Five questions instead of five models
“Top 5” here means the five factors that most often decide which robot fits a production task. They are payload, reach, speed, repeatability and cell layout. Specific products are not covered, and the figures in this article are hypothetical, used only to show the thinking.
Which basic robot type suits the work (jointed arm, SCARA, delta and so on) is a separate, earlier decision. This article assumes you have narrowed that down and now need to size the robot.
1. Payload
Payload is the weight the robot can carry at the end of its arm while still meeting its stated performance. The key point is that it covers everything that hangs on the arm, not just the part. The gripper, any sensors, cables and fixtures count too.
A hypothetical example: a part weighs 2 kg and the gripper weighs 3 kg. The total load is 2 + 3 = 5 kg. If the gripper must sometimes hold two parts at once, the load becomes 3 + 4 = 7 kg. A robot rated close to the first figure would have no room for the second case.
Payload ratings are also tied to how far the load is from the wrist and how it is shaped, because a long or off-center tool produces more turning force on the joints than the weight alone suggests. Manufacturers publish how the rating applies, so read the documentation and check equipment data plates and manuals rather than assuming the headline number covers your case. It is usual to leave a margin.
2. Reach
Reach is how far the robot can extend from its base. The useful question is whether every point the robot must touch lies inside its working envelope, with the tool angled the way the task needs, and not merely whether the farthest point is reachable.
Some points near the base or directly above it can be awkward or impossible to reach, and a long tool shortens the practical reach. A common approach is to sketch the base position, the pick and place positions, and the obstacles, then check each point. Many robot suppliers and software packages can simulate this before anything is bought.
More reach is not always better. A larger robot is usually heavier, costs more, needs more floor space and may be less precise at the far end of its travel.
3. Speed and cycle time
Speed specifications, such as joint speeds, do not directly tell you how many parts you will finish per hour. What matters is cycle time: the full sequence of picking, moving, placing, waiting and returning.
A hypothetical example: if one complete cycle takes 12 seconds, then in one minute the robot completes 60 / 12 = 5 cycles, or 300 per hour, before allowing for stops. Real output is lower once changeovers, jams, maintenance and waiting for upstream or downstream equipment are included.
Cycle time often depends on more than the robot. Gripper closing time, vision processing, curing or machining time and the rate of a conveyor can all set the pace. If a machine tool takes much longer than the robot’s motion, buying a faster robot gains little. Line balance and the wider flow matter, which is part of what manufacturing efficiency means in practice.
4. Repeatability
Repeatability describes how closely a robot returns to the same taught position again and again. Accuracy is different: it describes how close the robot gets to a position commanded in space. Many production tasks rely mainly on repeatability, because the robot is taught points and returns to them.
Required precision comes from the task. Dropping boxes onto a pallet generally tolerates more variation than inserting a small pin into a tight hole. Compare the repeatability figure with the tolerance of the job, and remember that the gripper, fixtures, part variation and thermal changes also add error. A robot with excellent repeatability cannot make up for a loose fixture. Where the task is very demanding, vision guidance or compliant tooling can help.
5. Cell layout
The cell is the robot plus everything around it: the base mounting, fixtures, feeders, conveyors, guarding, controls and access for operators and maintenance. Poor layout can undermine a good robot choice.
Points to consider include:
- How parts arrive and leave, and whether they are presented consistently.
- Whether the base is on the floor, a pedestal, the ceiling or a track.
- Space for maintenance and for the tool changes.
- Cable routing and the cable length available to the robot.
- Where people will stand, and how guarding, light curtains or other safeguards will separate them from motion.
- How the cell fits into the overall equipment layout of the plant.
Safety needs to be part of the design from the start, not added afterward. Risk assessment, guarding and emergency stop circuits should follow the manufacturer’s instructions and local regulations, and the work should be done by trained, qualified people. Any servicing should use lockout and tagout style procedures.
Looking beyond the specification sheet
Specifications are only part of a purchase. It also helps to consider:
- The total cost of ownership, which includes tooling, integration, training, spare parts and maintenance, not only the robot price.
- What testing and acceptance testing will be performed before the cell is accepted.
- How easy it will be to change over to a new product later.
- The support available locally, and the skills your own staff will need.
If you can describe the part weight, the points to reach, the target cycle time, the tolerance and the available floor area, you are usually in a good position to talk to suppliers. Treat their proposals as something to check against your own numbers, and ask to see the calculations behind any claimed output.