MARKET PLACE
Extending the possibilities
The features of a recently developed pick and place machine bring new precision capabilities to the medical device manufacturing industry. The machine is able to pick small discs randomly scattered on a glass plate and place them precisely into predefined locations. Initially, it was believed that a system of this type could be sourced from the microelectronics industry. For example, an off-the-shelf solution such as a miniature bowl feeder supplying the discs to a pick and place system designed for surface-mount components. However, neither a bowl feeder nor a pick and place system were found to be suitable for the requirement.
The brief
The system described here was designed for a large volume producer of diagnostic medical devices. The system was required to pick up small metal discs of 0.6-mm diameter and 0.075-mm thickness and place them into an array of 72 positions on a rectangular tray. The tray had to be populated in no more than 3.5 min (approximately 1 disc every 3 s). The discs were to be presented randomly to the machine, loose in a container. Because it was not possible for these parts to be supplied in a controlled form, it was decided that scattering them on a backlit glass plate would offer an efficient way of handling them as well as suitable way of presenting the disc for camera recognition.
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Figure 1: The low mass vacuum tip.
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Image analysis
One important feature of the machine is the ability to precisely locate isolated discs, determine their position and check their shape and diameter to ±10 µm. This is achieved using high quality vision hardware combined with image analysis tools that are integrated into the control software. Using precision linear motor actuators, the pick tray moves in the y direction so that a selected disc is brought into line with the y coordinate of a small vacuum head. The vacuum head (Figure 1) is then moved in the x direction until it is directly over the disc, as reported by the imaging software. The vacuum head then lowers to pick up the disc.
A problem overcome by the system is the possibility that, when the discs are spread on the pick tray, two discs could sit directly on top of each other and that this “double disc” could not be detected as being double by the imaging system. The solution chosen was to lower the vacuum tip (Figure 1) using a stepper motor driving a precision ground linear “snail” cam. This has the effect of converting the rotational input into a proportional vertical movement. The low-mass vacuum tip rests under gravity on the cam and is, therefore, free to move upward if obstructed. The height of the tip is then measured accurately using a linear optical encoder, which has the same resolution as the motor. Each time the tip lowers to a point nominally 25 µm above a disc to pick it, the height is measured by the encoder. If it is greater than 25 µm (plus the disc thickness), it means the tip has met an obstruction. In all likelihood this will be a double disc; the pick is then aborted and the system moves on to the next free disc.
Surface profiling
Because of the difficulty and cost of sourcing engineering materials and components with an overall accuracy better than ±10 µm, the use of the linear encoder to measure tip height has a secondary advantage. The tip can be used to “profile” the surface of the glass pick tray before use, thus cancelling out any small level errors of the glass or other components.
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Figure 2: Second camera mounted on gantry above bed plate.
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Between 6 and 12 mapping points are chosen over the area of glass (which in this case is approximately 50 mm x 230 mm) and these heights are stored in a software array. When the machine is run and a disc is selected for picking, the software knows its position and can, by interpolation between the closest mapping points, calculate the height of the glass surface at that point to a precision of ±2 µm.
Design Engineer Jim Crawshaw said, “Part of the philosophy behind the design of this system was rigidity to maintain accuracy.” This drove the decision to mount all the major mechanical devices on a thick aluminium tool plate base and use single axis linear actuators to drive the pick tray, the tip assembly and the placement tray (on which the discs are subsequently located) in a matrix formation. Another feature of the system is a second camera, also driven by a linear motor actuator, which checks the satisfactory positioning of the discs on the placement tray. This camera and its actuator are mounted together with the pick analysis camera on a substantial gantry above the bed plate (Figure 2).
Nonstandard shape handling
The pick and place system was designed to handle circular discs where there is no need for rotational orientation. A similar machine that would be able to handle metal rods of 300-µm diameter by approximately 3-mm long lying horizontally (also arriving in a random state) is being developed. This will require the vacuum tip to contact a rod on a curved surface, and then rotate it to a defined orientation prior to placing. Tests have already confirmed that this is achievable.
The system could be adapted for use in a range of applications where components cannot be presented to a picking head in a controlled or organised way. The system’s flexibility means that further applications beyond the original one are being identified. Another example of the system’s potential is that a colour camera is used to image the parts. If it is important which way up the parts are placed and they have a different colour on each side, it is possible to select only those sitting the correct way up on the pick tray.
For more information, contact Andrew Solly, Senior Projects Engineer, Horizon Instruments Ltd, Heathfield UK, tel. +44 1435 864 239, e-mail: sales@horizoninstruments.co.uk, www.horizoninstruments.co.uk
