Industrial robotics
Level: Advanced
7 min read
33 questions
An industrial robot is a programmable arm that repeats movements with great precision. Working with one takes more than knowing how to jog it with the teach pendant: you need to understand its axes, its coordinate systems, the types of motion, the safety of the cell and how it communicates with the PLC that runs the machine. This topic covers the essentials, valid for any brand.
Types of robot
| Type | Axes | Typical use |
|---|---|---|
| Articulated (anthropomorphic) | Usually 6 | Welding, handling, painting, machine tending, palletising. |
| SCARA | 4 | Fast assembly and handling in a horizontal plane. |
| Cartesian or gantry | 3 linear, sometimes with a rotation | Long travels, machine tending and palletising. |
| Delta | 3 or 4 | Picking and placing light parts at high speed, for example in food. |
| Collaborative (cobot) | Usually 6 or 7 | Tasks close to people, when the risk assessment of the application allows it. |
Axes and TCP
- Axes 1, 2 and 3: the base rotates, the arm goes up and down, and the forearm reaches out. Together they position the wrist.
- Axes 4, 5 and 6: the wrist orients the tool.
- TCP (tool centre point): the tip of the gripper, the welding torch or the suction cup. When a point is programmed, the TCP is what reaches that point.
Typical mistakeChanging the gripper or knocking the tool and not re-measuring the TCP. The
robot keeps taking its old TCP to the points, and every position ends up offset.
Coordinate systems
When jogging the robot with the teach pendant or programming points, you choose which coordinate system you work in. The names change from brand to brand, but the concepts are the same.
| System | What it is | What it is for |
|---|---|---|
| Joint (axis) | Each axis moves on its own. | Getting out of awkward positions and taking the robot home. |
| Base or world | Fixed X, Y, Z axes, usually at the robot base or in the cell. | Moving in a straight line relative to the floor. |
| Tool | Axes attached to the TCP, which turn with the tool. | Approaching a part in the direction of the tool. |
| User frame or work object | Axes placed on a table, a fixture or a pallet. | Programming the points relative to the part. |
On siteIf the points are programmed in a user frame and the table moves a few millimetres, you
only need to re-measure that frame: all the points move with it. If they were programmed in the base frame, they have to
be taught again one by one.
Types of motion
| Motion | What it is like | Usual names |
|---|---|---|
| Joint (point to point) | Each axis goes to its target; the TCP path is not a straight line. It is the fastest. | PTP, MoveJ, J |
| Linear | The TCP moves in a straight line. | LIN, MoveL, L |
| Circular | The TCP follows an arc defined by an intermediate point and the end point. | CIRC, MoveC, C |
- Joint motions are used for moving through clear areas. Close to the part or to obstacles, linear motions are used, because their path is predictable.
- For each point you choose whether the robot must stop exactly on it (fine point) or may blend the path so as not to slow down. Blending shortens the cycle, but the TCP does not pass exactly through the point.
Singularities
A singularity is a position in which two axes line up and the robot loses a degree of freedom. A typical example: with the wrist straight, axes 4 and 6 line up. In a linear motion passing close by, some axes would have to turn very fast, and the robot stops with a warning. It is avoided by changing the tool orientation or using a joint motion for that section.
Data that must be declared correctly
- Payload: the maximum weight the robot can carry, including the tool. The gripper counts too.
- Load data: mass and centre of gravity of the tool and the part. If they are wrong, the robot follows the path less accurately, wears faster, and collision detection gives false alarms or reacts late.
- Reach: the maximum distance the wrist can get to.
- Repeatability: how far the robot deviates when returning many times to the same taught point; it is usually hundredths of a millimetre. Absolute accuracy, going to a calculated coordinate, is considerably worse if the robot is not calibrated.
Programming
- With the teach pendant (teach-in programming): the robot is taken to each point and the point is saved. It is the most common method on the plant floor.
- Offline: the program is written and simulated on the computer with a model of the cell, and the points are then adjusted on the machine. It reduces production downtime.
- Each manufacturer has its own language: RAPID for ABB, KRL for KUKA, TP and KAREL for FANUC, for example. The ideas (points, motions, inputs, outputs, loops) are the same.
Safety
An industrial robot moves with great force and speed, and it can start without warning within its cycle. The reference standards are ISO 10218-1 (the robot) and ISO 10218-2 (the cell and its integration). For collaboration with people, the technical specification ISO/TS 15066 sets out force and pressure limits for each body region.
- Cell protection: fencing, interlocked doors, light curtains or laser scanners that stop the robot if someone enters.
- Operating modes: manual reduced speed (T1, with the TCP limited to 250 mm/s), manual high speed (T2, on robots that have it and with restrictions) and automatic.
- Three-position enabling device on the teach pendant: the robot only moves in manual with the switch half pressed. If it is released or squeezed fully, as happens in a fright, the robot stops.
- Emergency stop on the teach pendant and in the cell.
A cobot is not safe by itselfWhat is assessed is the complete application: the robot, the tool,
the part and the task. A cobot carrying a knife or a part with sharp edges may need the same protection as an
industrial robot. The recognised forms of collaboration are safety-rated monitored stop, hand guiding, speed and
separation monitoring, and power and force limiting.
Robot and PLC
On a machine, the PLC normally commands and the robot executes. They communicate over a fieldbus (PROFINET, EtherNet/IP) or through hardwired signals. A typical exchange:
| From the PLC to the robot | From the robot to the PLC |
|---|---|
| Start, stop and reset | Robot ready, in automatic, running |
| Program or task number | Program running and program finished |
| Part present, zone clear | Robot at home, robot out of zone |
| Alarm acknowledge | Alarm active and fault code |
- The exchange is done with a handshake: the PLC requests a task, the robot confirms it has started and reports when it finishes. The PLC does not give the next command until it gets that report. It is easy to model with a GRAFCET.
- When the robot shares space with another machine or another robot, zone signals are used: each one only enters if the other has confirmed it is out.
- Safety functions (emergency stops, doors) go through hardwired safety circuits or a safety protocol such as PROFIsafe, never through the normal signals.
Typical mistakes
- Not re-measuring the TCP after changing or knocking the tool.
- Programming every point in the base frame when the part sits on a fixture that can move.
- Declaring the wrong load: the robot follows paths badly and collision detection fails.
- Testing a new program straight in automatic and at high speed.
- Bypassing a cell door or light curtain “just to test”.
- Assuming a cobot is safe in any application.
- Handling safety functions with the normal bus signals.
Summary
- Articulated, SCARA, Cartesian, delta and collaborative: each type has its application.
- Axes 1 to 3 position the wrist and axes 4 to 6 orient the tool. The TCP is the point that is programmed.
- Coordinate systems: joint, base, tool and user. Programming on a user frame makes adjustments easy.
- Joint, linear and circular motions, with fine point or blending. Watch out for singularities.
- Safety according to ISO 10218: protected cell, T1 at 250 mm/s maximum and a three-position enabling device.
- The PLC commands with a handshake; safety goes through safety circuits.
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