Reducing packaging planned downtime: effective sensor application — Part 2

Sensor technology is constantly evolving, giving OEM packaging machine designers new ways to reduce the cost of their machine designs. In Part 1 we looked at reducing the cost of changeovers with linear position sensing. This time we look at simpler, more reliable power connections, rotary encoders and optical detection.

As we said in Part 1, product changeovers are the most common type of planned downtime in the packing industry. While reducing planned downtime is a core goal of lean manufacturing, there comes a time when procedure and process is not enough. This means we must turn to the latest technology to further limit planned downtime.

Power remote wireless connectivity: sub assembly flexibility

Another form of changeover is to replace an entire assembly on a machine with another sub assembly. This normally involves disconnecting and reconnecting control circuits, which opens us up to a range of undesirable problems such as misconnection, connector fatigue and unnecessary downtime. The answer is a power remote system. Power remotes transmit power to sensors and other components across an air gap of about 2 mm from one sub assembly to another. At the same time, control information is also transmitted back and forth across this air gap, to and from sensors, and the PLC is using a remote communicator for sensors, actuators or motors themselves. By removing entire assemblies from a machine including the sensors and putting another sub assembly in place, we can have a changeover time of seconds (Figure 1).

Figure 1: Power remotes provide wireless modular connections anywhere on the line.

Power remotes come in a range of sizes and power capacities. Some are as small as a 30 mm proximity switch and handle up to eight inputs. Larger versions handle more sensors and pass over 6 A of current across the air gap to run a motor. Power remotes also solve the problem of twisted, shorted out wiring and unreliable wiper-based contact technology in rotating assembly platforms, all without introducing conventional wireless complexity.

Rotary shaft encoders

Magnetic linear technology has combined with rotary technology to make an incremental shaft encoder. This technology has the ability to convert 46,000 magnetic segments per revolution into 360°of rotation to convert rotary motion into exact linear measurement of the conveyor. It can be used to keep track of position for a rotating table to degrees of rotation of a machine. Its advantage is its magnet. It does not have a glass disk and it will run in almost any environment. It can even run in a washdown environment with proper protection.

Mounted in a hollow shaft configuration at a cost of under $400, it reduces the cost of the encoder and improves the machine’s accuracy. Typically today, we measure machine position indirectly. We do this by installing a sprocket on the main drive and connect the encoder mounted separately through a chain (Figure 2). This assembly costs about $1220, with $400 for the encoder, two sprockets, chain and a custom bracket, plus the cost to mount and wire the device. We must also worry about overtensioning the chain, ruining the bearings in the encoder, while hoping we don’t drop a wrench on the encoder and break it. In contrast, a magnetic hollow shaft encoder mounts directly on the main drive shaft, has no additional parts for mounting and reads the machine position directly without chain slop (Figure 2a). The set-up for the rotary magnetic encoder is simple. We feed the quadrature input into a high-speed counting card, or standard PLC input for slow applications, and convert the 46,000 pulses into 360°of machine position. From here we can have an infinite number of cams in the machine — some cams for machine internal decisions and some to control actual machine function, for example, establishing a point for the machine to cycle stop.		Figure 2: Traditional chain and encoder positioning.
		Figure 2a: Rotary encoder positioning provides for scores of control functions on the line.

Distributed detection with single station rejection

Because the pulses from the rotary encoder come in a stream to the PLC, we can use rotary encoders for other tasks such as tracking the position of a part, and rejecting it downstream from a quality check. Let’s say we are using this device to track degrees of rotation for a labelling machine. The PLC can use the same pulses that it is converting into degrees of rotation and convert them to linear distance to track the product to the reject station.

This feature also works for multiple detection devices and one reject station. A recipe-driven application can determine which detection devices are for the specified product, using the same reject station for all detection stations. When the recipe for the changeover is selected, it will determine which error-proofing sensors to activate and send rejects from them to the same reject point. This is another example of reducing total delivered cost (TDC). It also helps sustainability because we have fewer components to fail within the system (Figure 3).

Figure 3: Rotary encoders allow multiple sensor error proofing with single station rejection. Here a misapplied cap is rejected.

Colour detection

Advanced colour sensors can be programmed to either look for a single colour or look for several colours to error proof different products. Again, the goal is reduced planned downtime. Colour sensors can be programmed by simply selecting the new product recipe when the colour sensor is set up for a new product. This way of reducing planned downtime includes full colour detection of parts and avoids re-teaching of sensors to recognise new colour parameters.

Today’s advanced full colour sensors are capable of being programmed with multiple outputs, each corresponding to the correct colour (or colours) of the product batch being run down the line.

This error-proofing detection step fits into the recipe-driven changeover scheme by allowing the code writer to assign specific inputs (or colours) for each recipe. In turn, the control system will look for the correct colour as targets are presented and will automatically reject a ‘bad’ or incorrect colour product, thus keeping the line running continuously. Some full colour sensors feature multiple outputs which are easy to program and operate at speeds sufficient to keep up with today’s fast-moving applications (Figure 4).

Figure 4: Colour sensors add an added dimension of error-proofing capability.

Optical sensors New vision-based optical sensors use simplified vision technology to bridge the gap between vision systems and sensor technologies. They provide a simple, practical and cost-effective way to error proof production by simultaneously checking several aspects of the product with a single device. Using a simple configuration interface that can be learned and used quickly by in-house staff, new optical sensors provide more information than a single function ‘smart camera’ or a group of standard discrete sensors. At the same time, they avoid the traps of complex vision systems in cost, complexity and high operator expertise (Figure 5). Optical sensors operate more like a smart sensor than a vision system. Just like a sensor, they are configured to look for certain attributes of a part or product, to make sure those aspects of the product are present, the part is configured correctly and positioning is verified. And like a vision system, they can error proof parts or configurations presented to them in different positions (Figures 5a and 5b). The result is a perfect solution for advanced error proofing. With the combination of both technologies and the simplified ‘sensor like’ approach to configuration and usage, the user can apply higher level sensing at a lower cost point, allowing these new optical sensors to be applied more readily in a true error-proofing scheme. The optical sensor provides an inexpensive new capability that was not available before for reducing planned downtime, easier line changeovers and accommodation of flexible or ‘build-to-suit’ manufacturing.		Figure 5: Instead of multiple sensors, as in Figure 3, optical sensors can reduce the number of sensors needed for error proofing.
		Figure 5a: Traditional sensor-driven trailing open flap error proofing uses at least four sensors.
		Figure 5b: One optical sensor and one keying sensor can simplify the process.

The bottom line

The goal in all of these applications is a lower total delivered cost, lower cost of ownership, and a more flexible machine with superior sustainability. Modern sensor technology combined with seasoned application expertise can get you there.

Combining machine positioning — both linear and rotary — along with programmable sensors for different machine set-ups, plus RFID technology to confirm the use of the proper change parts, allows entire sub assemblies to be removed from a machine. This provides more run time to make more product in the same manufacturing time.

OEM designers need to incorporate new sensor and linear positioning technologies into their packing machines to achieve evolving industry-driven performance/cost ratios. The resulting equipment will be less costly, more flexible, more sustainable and provide lower cost of overall operation. This allows OEMs to commit to higher throughputs to reduce machine cost per manufactured case.

Balluff-Leuze Pty Ltd
www.balluff.com.au