Reducing packaging planned downtime: effective sensor application — Part 1

Sensor technology is constantly evolving, giving OEM packaging machine designers new ways to reduce the cost of their machine designs while decreasing planned downtime during machine changeovers.

Reducing downtime is one of the most effective ways packaging end users can achieve profitability in today’s highly competitive marketplace. For OEMs to help end users minimise downtime, they must develop equipment that has the operational flexibility and technical attributes able to:

• minimise planned downtime
• reduce or eliminate unplanned downtime
• be competitive in the marketplace.

The easiest way OEMs can accomplish this is to turn away from obsolete methods and expensive technology such as servomotors. New, more economical sensor and encoder technology will make their packing products more changeover friendly, more repeatable, more sustainable and less costly over their production life.

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. Yes, we can pre-stage change parts, or continue a changeover through a break or lunch to get production up sooner, but when we have optimised all we can, we must turn to the latest technology to further limit planned downtime. Today, that means using sensors to help shorten changeover downtime while increasing machine repeatability and flexibility at a cost the end user will pay.

What is a changeover?

The industry defines a changeover as running at a known level of efficiency on one product to running at a planned level of efficiency on a new product. The downtime involved includes the machine tuning time necessary to achieve a specified level of performance.

Of course, the time a changeover takes results in lost production revenue.

The changeover process

Let’s assume a changeover is about to begin. In an optimised lean plant, the change parts will be positioned on a cart of one colour and an empty cart of another colour will be positioned as well to hold removed parts. As the changeover begins, we know the package size we are currently running, and we know what size we want to go to. The mechanical and electrical changing process continues with parts being removed and replaced as fast as possible. Once this is done, we must complete the rest of the changeover by eye, turning cranks and looking at linear scales, and then eyeballing the proper position.

Once we have completed this process, we run the machine and tune it for actual operation and optimisation. Tuning is essential to a successful manual changeover, but the less tuning is involved, the more repeatable the changeover is.

Linear position sensors enable low-cost automatic size changing

To automate previously manual size-changing operations, the challenge has been to overcome the inaccuracy and delay inherent with human interpretation of mechanical measuring scales, without resorting to high-cost servomotor technology. While servos provide the necessary level of accuracy, their high cost per axis eliminates them from consideration for automatic size changing. Also, their performance envelope far exceeds the requirements for size changing — they’re simply overkill for the application.

Yet servos remain compelling to designers because they provide both motion and control. Other prime mover technologies, such as DC motors or pneumatic cylinders, provide motion but lack appropriate control. This is where low-cost yet accurate and reliable linear position sensors can help. By providing the missing continuous position feedback signal, the linear position sensor suddenly allows a low-cost DC motor or pneumatic cylinder to emerge as a cost-competitive functional alternative to a servo system for automatic size changing. It’s even possible to consider leaving the human operator as the prime mover, yet employ a linear position sensor and an appropriate HMI interface to automatically indicate to the operator when the mechanism has reached the proper position according to the current control recipe. Linear position sensing enables full control of the resizing sequence with complete repeatability, based on low-cost prime mover technologies.

Of course, taking full advantage of the economic benefits of this new approach is most effectively accomplished during the OEM design process, not on the factory floor. The key person in this process is not the electrical engineer, but the mechanical engineer who designs the motion technology into the OEM machine to begin with. Without proper forethought applied to the integration of the sensors into the machine design, sensor technology will never be applied to maximum effect. This is why the trend towards mechatronics is so important. Mechatronics is the combined application of mechanical, electronic and software engineering to produce an effective machinery control system. The emergent trend is that the machine’s mechanical designer is a sensor expert and a control code writer as well, so that the complete vision of the machine’s mechanical design and functional capabilities rests with one highly capable person.

A new approach to reducing planned downtime

Recipe-driven size changing is not a new idea, but the approach of using sensors to validate the repeatability of the process is — and as a plus, the sensors to do this are much more affordable compared to servo technology.

As for keeping track of change parts, we can use an RFID tag with a scanner to verify only correct parts are applied to the machine. With respect to other changeover steps, a recipe-driven process built into the operator interface allows the changeover to be made in the proper sequence — including prompts to hand-change mechanical parts in the middle of the automatic changeover process.

