Schunk robot arm with hand copyThe first question here is what constitutes an industrial robot?

Industrial robot as defined by ISO 8373: An automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes, which may be either fixed in place or mobile for use in industrial automation applications.

So a robot must be automatic, reprogrammable, multipurpose and multi-axis. This makes robots separate from single purpose machines such as CNC mills although in terms of components and controls they may be very similar. There are Beam robots which are multi axis linear actuators, but here we are looking at robot arms.

Controllers

Universal Robots Controller with Teach pendant. photo: Universal Robots
Universal Robots Controller with Teach pendant. photo: Universal Robots

The controller is the brain of the robot. Often in a separate cabinet and connected by umbilical cables to the arm the controller has a Central Processing Unit, memory, Power supplies and servo controllers. The controller usually has various buttons and displays to allow control of the robot. Most robot controllers have a PLC (Programmable Logic Controller) built in that allows the robot to receive inputs and process information from sensors and also set outputs and relay other information. Robots are designed with a safety circuit that stops the robot in an emergency. This is often linked to guarding and entry doors to prevent the robot from moving when a person is in the cell with the robot (see collaborative robots that don’t need this). These safety circuits are often dual, that is they have a 24v and 0v chain so if either is cut or shorted out the robot will fail safe and stop.

Teach Pendant Unit

A current Mitsubishi Teach Pendant Unit
A current Mitsubishi Teach Pendant Unit

Although not every robot needs a TPU to run in automatic almost all use them to program and teach the robot. Typically a TPU will have a large display and an Emergency Stop and motors on switches. Most are now touch screen and full colour. Some have joysticks or dedicated axis movement buttons. TPU’s usually have a mechanical dead mans handle, or enable switch. This is pressed in by the programmer when teaching the robot, so that the robot can only move deliberately. The dead mans handle version has three positions and must be held at a midway point. The idea is that if an operator was to get a shock, and they clench onto the handle, the robot stops.

The robot arm 

Fanuc hollow arm robot copy

Often constructed from aluminium for lightness and strength other materials used include carbon fibre and cast or fabricated steel. The arm must be very stiff, especially for larger robots to maintain high accuracy. The arm must also be as light as possible. Some are hollow to allow cables and hoses to be run through to the tooling on the end of the arm.

Motors  

Almost exclusively AC servomotors. Small robots may use stepper motors, but steppers are not powerful enough for most robotic applications and are generally designed to be open loop, that is the stepper already knows where it is by counting steps and does not have any feedback. Servo motors have some form of feedback, usually a resolver or encoder mounted on the end of the motor, that allows the CPU to know the position of the arm. This completes the feedback loop of the servo system.

Encoders or Resolvers

These are usually mounted on the end of the robot and give feedback on motor position. Encoders are usually optical, with a laser or light source shining through a disc with clear and opaque sections. These are registered and counted by a processor to give the position and number of revolutions of the motor and hence the axis position. Resolvers work electromagnetically with three coils, one moving with the motor armature and fed with a known signal, and the other two fixed and mounted at angles to the first. By monitoring the resultant signals the angle of the first coil can be measured. Again a processor counts and calculates the number of rotations.

Encoders are used by many robot manufacturers and they are very effective and not too expensive. Optical encoders are quite fragile and must be treated with care. Resolvers are tougher and give a true analogue output and are favoured by ABB.

Brakes 

With few exceptions Industrial robot motors have a brake attached to them. This is used for holding the robot arm still when it is not powered and also in emergency stops. The brakes are disc brakes that are electromagnetic; with a coil to pull them open and a mechanical spring to shut them on. As they are designed to hold a still robot their use as an e-stop causes a great deal of wear and should be avoided – except in emergency! Older robots will sometimes lose the power of their brakes which can be dangerous. It is also possible for brakes to bind in wet and corrosive atmospheres if they are not protected.

Gearboxes

Industrial robots use many gearbox types depending on what is needed but most industrial robots use one of two types of gearbox to achieve the speed and accuracy needed. This is one of the reason most robots are similar prices regardless of manufacturer, they often have the same gearboxes inside. Smaller robots use Harmonic drive gearboxes. As used on the wheels of the lunar rover these gearboxes are very compact and have excellent characteristics for robotics with very low backlash. Despite their apparent simplicity Harmonic drives require a lot of precision and materials technology to manufacture and their patents are well guarded as such they are quite expensive.

Larger robots use Cycloidal gearboxes. Again precision made with low backlash these are quite complicated mechanisms that give a large ratio in a compact space. They are also very reliable as long as correct lubrication is used – the loadings and forces within a cycloidal box can destroy the wrong lubricant very quickly causing premature failure.