Industrial robots

This section discusses industrial robots in more depth, looking at the worldwide market drivers, different robot types, typical applications, basics of programming and safety considerations.

MARKET TRENDS

Industrial robots are driving competitive advantage across manufacturing, enabling companies to respond to the challenges of faster business cycles, greater variety in customer demand, competitive pressure and the challenge of reducing emissions. The falling costs of industrial robots has lowered capital investment requirements, increasing adoption across all industrial sectors.

Estimates predict that the number of industrial robots operating worldwide will rise from 1,828,000 units at the end of 2016 to 3,053,000 by 2020, with the bulk of this growth in Asian factories, particularly in China, where it is forecast that over 950,000 units will be deployed. China, the Republic of Korea, Japan, the United States and Germany accounted for 74% of industrial robot production in 2016, and Figure 1 shows the growth in the supply of industrial robots by industry over the period 2014 to 2016.

Collaboration – inter-robot, and between robots and humans – is increasing in importance, driven by a trend towards low-volume production runs with a high component mix, requiring more variability and more human intervention.

Collaborative automation allows people and robots to each contribute their unique strengths – with people providing insight and improvisation, and robots offering speed and repetition. ABB’s IRB 14000 series is a prime example of a new generation of collaborative robots, developed initially in 2015 for small parts assembly applications. The importance of collaboration is demonstrated by the recent venture between ABB and Kawasaki to develop the next generation of ‘cobots’.

Figure 1: Overall, usage of industrial robots has increased year-on-year (Source: IRF 2017 Report)

Types of industrial robots

Most industrial robots are classified as stationary robots, with a robotic arm moving above a stationary base. Stationary robots break down into six main types, shown in Table 1, with the most commonly used being articulated, SCARA, delta and cartesian coordinate robots.

Type	Description	Typical application
Cartesian/gantry	Operate within x-, y- and z-axes using linear guide rails	Pick-and-place work, sealant application, arc welding
Cylindrical	Rotary joint combined with prismatic joint. Movements occur within a cylindrical work envelope	Assembly operations, spot welding, machine tool handling
Spherical	Combined rotational joint, two rotary joints and a linear joint to achieve a spherical work envelope	Spot welding, die casting, gas and arc welding
SCARA	Compliant arm is cylindrical in design and comprised of two parallel joints providing compliance in one plane	Pick-and-place work, sealant application, assembly operations, machine tool handling
Articulated	Rotary joints connect the links in each arm; each joint is a different axis, providing an additional degree of freedom. Articulated robots have four or six axes	Assembly operations, die casting, gas and arc welding, paint application
Parallel or delta	Built from jointed parallelograms connected to a common base. Parallelograms move a single endof-arm tooling in a dome-shaped envelope	Pick-and-place operations requiring precision

Table 1: Main types of industrial robot
Robots are also specified in terms of various operating parameters, as summarised in Table 2.

The working envelope needs to be considered for each robotic application

Parameter	Description
Number of axes/degrees of freedom	Two axes are required to reach any point in a plane, three to reach any point in space. Three more axes (yaw, pitch and roll) are required to fully control the orientation of the end of the arm (the wrist)
Working envelope	The region of space a robot can reach
Kinematics	The arrangement of rigid members and joints in the robot, determining the robot's possible motions. Classes include articulated, cartesian, parallel and SCARA
Carrying capacity/ payload	How much weight a robot can lift
Speed	The speed at which the robot can position the end of its arm, defined in terms of the angular or linear speed of each axis or as a compound speed
Acceleration	How quickly an axis can accelerate. Since this is a limiting factor a robot may not be able to reach its specified maximum speed for movements over a short distance or a complex path requiring frequent changes of direction
Accuracy	The absolute position of the robot compared to the commanded position is a measure of accuracy. Accuracy can be improved with external sensing, e.g. vision systems or infrared. Accuracy can vary with speed and position within the working envelope and with payload (compliance)
Repeatability	If a position is taught into controller memory and each time the robot is sent there it returns to within 0.1 mm of that taught position, then the repeatability will be within 0.1 mm
Motion control	For applications such as simple pick-and-place assembly, the robot only needs to return repeatedly to a number of pre-taught positions. For applications such as welding and finishing, motion must be continuously controlled to follow a path in space, with controlled orientation and velocity
Power source	Examples include electric motors and hydraulic actuators
Drive	Some robots connect electric motors to the joints via gears, others connect the motor to the joint directly (direct drive). Smaller robot arms often use high-speed, low-torque DC motors, requiring high gearing ratios with the disadvantage of backlash
Compliance	The amount of angle or distance that a robot axis will move when a force is applied to it. When a robot goes to a position carrying its maximum payload it will be at a position slightly lower than when it is carrying no payload

Table 2: Operating parameters of industrial robots

Example applications

Here we look at two examples of industrial robots which have been developed by their manufacturers to meet the specific needs of their intended application.

