6-axis industrial robots are one of the most flexible and powerful tools available today. In their simplest terms they consist of two units; the robotic arm and the control system. The arm has 6 joints or axis which allow it to move to any point within a working envelope. Movement is driven by electric motors. The control system is a computer that allows precise movement of the arm by controlling the power supplied to the motors.
Robotic arms typically have capacities of between 1kg to 300kg and are accurate to 0.1mm or better. Robots are sometimes developed for specific tasks such as painting or welding but the standard industrial robot is easily programmed to fulfill a number of roles, see the applications page for more details.
Used robots is a fast growing sector of the automation market. The technology and build quality of robots made over 10 years ago means that they are a viable alternative to new robots with little or no difference in capability in many applications. Just a big saving in cost. For further information please contact us.
Robot Movement
Industrial robots are classified by the International Standards Organization as:
Automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes.
However 6-axis anthropomorphic robot arms are considerably more flexible, adaptable and standardised than others. As the name suggests anthropomorphic arms mimic the movements of a human arm.
As the diagram below shows, axis 1 and 2 are effectively a shoulder, axis 3 and 4 elbow and forearm and axis 5 and 6 are the wrist of the robot. 
The 6 axis system allows the robot to have an expansive work envelope and allows the tool on the end of the arm to be manipulated in almost anyway within that envelope.
Other terms for these axis include degrees of freedom or DOF, joints or axels. Most manufacturers number them 1 to 6 starting at the bottom of the arm. However some manufacturers use letters for different axis. Motoman term them S, L, U, R, B and T for axis 1 to 6 respectively. Fanuc called them J1 to J6.
Some robots have greater degrees of movement than others, especially on axis 2 and 3 which are often limited by the mechanical structure of the arm. The best arms are developed to be well balanced and require the minimum of force from the motors possible. Various types of robots use counterbalance weights, gas springs and mechanical springs to help the achieve this balance.
All robot axis are braked on almost every robot type. This means that even if the power is switched off the robot will retain its position. Also if an emergency stop is engaged in any part of the system these brakes will immediately engage and prevent movement.
The robot computer knows the position of the arm from feedback from each of the axis in the robot arm. The computer uses this information to control the movement of the robot.
The next diagram describes the different types of robot movements that are commonly available with a 6 axis machine. The teach pendant (see below) will often have a button that selects the movement type.
Axis or joint movement is the simplest to understand and fixed co-ordinate is again quite straightforward, it simply moves the robot in relation to the base of the robot in X, Y and Z co-ordinates.
More complex are the wrist and tool oriented systems, these use the centre of the faceplate at the end of the wrist or a specified point on the tool (the Tool Centre Point or TCP) as their reference. This is typically used repositioning the angle of the tool, for example changing the angle of attack on a grinding wheel.
Overall this gives the programmer excellent and hopefully intuitive control over the robots positioning.
The computer calculates not only which motors to turn on but also calculates the acceleration and deceleration of the arms movement. This means that the robot uses information about the mass of the arm and indeed the mass of its load to move in a smooth and accurate path to the required position. Path or movement accuracy has been increased over the years as computing power has increased. Almost any robot manufactured in the past 15 years will have excellent positional accuracy and most of the main manufacturers robots have excellent path or movement accuracy but this can be at the expense of speed.
How Robots are programmed
The computer control system of a robotic arm will vary from manufacturer to manufacturer and indeed model to model, however a standard set of movement principles has evolved that most of the major manufacturers use. Although offline programming has advanced hugely much of the programming is done using the teach pendant. This is a handheld device that allows the robot to be moved and allow the input of commands and instructions. Movements are sometimes controlled by a joystick (ABB robots) or by buttons. While in "teach" mode the robot is limited to a maximum speed of 250mm/s to ensure safety. The robot can be moved axis at a time or can be moved in relation to itself.
This is part of the reason why offline programming is not as popular as one would imagine; if the only concern was the location of the end of the robot then it would be straight forward. However the positioning of the entire arm has to be considered. Not only can the robot's position be determined but also its movement between these positions. The movement can be specified in the same way as the movement type is changed when teaching the robot. Joint or axis movement is fast but obviously follows an arc or arcs. Co-ordinate movement follows straight lines but requires the movement of many more axis and more computing power. Circular motions are also possible as are changing the types of positions to those that are stopped at or ones that are "flown through". The speed of each movement can also be specified, this can be slow, almost imperceptible movement to over 2.5m per second depending on robot type. However as a robot is almost always accelerating or decelerating a manufacturers quoted top speed is rarely seen in practice.
In addition to the movement of the robot there are many functions that allow the robot to interact with its surroundings. These are normally the inputs and outputs of the robot. These are usually digital 24v signals that the robot can send and receive. A command may be a simple "stop if a signal input is not received" or "turn on an output that illuminates a warning beacon", to a binary signal input that selects a robot program. In addition there are functions that allow the robot to function as a PLC.
Programming robot positions and the movement between them while simple to grasp and achieve can take years to fully master. Experienced programmers will often try to use joint moves as much as possible as they are generally quicker for the robot to execute and will sometimes slow down the robots maximum permissible speed in a small move to actually increase the speed of the arm.
Robot Safety
Industrial robots can be dangerous. They are exceptionally powerful devices, especially models with larger capacity and reach. This means that safety is paramount during the installation and in production. Safety guidelines vary from country to country and it is essential to ensure that any installation complies with local legislation. That said, robot safety systems are highly advanced. Mostly safety is about isolating personnel from the robot's work envelope and ensuring that movement can be easily halted in an emergency. To this end robots have inbuilt dual safety chains or run chains. These are two parallel circuits that when broken will prevent the robot from moving external connections including emergency stops are also catered for. It should also be noted that almost all robots have electrically operated disc brakes on each axis. These are on whenever power is not applied to release them. Therefore in the event of a power faliure or if the emergency stop is applied the robot stops dead, within a split second, in position. It does not collapse and it should retain positional and program data. A great reference to industrial robot safety is available from the UK Heath and Safety Executive - Publication HSG43. This is available at http://www.hsebooks.com/Books/product/product.asp?catalog%5Fname=HSEBooks&category%5Fname=&product%5Fid=2291.
Robot Economics
Industrial Robots are becoming cheaper and cheaper and our used industrial robot prices reflect this. Consider that an ABB IRB 6000 that we retail for £4000.00 cost over £90,000.00 when it was new in 1993! A new equivalent robot will cost between £30,000 to £60,000 depending on manufacturer and specifications. We aim to provide both intergrators and end users a reliable and consistant supply of good quality robots and give guidance on what robot is suitable for what process, because we stock a range of types of robot by different manufacturers we are not limited to the latest thing or just a few models. If you don't need the latest, most hi-tec robot in the world why pay for it?
As this extract from Forbes Website (Jan 2006) describes, even a new robot is cheaper than the cheapest labourer, the savings with a used robot are even greater!
http://www.forbes.com/logistics/2005/12/30/robots-logistics-warehouse-china-cx_rm_0103robots.html
For further information please contact us.
Click here to return to the stock page.
