How to choose the right robot: 12 important parameters explained
Sometimes it looks like managers and engineers speak a completely different language. Industrial machines are a complex subject and it is completely normal to feel lost if this topic is new to you. The communication between engineers and their clients is crucial, which is why we always try to inform and educate our clients so that they can make the right choice. This is especially important if you are interested in investing in some kind of robotic equipment, whether it is a robotic palletizer or some other modular robotic add-on for your packaging line. In order to help you make the right decision, we decided to post this short guide about defining parameters of industrial robots, so that you can have a better understanding of technical descriptions and specifications of each robotic arm. There is a quite a number of parameters used to describe the features and possibilities of a robot and here are the most important ones.
- Number of axes or Degrees of freedom: Two axes are required for a robotic arm to reach any point in a plane. To reach any point in space – three axes are required. To fully control the orientation of the end of the arm, and to enable the wrist rotation, three more axes called yaw, pitch, and roll are required. Some designs such is the SCARA robot with 4 axes exchange limitations in motion possibilities for accuracy, speed and cost.
- The working envelope is a term used to define the region of space a robot can reach.
- Kinematics are determined by the way rigid members and joints in the robot are arranged, which limits the robot’s possible motions. Classes of robot kinematics include articulated Cartesian, parallel and SCARA robots.
- Carrying capacity or payload defines how much weight can be lifted by a robot.
- Speed is the parameter connected to how fast the robot can position the end of its arm. This may be determined in terms of the linear or angular speed of each axis or as the speed of the end of the arm when all axes are moving.
- Acceleration is a measure telling us how quickly an axis can accelerate. Distance and movement pattern can limit acceleration: a robot may not be able to reach its specified maximum speed for movements over a short distance or over a complex path that requires frequent changes of direction.
- Accuracy is defined as a measure that shows how closely a robot can reach a commanded position. When the actual position of the robot is measured and compared to the commanded position, the error between these two is a measure of accuracy. External sensors such as a vision system or Infra-Red can improve accuracy. Accuracy can depend on speed and position within the working envelope and on the payload.
- Repeatability is a dimension that tells us how well the robot will return to a programmed position. This is not to be confused with accuracy. When the robot is told to go to a certain X-Y-Z position, and the robot reaches a position that is one 1 mm away from the required position – that would be its accuracy which can be improved by calibration. But if that position is stored in controller memory and each time robot is sent there it returns to within 0.1mm of the required position then the repeatability will be within 0.1mm.
As already stated, accuracy and repeatability are two different measures. Repeatability is the most important criterion for a robot. ISO 9283 proposes a method where both accuracy and repeatability can be measured. A robot is sent to a taught position a number of times and the error is measured at each return to the position after visiting 4 other positions. Repeatability is then quantified using the statistical measure called the standard deviation of those samples in all three dimensions/ axis. The repeatability is not the same in different parts of the working envelope and it also changes with speed and payload. ISO 9283 demands that accuracy and repeatability should be measured at maximum speed and at maximum payload, but these results are often pessimistic because the robot could be much more accurate and repeatable at light loads and speeds.
- Motion control: defines the level of control required for some tasks. For a simple pick-and-place assembly, typical for robotic palletizing, the robot needs to merely return repeatedly to a limited number of pre-taught positions. For more sophisticated applications, like welding and spray painting, movement must be constantly controlled to follow a path in space, with controlled orientation and velocity.
- Power source: Some robots use electric motors, some use hydraulic actuators. Electric robots are faster, hydraulic robots are stronger and better in more complex applications such as spray painting.
- Drive: some robots connect electric motors to the joints via gears; others connect the motor to the joint directly (direct drive). Using gears causes measurable ‘backlash’ (loss of motion in a mechanism caused by gaps between the parts). Smaller robotic arms often work at high speed, low torque DC motors. These motors generally require high gearing ratios which have the disadvantage of backlash. In such cases, the harmonic drive is often used.
- Compliance is a measure connected to an angle or distance at what robot axis will move when a force is applied to it. Compliance causes that when a robot goes to a position carrying its maximum payload, its final position will be slightly lower than the position when it is carrying no payload. Compliance can also be responsible for overshoot when carrying high payloads in which case acceleration would need to be reduced.
We hope that this guide helped you understand our mechanical friends better. Well-chosen robotic packaging equipment, purchased from right manufacturers and integrated by the right experts can mean a world in terms of efficiency. If you want to learn more about the way we utilize robots, check out Tishma Technologies Tetristack Robotic Palletizers, available in 6 different variations created to respond to anyone’s particular needs.