Precision pick-and-place system handles 0.6mm discs

The development of a precision pick-and-place machine, for picking small discs randomly scattered on a glass plate and then placing them precisely into predefined locations, is one of the latest projects to be completed by UK based Horizon Instruments. Initially Horizon Instruments, working closely with its customer, believed that a system could be sourced within the micro-electronics industry to meet the needs of the application - for example, an off-the-shelf system, 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.

Horizon Instruments is a well established company that designs and manufactures special-purpose test and production equipment for pharmaceutical and medical diagnostic applications. The system described here was initially designed for a large-volume producer of medical test and analysis devices.

Brief

The brief was to design a system that would place small discs precisely into predefined locations in a plastic moulding and, specifically, the discs were to be presented to the system in a random manner, loose in a container. As it was not possible to arrange for these parts to be supplied in a controlled form, Horizon proposed a means of handling them by getting the operator to scatter the parts on a back-lit glass plate. The requirement was to pick and place small metal discs 0.6mm (600um) diameter x 0.075mm (75um) thick into an array of 72 positions on a rectangular tray. The whole tray had to be populated within a period of no more than 3.5 minutes (approx 1 disc every 3 seconds).

One important feature of the machine is its ability to precisely locate isolated discs, determine their position, and then check their shape and diameter to +/-10um. This is achieved using high-quality vision hardware combined with image analysis tools that are fully integrated into the control software. Using precision linear motor actuators, the pick tray moves in the y direction such that a selected disc is brought into line with the y co-ordinate of a small vacuum head. The vacuum head 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.

Height detection

Another problem overcome by Horizon engineers was the possibility that, when the discs are spread on the pick tray, two discs could sit, one directly on top of the other, and that this 'double disc' could not be detected as such by the imaging system. The solution chosen was to lower the vacuum tip 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 very accurately using a linear optical encoder having the same resolution as the motor. Every time the tip lowers to a point nominally 25um above a disc to pick it, if the height measured by the encoder is greater than 25um (plus the disc thickness) this means the tip has met an obstruction. In all likelihood this will be a 'double' disc; so the pick is aborted and the system moves on to the next free disk.

Because of the difficulty and cost of sourcing engineering materials and components with an overall accuracy better than +/-10um, 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, to cancel out any small level errors of the glass or other components.

Between six and twelve mapping points are chosen over the area of glass (in this case about 50mm x 230mm) 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 +/-2um.

Stiffness and accuracy

Design Engineer Jim Crawshaw said, "Part of our philosophy behind the design of this system was rigidity in order to maintain accuracy and this drove our decision to mount all the major mechanical devices on a thick aluminium toolplate 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."

Second camera

Another feature of the system is a second camera, also driven by a linear motor actuator, that checks the satisfactory positioning of the discs on the 'placement tray'. This camera, with its actuator, is mounted with the pick analysis camera on a substantial gantry above the bed plate.

Daniel Bolton, Projects Manager at Horizon Instruments adds: "We always liaise closely with our customers to ensure our equipment meets their exact requirements. We take pride in the aesthetics of the machines as well as ensuring they meet all build and operational regulations."

With reference to the vigorous demands of the medical device sector for systems to be FDA/GAMP4 compliant, Daniel says: "Today's market demands the validation and documentation of all aspects, both software and hardware, when designing industrial systems. This is an area that we take very seriously and is an integral part of our service."

Next project: small rods

The pick-and-place system was designed to handle circular discs where there is no need for rotational orientation. However, Horizon is now in discussion with a client for a similar machine that would be able to handle metal rods 300um in diameter by about 3mm long lying horizontally (also arriving in a random state). This would 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 taken place to confirm that this is achievable.

Daniel Bolton comments: "The system could be adapted for use in a wide range of applications where components are not able to be presented to a picking head in a controlled or organised way. We think the system has the potential to evolve much further and its flexibility is ensuring that further ideas, over and above the original application, are being identified all the time. 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."