Following a lull three or four years ago, sales of robots and robotic systems are again on the rise on both sides of the
North American manufacturing companies ordered 5,316 robots worth over $300m (£168m) in the January–March period. An additional 272 robots worth $18m were exported to manufacturing companies outside
‘The strong first quarter results, continuing the double-figure gains of the past two years, show that north American firms increasingly recognise the benefits robots provide in terms of quality improvement, productivity, and cost savings. If economic conditions remain healthy, we expect 2005 to be a very good year.’
According to the company, a buoyant market is being created by the increasing willingness of firms to invest and the growing trend to robot-based automation.
‘We recognised this shift very early on and can now reap the rewards of the drive that we initiated two years ago towards presenting robots at more and more trade fairs and customer events,’ said Reiner Hänel, Mitsubishi’s robotic systems product manager.
With several types of robotic arm on the market, it is increasingly important to understand the differences between them to make the correct selection. Of the many versions available, Scara (Selective Compliant Assembly Robot Arm) and Cartesian robots have proved to be the most popular for straightforward manufacturing applications.
In general, traditional Scara units — sometimes referred to as Selective Compliant Articulated Robot Arm — are four-axis robot arms which can move to any X-Y-Z co-ordinate within their work envelope (the geometric area in which the arm can operate).
There is a fourth axis of motion which is the wrist rotate (Theta-Z). The X, Y and the Theta-Z movements are obtained with three parallel-axis rotary joints. The vertical motion is usually an independent linear axis at the wrist or in the base.
By virtue of the Scara parallel-axis joint layout, the arm is slightly compliant in the X-Y direction but rigid in the Z direction, hence the term ‘selective compliant’. This is advantageous for many types of assembly operations, such as inserting a round pin in a round hole without binding. Scara’s second attribute is the jointed two-link arm layout which is similar to our arms, hence the use of the word articulated. This allows the arm to extend into confined areas and then retract or ‘fold up’ out of the way, providing benefits for transferring parts from one cell to another, or for loading/unloading process stations that are enclosed.
They are generally faster and less intrusive than comparable Cartesian systems. Their single pedestal mount requires a small footprint and provides an easy, unhindered form of mounting.
On the other hand, Scara can be more expensive than Cartesian, and the controlling software requires inverse kinematics for linear interpolated moves (although this software typically comes with the Scara).
One disadvantage is that their work envelope tends to be difficult to control. Cartesian robots have three specified directions X, Y and Z, with co-ordinate directions at right angles to each other.
The main advantage of Cartesians is that they are capable of moving in multiple linear directions. They are also able to carry out straight-line insertions into furnaces and are easy to program. They also have the most rigid robotic structure for a given length, since the axes are supported at both ends.
Due to their rigid structure they can manipulate relatively high loads, so are often used for pick-and-place applications, machine tool loading, and stacking parts into bins. Cartesians can also be used for measuring, and assembly, where electronics parts insertion is a major growth area.
The major drawback is that they require a large volume of space in which to operate, even though it is not all used.
But whatever choice is made, robotics is an increasingly popular choice for companies seeking to reduce operating costs.
One example is cable management specialist C&C Marshall, which has installed two Toshiba Scaras at one of its
The installation was prompted by the need to reduce the man-hours spent on the shop floor and increase production.
The now automated functions used to be performed manually, but this was inefficient and caused problems, such as repetitive strain injury (RSI), among workers.
‘Often, manufacturers who have never worked with robotics discover unexpected benefits when they do, such as avoiding costly compensation claims for RSI,’ said Nigel Smith, managing director of TM Robotics. ‘More tangible benefits are production increases and the ability to function on a genuine 24/7 basis.’
In one application the robots work in conjunction with a plastics moulding machine and a Cartesian robot. The Scara positions swivel clamps in dry lining boxes, for use in electrical installations. When installed, the box sits between the socket plate and the wall.
It contains the switchgear needed to operate the socket, keeping the system insulated.
The process uses a Cartesian robot to move four dry lining boxes from the moulding machine into inserting nests, where the swivel clamp is inserted. The Toshiba machine robot is bowl-fed with the clamps, and places each one in the correct position in these mouldings or boxes. The Cartesian then pick ups the boxes and drops them on to a carrier before they are placed on to a packing carousel. The gap, into which the clamps are inserted, is less than 5mm wide, so the repeatability of Toshiba’s Scara is therefore vital.
When choosing such a robot the distinction between accuracy and repeatability is of major importance.
Accuracy applies to just one movement, while repeatability ensures that accurate movement is repeated on every occasion.
Operating on its most common programme, the system shifts 10,000 dry lining boxes a day. Before automation, the clamps were manually inserted into the boxes. Just three people now operate the entire Scara system, whereas previously eight were required.
Given that five to six tonnes of boxes are produced per week, this represents a significant improvement on the factory’s bottom line.
‘The typical payback on a Scara is 18 months, but this system paid for itself in six,’ said Smith. ‘Obviously, the smaller the manufacturing runs, the longer the payback time. The reluctance to employ automation lies not with the plant engineers, but in the boardroom with financial directors who often know little of engineering.
‘Thus, it is understandable that these people might believe that an extra injection-moulding machine could be the best way to improve productivity. However, as we have demonstrated, this isn’t always true. An improved process can be even more valuable.’