Large multi-axis robots are fairly familiar - we see them on television welding or spraying cars. What may not be so familiar is the success of smaller benchtop robots, which since the mid-1990s have taken on assembly tasks on more modestly sized products, and in the SMEs now a significant factor in regeneration of the UK’s manufacturing industry
Small robots with footprints approximately A4 in size and working areas of 200 x 200mm have all the functionality of large machines. Bigger versions cover 800 x 600mm with a z-axis of 200mm. They are robust, fit for shop floor use and maintenance is relatively simple - often with internal computers and self-diagnostic protocols.
Peter Swanson, MD of Intertronics explains: “Typically, programming is via a simple teach pendant with prompted inputs, and programs are stored internally or kept on a computer. Software may be generic, allowing programmer control of functionality, or function specific (e.g. dispensing). Customised software is even more user-friendly, with selectable point-to-point or continuous path motion. Whilst two-axis machines are available, three-axis are the most popular, with full interpolation of lines, arcs and circles through all three axes. A fourth, rotational axis is used for more complex, non-planar jobs. Positional repeatability is typically 0.01mm and movement speeds can reach one m/sec. Inputs and outputs allow interfacing with external devices (e.g. pick and place pneumatics). Sophisticated camera based position correction systems are now available.”
Types of benchtop robot
There are three major types of robot recognised in the benchtop format:
Semi-automatic, cartesian robot - the work is placed on a moving plate on the bed of the machine, which provides x-axis movement. Overhead, a beam mounted traveller gives the y- and z-axis movement.
SCARA robot - Selectively Compliant Articulated Robot Arm, or sometimes Selective Compliant Assembly Robot Arm. With joints much like a human arm (shoulder, elbow and wrist axes), it performs pick/place, palletising or dispensing functions.
Gantry robot - an overhead beam moves back and forth over the work on a fixed base. A z-axis component slides on the gantry.
All types are suitable for semi-automatic batch operation, where the work is manually loaded/unloaded. The SCARA and Gantry types will also work with automatic feed by, for example, being placed next to/over a conveyor. They can find application in dispensing; soldering, brazing, welding; pick and place; screwdriving; engraving, cutting and testing/calibration.
Adhesives, sealants, encapsulants, potting compounds, coatings, inks, paints, greases, oils, solder pastes etc may be accurately metered onto workpieces - dispensed from barrels or cartridges mounted on the machine z-axis, or remotely located and fed through a robot mounted dispensing valve. Beads, coatings or micro-dots are common. Metering and mixing equipment can be interfaced to dispense two part systems.
One popular application is the dispensing of form-in-place (FIP) gaskets onto a component, which then cures into a soft, sealing gasket. The bead must be the same diameter over its entire length, and so the dispense needle travel-speed must match the FIP gasket flow rate, even around corners in three dimensions. A robot can be programmed to do this, whereas a human would find it impossible.
Test and calibration
Typically a camera is sequentially positioned over a number of small parts. The image is then examined by computer, to detect a pass or fail. Reject parts may be identified, marked or removed whilst on the robot. Probes can be accurately positioned to measure electrical or thermal characteristics: e.g. thermostat calibration - using a robot, interfaced to the testing equipment, which adjusts the calibration screw until the appropriate electrical output is achieved.
The engineers at Intertronics have noted the general qualities of benchtop robots falling into some very advantageous categories:
Quality - robots are more consistent and accurate, do not get distracted nor interrupted. Therefore product integrity is enhanced.
Production yield - generally, reject rates go down.
Throughput - robots can work continuously and can be much faster than humans.
Necessity - sometimes the job just cannot be accomplished by a human.
Material savings - there can be significant savings in material usage. Expensive materials are dispensed accurately and multi-part material mixing may require purging or nozzle changes unless dispensed continuously.
Consequently with technology costs now lower than ever in real terms it is clear that benchtop robots can achieve a high level of functionality, are readily available, simple to implement (no need to understand robot/machine language/PLC programming, no previous experience required) and surprisingly inexpensive with prices starting at about £4,000. At this level, cost justification by higher throughput and yields, fewer rejects, better quality or reduced labour content is relatively straightforward. Payback times can be as little as a few weeks - vital to companies which would like to see domestic manufacturing both stay in the UK, and lead the economic recovery.
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