The IRB 5500 series by ABB

The IRB 5500 series by ABB is an articulated robot with six axes of movement, developed for spray painting on car assembly lines.

The IRB 5500 has three characteristics which make it suited to its chosen application:

Large work envelope, removing the requirement to have two robots for paint application across a horizontal surface such as a car bonnet, which creates a ‘stitching’ effect in the centre. A single robot removes this quality control issue entirely.
High acceleration: for sophisticated applications, such as welding and spray painting, motion must follow a path in space, with controlled orientation and velocity. If a robot slows down too much when reversing, excess paint will accumulate in those regions of the vehicle where the slow movement takes place.
High payload, enabling closer integration of the processing equipment with the work surface, reducing waste.

The Quattro 800 series by Omron:

This is a parallel robot designed for high-speed manufacturing, packaging, material handling and assembly. With the actuators all located in the base, the arms can be made of a light composite material, resulting in moving parts with low inertia, allowing for very high-speed acceleration. Having all the arms connected to the end effector increases the robot’s stiffness, but reduces the size of its working envelope.

The following characteristics of the Quattro lend themselves to the targeted applications:

Speed – 10 m/s (vs 1m/s for IRB 5500)
Repeatability – 0.1 mm (vs 0.15 for IRB 5500)
Working envelope – operates within a 1300 mm cylindrical area suited to the size of food production lines

Programming industrial robots

Programming a robot involves the establishment of a physical or geometrical relationship between the robot and the equipment or task to be serviced by the robot. In doing so it is necessary to control the robot manually and physically teach it the coordinate points within its working envelope.

There are three commonly used programming or teaching methods:

Lead-through programming or teaching

A handheld control and programming unit, or teach pendant, is used to manually send the robot to a desired position. It can also change to a low speed to enable careful positioning, or while testing through a new or modified routine. A large emergency stop button is usually included as a safety measure.

Walk-through programming or teaching

With the robot in ‘safe mode’ the user moves the robot by hand to the required positions and/or along a required path while the controlling software logs these positions in the controller memory. The program can later run the robot to these positions or along the taught path. This technique is popular for tasks such as paint spraying.

Offline programming or teaching

The robot and other machines or instruments in the workspace are mapped graphically, allowing the robot to be moved on screen and the process simulated. Simulators can thus create programs for a robot without depending on the physical operation of the robot arm, saving time during application design. Additionally, various ‘what if’ scenarios can be tried and tested before the system is activated, increasing operational safety levels.

A combination of methods will often be used. Programs created using lead-through or walk-through methods can be reviewed and refined using offline simulators. Operator control panels can also be used to switch programs, make adjustments within a program and also operate a host of peripheral devices that may be integrated within the same robotic system. A computer is often used to ‘supervise’ the robot and any peripherals, or to provide additional storage for access to numerous complex paths and routines.

ABB’s SRP programming toolset for the IRB5500 enables lead-through programming using a simulated paint spray gun with the simulation and offline programming software, RobotStudio, being available for offline review and further development

Safety considerations and systems

Various safety concerns must be considered when implementing robotic production systems. Care must be taken to ensure separation of humans from the operating envelope of the robot, as the end effector is capable of rapid acceleration resulting in high forces. Physical barriers and locking systems should be deployed to prevent the operator or other personnel from entering the work zone while the robot is operational.

operator must be in physical contact with the robot, so safety devices should be in operation, such as a teach mode where the speed of the robot is limited, along with emergency stop buttons. Sensors can also be integrated into the design of the robot to prevent excessive force or limit proximity to unexpected objects.

Additionally, the type of robot should be chosen to match the characteristics of the environment in which it will operate – e.g. care should be taken when deploying electric motors in environments where combustible materials or gases may be ignited by static or sparks.

In addition to emergency stop buttons, light curtains protect human operators from accidental entry into a robot’s working envelope

Conclusion

This section has provided an overview of the different types of industrial robot that may be deployed, along with a description of how these robots and their characteristics are adapted for specific applications. Consideration has also been given to programming of robots as well as safety aspects, and two robots developed for two very different applications have been discussed.

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