Industrial robots could be used for more precise machine milling for things such as aerospace components, after recent trials of new technology.
One of the key goals of the near-complete European COMET project is to develop robots that can compete against five-axis machine tools.
‘Up to now the problem with robots has been that they’re not quite accurate enough; they’re somewhere in the 2–5mm range,’ said Roland Krain of project partner TEKS. ‘If you calibrate it you can probably get down to a millimetre but it’s still not quite good enough for machining.’
The major current obstacles for fully automated machining are play, mechanical flexibility, thermal effects and particularly backlash.
‘A lot of companies have got robots that handle parts that need to be milled, but if they want to do milling they have to spend between £90,000 and £150,000 on a five-axis machine tool,’ Krain said. ‘If we can add a high-quality spindle into the mix, the robot is already there and so suddenly you’ve got a milling solution for a fraction of the cost — if we can get the accuracy of course.’
The cornerstone of this accuracy will be adaptive tracking, which is being developed in collaboration with COMET partner Nikon Metrology of Tamworth. It has devised a stepwise solution for increasing accuracy.
The first offline solution involves measuring the robot in more than 300 poses to determine where it deviates from where it should have been. That information is then fed back into the CAM software. When running, the software then compensates for backlash by driving the robot to what it thinks is the ‘wrong position’.
This can be complemented by an online, real-time system comprising three linear built-in charge coupled device (CCD) cameras with cylindrical lenses that measure the location of infrared LEDs mounted on the robot head. Essentially, it measures where the robot is, then checks where it should be and sends a compensation down to the robot controller in milliseconds.
Preliminary tests of these systems were performed last week in the UK demonstrating sub-1mm accuracy. The final phase of testing will be delivering case studies later this year such as machining the final leading edge of an aircraft wing and deburring parts on the turbine disk.
Meanwhile, TEKS is now exploring applications of COMET technology outside of industrial machining. One comes from the civil engineering sector for surveying quarries and mines using long-range laser scanning.
The new technology has proved difficult in that large amounts of rather unmanageable date are produced, which is difficult to visualise with existing computer hardware and software.
Using COMET technology, TEKS bypassed the computer to machine lightweight models of sites in polystyrene.
The main partners of the COMET project are BTU, Delcam, Nikon Metrology, Fraunhofer IPA, TEKS, SIR and AMRC Manufacturing.
What a clever sytem! This could be an extremel useful system when it is fully resolved. It does remind me though of the legendary discription of the workings of the Bloodhond AA missile, which went something like this:-
An excerpt from a report explaining the operation of the Ferranti
Bloodhound Inertial Guidance System.
The missile knows where it is at all times. It knows this
because it knows where it isn’t. By subtracting where it is from
where it isn’t (Or where it isn’t from where it is, depending on
which is the greater) it obtains a difference or deviation. The
inertial guidance system use deviations to generate corrective
commands to drive the missile from the position where it is to the
position where it isn’t. The missile arrives at the position
where it wasn’t, consequently the position where it was is now the
position where it isn’t. In the event that the position where it
is now is not the same as the position where it originally wasn’t
the system has acquired a variation, (variations are caused by
external factors and the discussion of these factors is not
considered to be within the scope of this report) the variation
being the difference between where the missile is and where the
missile wasn’t. If the variation is considered to be a
significant factor it too may be corrected by the inertial
guidance system. Moreover, the missile must now know where it
was also. The “Thought Process” of the missile is as follows:
Because a variation has modified some of the information which the
missile had obtained, it is not sure where it is. However, it is
sure where it isn’t and it knows where it was. It now subtracts
where it should be from where it wasn’t (or vice versa) and by
differentiating this from the algebraic difference between where
it shouldn’t be and where it was, it is able to obtain the
difference between it’s deviation and it’s variation, this
difference being called the Error.
I am sorry to say, this article is not entirely accurate. Industrial robots nowadays are extremely accurate. Take example of ABB robots. They have a position deviation of 0.1mm. Even when moving at 1,000 mm/sec, they are accurate within 1 mm.
Mr Chang,
Certainly some of the latest robots will reach the accuracy you stated. However we found that this is not necessarily true for all poses. Last week we used a Nikon K600 series 3D measurement system to check the accuracy of 90 poses. This was done on two second hand robots, a IRB 6640 and an IRB 2400l, we saw a spread of between 0.4mm and 2.2mm deviation between intended and actual position.
Best Regards
Roland