job. What it costs you in return is the work of building the cell around it (see Cell Design, PLC Integration and Feeding).
The main types
| Type | Motion | Typical strengths | Typical uses |
|---|---|---|---|
| Articulated (6-axis) | Six rotary joints; any position and orientation within reach | Flexible and widely available; payloads from a few kg to over 1 000 kg; reach from about 0.5 to over 4 m | Welding, painting, machine tending, palletizing, assembly |
| SCARA | Two rotary joints in a horizontal plane, a vertical (Z) axis and a rotation | Fast and stiff in the horizontal plane; repeatability about ±0.01 to ±0.02 mm; compact | Fast assembly, pick and place, screw driving, small parts |
| Delta (parallel) | Three arms from a fixed base move a light platform | Very fast with low moving mass: well over 100 picks a minute with light payloads | Food and pharmaceutical packaging, sorting |
| Cartesian (gantry) | Linear axes at right angles (X, Y, Z) | Simple, rigid, scalable to long reach, easy to program and predict | Machine loading, large pick and place, palletizing, 3D printing |
| Collaborative (cobot) | Usually 6-axis with built-in force and speed limiting | Can work near people after a risk assessment; easy to teach; payloads typically a few to about 25 kg; slower than industrial arms | Light assembly, machine tending, packing, inspection |
| Mobile manipulator | An arm on an autonomous mobile base | Moves between workstations | Intralogistics, tending several machines |
| Cylindrical and polar | Rotary base with linear reach and lift | Simple and strong; mostly older designs | Press loading, heavy handling |
The mechanisms inside these arms (servo motors, gearboxes, belts and screws) are described in Motion Control and Actuators and Gears and Power Transmission.
Work envelopes
The work envelope is the set of points the tool can reach. Its shape is set by the link lengths and the joint limits, and it is never a simple sphere: there is a dead zone around the base, another overhead, and limits behind the arm. The picture shows envelopes calculated by sweeping the joint angles within assumed limits.
Worked example: can one robot serve all four points?
A robot with 750 mm reach and a dead zone of about 120 mm around its base must serve these points (x, y in mm from the base axis):
| Point | Position | Distance | Fraction of reach | Verdict |
|---|---|---|---|---|
| Pick (conveyor) | 520, 180 | 550 mm | 73 % | good |
| Place (fixture) | -310, 440 | 538 mm | 72 % | good |
| Rest and vision | 0, 700 | 700 mm | 93 % | too close to the limit |
| Reject bin | 420, -330 | 534 mm | 71 % | good |
- The furthest point is 700 mm away, 93 % of the reach. Keep working points between about 30 % and 85 % of the reach: near full extension the arm is at its weakest and slowest (see Coordinate Frames and Kinematics on singularities), and the tool length and orientation also eat into the reach.
- Check the tool orientation at every point as well as the position: a pose reachable with the tool pointing down may be unreachable with it pointing sideways.
- Do this on a CAD or simulation model of the cell before buying (see Robotics Tools).
Reading a datasheet
| Specification | What it means | Watch out for |
|---|---|---|
| Payload | Maximum mass at the wrist that the robot can carry, at a stated position | It includes the gripper, tool changer and cables as well as the part. Capacity falls as the center of gravity moves away from the flange (see Payload, Inertia, Speed and Cycle Time) |
| Reach | Maximum distance from the base axis to the wrist | The usable envelope has dead zones, and the tool length reduces it |
| Repeatability | How closely the robot returns to a taught point (ISO 9283), typically ±0.02 to ±0.1 mm for industrial arms | This is not accuracy (below) |
| Axes (degrees of freedom) | Number of independent joints | Six are needed for any position and orientation in space |
| Axis speeds and ranges | Maximum speed and travel of each joint | Tool speed depends on the pose; the wrist axes are often the limit |
| Moment and inertia limits | Maximum torque and inertia at the wrist axes | A long or heavy tool can exceed these even when the mass is within payload |
| Protection and environment | IP rating, temperature range, cleanroom, washdown or explosion-proof versions | Check the whole cell: cable, controller and tool |
| Mounting | Floor, wall, ceiling or inverted | Some robots limit payload or motion for some mounting angles |
| Controller and interfaces | Fieldbuses, I/O, safety inputs, languages, vision and force options | The right interface can save weeks of integration (see Cell Design, PLC Integration and Feeding) |
| Mass and footprint | Arm and controller weight and size | Floor loading, a base plate and the foundation |
Worked example: how big a payload?
| Item at the wrist | Mass |
|---|---|
| The part | 2.1 kg |
| Gripper | 1.4 kg |
| Tool changer | 0.6 kg |
| Cables, hoses and sensors on the tool | 0.3 kg |
| Total | 4.4 kg |
Add a 20 % margin for tool revisions: 4.4 × 1.2 = 5.28 kg. The next standard size is a 6 kg robot, and then the payload chart (not just the headline rating) must be checked at the tool’s center of gravity (see Payload, Inertia, Speed and Cycle Time). The part is only 48 % of what the robot carries: a lighter gripper often allows a smaller, cheaper robot.
Repeatability is not accuracy
Repeatability is how closely the robot returns to a point it has been taught, for example ±0.03 mm. Accuracy is how close it gets to a point it has been told by a model (a CAD position, a camera measurement, an offline program). Without calibration, accuracy is typically several times worse than repeatability, because every link length, joint zero and gearbox deflection has a small error that adds up. A taught point cancels those errors; a computed one does not. See Vision, Calibration and Accuracy for how to close the gap.
- Teach, do not compute, the critical points where you can, or calibrate the robot and tool.
- Repeatability is stated for specific conditions (speed, load, temperature, pose); the figure at full load and full speed can be worse.
