Robot datasheets can be intimidating. This guide explains the key specifications you'll encounter when evaluating robot arms and mobile robots, helping you match specifications to your application requirements.
Payload Capacity
Definition: The maximum weight a robot can handle at its end effector, including the weight of the gripper or tool itself.
Payload is often the first spec people check, but it requires careful interpretation:
Maximum vs. Rated Payload
Maximum payload is achievable only at specific positions and speeds. Rated payload is sustainable across the full workspace. Design for rated, not maximum.
Include Gripper Weight
A gripper might weigh 2-5kg. If your robot has 10kg payload and a 3kg gripper, you can only handle 7kg parts.
Center of Gravity Matters
Payload ratings assume the load's center of gravity is at a specific distance from the flange. Off-center loads reduce effective capacity.
| Robot Type | Typical Payload |
|---|---|
| Collaborative robots | 3-16 kg |
| Small industrial arms | 6-20 kg |
| Medium industrial arms | 20-80 kg |
| Large industrial arms | 100-500+ kg |
| Delta robots | 0.5-8 kg |
Reach (Working Envelope)
Definition: The maximum distance from the robot's base to its tool center point (TCP). The working envelope is the 3D space the robot can access.
Reach determines what the robot can physically access:
- Maximum reach: The furthest point, typically with arm fully extended horizontally
- Minimum reach: Areas the robot cannot access, often directly below the base
- Working envelope shape: Varies by robot type—articulated arms create roughly spherical envelopes; SCARA robots have cylindrical envelopes
Practical Tip
Don't select a robot where your work is at the edge of its reach. Performance degrades at envelope extremes. Target tasks within 70-80% of maximum reach for optimal performance.
Repeatability vs. Accuracy
Repeatability
How consistently the robot returns to the same position. If you teach a point and return 1000 times, repeatability is the spread of actual positions.
Typical values: ±0.01mm to ±0.1mm
Accuracy
How close the robot gets to the commanded position. If you command X=500mm, accuracy is how close to 500mm you actually achieve.
Typical values: ±0.1mm to ±1mm (worse than repeatability)
Key insight: Repeatability is usually much better than accuracy. Most applications care more about repeatability—if the robot consistently goes to the same place, you can teach that place once and trust it. Accuracy matters when you need the robot to go to calculated positions it hasn't been taught.
| Application | Repeatability Needed |
|---|---|
| Palletizing, material handling | ±0.5mm - ±1mm |
| General assembly | ±0.1mm - ±0.5mm |
| Precision assembly, electronics | ±0.02mm - ±0.1mm |
| Micro-assembly, semiconductor | ±0.005mm - ±0.02mm |
Degrees of Freedom (DOF)
Definition: The number of independent axes of motion. Each DOF adds flexibility but also complexity and cost.
Can position the end effector in X, Y, Z space but cannot change orientation. Common in Cartesian robots and simple pick-and-place systems.
Adds one rotation axis. SCARA robots typically have 4 DOF—ideal for vertical insertion tasks like assembly and dispensing.
Full position and orientation control. Standard for industrial articulated arms. Can reach any point from any angle (within the workspace).
Redundant DOF allow the robot to reach the same point via multiple configurations. Useful for obstacle avoidance and complex workspaces. Common in cobots and research arms.
Speed & Cycle Time
Speed specifications can be misleading. Look for:
Joint Speeds
Measured in degrees/second (°/s) or radians/second. Each joint has a maximum speed. Outer joints are usually faster than inner joints.
TCP Speed
The speed of the tool center point in mm/s or m/s. More meaningful for applications than joint speeds. Typical range: 1-10 m/s for industrial robots.
Cycle Time
The real measure of productivity. How long to complete one operation cycle? Includes acceleration, motion, and settling time. Ask vendors for cycle time estimates for your specific application.
Acceleration
Often more important than top speed for short moves. High acceleration means faster cycle times even if maximum speed is never reached.
Cobot Speed Limits
Collaborative robots are intentionally slower for safety. ISO/TS 15066 limits contact forces, which effectively limits speed when humans are present. Expect 250-1000 mm/s in collaborative mode vs. 4000+ mm/s for industrial arms.
IP Rating (Ingress Protection)
Definition: A two-digit code indicating protection against solids (first digit) and liquids (second digit). Higher numbers mean better protection.
| IP Rating | Protection Level | Typical Use |
|---|---|---|
| IP40 | Protected from objects >1mm, no water protection | Clean, dry environments |
| IP54 | Dust protected, splash resistant | General manufacturing |
| IP65 | Dust tight, water jet resistant | Washdown environments |
| IP67 | Dust tight, submersion resistant (1m/30min) | Food processing, harsh environments |
Safety Standards
Look for compliance with relevant safety standards:
ISO 10218-1/2
Safety requirements for industrial robots. Covers design, safeguarding, and information for use. The baseline for all industrial robot safety.
ISO/TS 15066
Technical specification for collaborative robot safety. Defines force and pressure limits for human-robot contact. Essential for cobot applications.
CE Marking / UL Listing
CE marking (Europe) and UL listing (North America) indicate the robot meets regional safety requirements. Required for legal deployment in most jurisdictions.
SIL/PL Ratings
Safety Integrity Level (SIL) and Performance Level (PL) indicate the reliability of safety functions. Higher ratings (SIL 3, PL e) mean more reliable safety systems.