As we reconfigure the machine, we verify the change through the feedback loop from our sensors regardless of whether the prime mover is a motor, air cylinder or a person. If the prime mover is something other than a person, the size-changing prime mover stops automatically once the planned position is achieved. If it is a person, we can provide an audible sound or visual indication that the position has been reached. Once the changeover is accomplished, a simple check list can be used to confirm that the correct parts have been installed and the machine is positioned properly. As the machine continues to run, and minor changes are required to adapt to packing material variations, we can save and store the final settings before going to the next new size. This allows maintenance people the opportunity to decide if the recipe for a specific package size should be permanently altered for changeovers in the future.

Linear positioning technology

Affordable linear positioning is fast gaining ground as a preferred technology to speed machine size changing and is available in several form factors to fit most machine applications. While it is an effective method of monitoring machine position during a changeover, it is also cost effective because it can replace expensive servomotor controls without negative impact on changeover repeatability.

Linear positioning technology comes in absolute analog or digital versions, each with the ability to directly or indirectly measure machine positioning. These sensors take an open-ended prime mover such as a motor or air cylinder and make it a closed loop device at an affordable price (Figure 1).

Figure 1: Linear positioning technology provides recipe-driven exact placement capability.

Figure 2: Traditional hand-turned chain and sprocket methodology depends on the skill of the operator.

The advantages of this technology are its accuracy, repeatability and low installed cost. For less than $650 per 30 cm stroke measurement, linear positioning provides an infinite number of stop locations along its measured axis.

Changeover evolution: reducing planned downtime A typical changeover application is moving the entire side of a cartoner or case packer. Today, we do this with a crank, a bunch of chains and some position counters or linear scales. There are several ways to reduce the cost of this planned downtime, improve the technology of the machine and execute changeover faster with more repeatability (Figure 2). Take the example of the hand crank and chains. We can put a linear sensor on the machine to monitor its movement and give an audible signal to the person operating the crank as to when to stop (Figure 2a). This becomes an effective retrofit for existing machines. The next step is to replace the crank with a motor and gear box and continue to use the linear sensor as a feedback loop (Figure 2b). However, the best solution is to replace the chains, cranks and all of the supporting mechanism and use a couple of rod-lock style air cylinders together with a linear sensor and have a complete recipe-driven automatic changeover system (Figure 2c).

Figure 2a: Adding a transducer can provide higher position accuracy.

Figure 2b: A simple motor used with a linear feedback device closes the feedback loop.

Figure 2c: Air cylinders and a transducer provide accuracy and precision at less cost.

Linear motion evolution: increasing flexibility while lowering cost

Let’s take the example of an air cylinder or mechanical system that is pushing a collated group of cartons into a container to make a case of product.

In the original air cylinder-based solution, the collated product is pushed in front of the loading ram, and then pushed a known distance into a container. Let’s say we must push a second load into the container as well. Typically, we would have pushed at the same stroke distance and depended on the second load to push the first load all the way into the container (Figure 3).

Then came the servo motor; that allowed us to have two stroke distances and push the load into the container at a purchase cost of about $4250. This was a step improvement over the air cylinder. With the servomotor, we could make the acceleration and deceleration of the load gentler. The next step was a sensor-driven solution at a much reduced price. Proximity sensors can provide discrete position information. In Figure 3a, four sensors provide the positioning information to allow a two-position box loading operation. Today, using a linear transducer coupled with an air cylinder, we can obtain the same machine function for one third the cost of a servomotor — an excellent example of sustainability and total delivered cost reduction (Figure 3b).

Figure 3: Air cylinder technology alone can only push to one distance.

Adding more flexibility

Often machines designed for specific package sizes must now handle new packages within their size capability. The answer is to convert binary control to linear control. This means using more flexible sensor technology — in other words, converting proximity switches (Figure 3a) to linear positioning technology (Figure 3b) for increased machine flexibility.

Figure 3a: Inductive sensor arrays give multiple discrete distance positioning capability.

Figure 3b: Analog linear technology can provide infinite distance positioning capability.

The good news is that linear technology is inherently less expensive than using conventional sensors. Let’s assume we have planned four different package sizes and we are adjusting a machine component to a known position by using proximity switches. The total delivered cost of each proximity switch is about $330. The proximity switch may cost $65, the bracket $25, the cable is another $25, the PLC input will cost $130 and the labour to install the device is about $85 including mechanical and electrical. The cost of four devices is $1320. If we substituted a linear positioning device, the total cost for a more flexible and efficient machine would be about $780.

In Part 2

In the second part of this article, we will continue to report on evolving sensor technology for the packaging industry, looking at power remotes, rotary shaft encoders, colour detection and optical sensors.

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