- Heat moves the robot: the arm grows as it warms, often by 0.1 mm or more in the first hour; warm it up before precise work.
What is inside the arm
- Servo motors with absolute encoders at each joint, and brakes that hold the arm when power is removed.
- Precision gearboxes: harmonic (strain-wave) drives and cycloidal (RV-type) reducers give high ratios with almost no backlash. Typical ratios are 50 to 160.
- Controller with the motion planner, the drives and the safety circuit, sometimes in a separate cabinet and sometimes built into the base.
- Cables and hoses through hollow wrist shafts or external dress packs: the most common wear item (see Commissioning, Maintenance and Troubleshooting).
Worked example: how fast does a joint turn?
A motor running at 3 000 rpm through a 100:1 gearbox turns the joint at 3 000 ÷ 100 = 30 rpm, which is 180 degrees per second. The gearbox multiplies the torque by about the same ratio (less its efficiency), which is why a small motor can hold a heavy arm. The same reduction that gives torque limits speed: this is the trade-off every robot joint is built around (see Payload, Inertia, Speed and Cycle Time).
Choosing a robot
Start from the task, not the brand. A fast comparison is the cycle time of the same pick and place move on different robots, using a simple trapezoidal speed profile (see Motion, Paths and Programming).
Worked example: the same move on four robots
A pick and place with a 25 mm lift, a 300 mm transfer, 25 mm down, then back, plus 0.3 s each to close and open the gripper. The speeds and accelerations below are typical and illustrative:
| Robot | Speed and acceleration used | Cycle | Rate |
|---|---|---|---|
| SCARA (light) | 2 000 mm/s, 12 000 mm/s² | 1.60 s | 38 per minute |
| Delta | 3 000 mm/s, 30 000 mm/s² | 1.23 s | 49 per minute |
| 6-axis articulated | 1 500 mm/s, 6 000 mm/s² | 2.01 s | 30 per minute |
| Cobot | 500 mm/s, 2 000 mm/s² | 3.19 s | 19 per minute |
- The SCARA and the delta are faster because of low moving mass and high acceleration; the short 25 mm moves never reach top speed, so acceleration, not top speed, sets the cycle (see Payload, Inertia, Speed and Cycle Time).
- The cobot is slow by design: its speed is limited so that contact with a person is safe (see Safety: Standards, Safeguarding and Cobots). Choose it for flexibility and safety, not rate.
- The gripper time (0.6 s) is a large part of the fast robots’ cycle: shortening it gains more than a faster arm.
| If the work is… | Consider |
|---|---|
| Light parts, planar, high speed, high repeatability | SCARA |
| Very light products at over 100 picks a minute | Delta |
| Many orientations, long reach, welding, painting, machine tending | Articulated 6-axis |
| Large rectangular area, long travel, heavy loads | Cartesian gantry |
| Shared space with people, frequent changeover, low rate | Cobot |
| Several machines or work stations spread around | Mobile manipulator, or a robot on a linear track |
- Check availability and support: spares, training, local service and the language your staff know.
- Check the controller: fieldbus options, safety I/O, vision and force interfaces, and whether it fits your PLC (see PLC and Logic Control).
- Four axes may be enough: for flat palletizing and for planar pick and place a 4-axis robot is faster and cheaper than a 6-axis one.
What a robot really costs
Worked example: the arm is a fraction of the cell
| Item | Cost (typical, illustrative) |
|---|---|
| Robot arm and controller | $28 000 |
| End-of-arm tool | $6 000 |
| Fixtures, feeders and conveyors | $8 000 |
| Guarding and safety devices | $9 000 |
| Integration: panel, wiring, PLC and vision | $18 000 |
| Programming, commissioning and training | $7 000 |
| Total installed | $76 000 |
The installed cell costs 2.7 times the price of the arm. The common rule of thumb is two to three. Plan the budget for the cell, not for the robot, and use the payback calculation in Commissioning, Maintenance and Troubleshooting to decide.
Related
- Industrial Robotics: the overview of this guide.
- Coordinate Frames and Kinematics: how the chosen robot moves.
- Payload, Inertia, Speed and Cycle Time: checking payload, inertia and cycle time.
- End-of-Arm Tooling and Grippers: the tool at the wrist.
- Motion Control and Actuators: the servos and gearboxes inside.
FAQ
What is the difference between a cobot and an industrial robot?
A cobot has built-in force and speed limiting so that it can work near people after a risk assessment, usually at lower payload and speed. An industrial robot is faster and stronger and normally works behind safeguarding. Neither is safe by itself: the whole application must be assessed.
Do I need six axes?
Only if you need to control both position and orientation in space. For work in a plane with a vertical motion, a four-axis SCARA is faster, stiffer and cheaper; for flat palletizing four axes are also enough.
How do I choose payload?
Add the part, the gripper, the tool changer and the cabling at the wrist, add a margin of about 20 %, and then check the manufacturer’s payload chart for the tool’s center of gravity, since capacity falls as the load moves out from the flange.
Is a more repeatable robot a more accurate one?
No. Repeatability is returning to a taught point; accuracy is reaching a calculated one. An uncalibrated robot is typically several times less accurate than it is repeatable.
References
- ISO 8373, Robotics: vocabulary; ISO 9283, Manipulating industrial robots: performance criteria and related test methods.
- International Federation of Robotics (IFR), World Robotics (statistics and definitions); robot manufacturers’ datasheets (ABB, FANUC, KUKA, Yaskawa, Universal Robots, Epson, Denso).
- Craig, J. J., Introduction to Robotics: Mechanics and Control, Pearson; Siciliano, B. and others, Robotics: Modeling, Planning and Control, Springer.
Last reviewed: October 2026. Speeds, costs and reach figures are typical and illustrative: use the manufacturers’ data for a real selection.