Inside machines: Motion control moves bacon faster

Case study: When a PLC cannot move bacon quickly enough, consider a dedicated motion controller.

10/29/2012


When a New Zealand manufacturer of food slicing machines needed to squeeze more performance from its bacon slicer design, it switched from using a programmable logic controller (PLC) for total machine control to a distributed arrangement that combines a small PLC with a motion controller. Evaluating the trade-offs between central and distributed control is a common practice for machine designers. Whether centralized control architecture will work depends on the application’s requirement for determinism, the need for responding to stimulus in an absolutely predictable and timely manner. Deterministic systems produce repeatable results; the same conditions and timing result in identical responses from machine cycle to machine cycle.

For a very high-speed process to be controlled with determinism, the controller needs to be able to evaluate its inputs and respond appropriately in less than the time allotted by the process. As the need for higher productivity drives a machine to run faster, timing margins that used to be sufficient may no longer allow for completing the necessary control loops in time and the system will fail to function as desired.

Figure 1. The Titan 500 bacon slicer machine. Courtesy: Delta Computer SystemsThat’s the problem that was encountered by Titan Slicer Ltd. of Nelson, New Zealand. The company needed to increase the cutting speed of its Titan 500 bacon slicer (Figure 1) to produce 1600 slices per minute, but the PLC that controlled all machine functions couldn’t keep up. The challenge was not in the cutting of the raw bacon slab, but in controlling how the slab is fed into the slicer. To ensure that the size of the initial slice in the package is uniform with that of the other slices, the bacon slab must be fed into the slicing blade at exactly the right time. Too soon or too late will produce an initial slice that is too thick or too thin, which is exactly the problem the PLC was having.

Figure 2. The RMC75E motion controller from Delta Computer Systems controls up to two motion axes simultaneously. Courtesy: Delta Computer SystemsTo resolve the problem, Titan Slicer executives called in an experienced control system integrator, Tui Technology of Rotorua and Whangarei, New Zealand. Tui’s engineer, Malcolm Jones, had worked with motion controllers in the past and came up with a new control system design for the bacon slicing machine using a dedicated motion controller (see Figure 2). A PLC was used in the new design, but for less time-critical functions, such as issuing on/off controls for the blade motor.

The motion controller is programmed to do closed-loop position control using encoder feedback from the servo drive that moves the bacon slab into the cutting blade. The motion controller sends a +/- 10 V analog output to the servo drive that moves the bacon slab. Bi-directional motion control is important so that the bacon can be moved back from the cutting position when the spinning blade is not at the right position for cutting the first slice of a new bacon pack.

Note that the blade is not round, but is instead shaped to slice the bacon as it moves under it (see Figure 3). Different versions of the machine move the bacon slab via belts above and below the blade or via a lower belt and a gripper that grabs the slab’s trailing end. So that the motion controller can track the speed and angular position of the turning blade, the motion controller connects to a second encoder, mounted on the blade motor, as a reference.

Figure 3. The slicer blade of the Titan 500 is at the center of the image. The bacon slab is fed in from the right and the slices are carried out of the machine on the conveyor to the left. Courtesy: Delta Computer SystemsThe motion controller’s fast cycle time allows the cutting operation to produce repeatable results. At 1600 rpm, there is a 38 ms time window in which to decide whether the bacon slab is to be moved forward to set up for the next cut or backward to clear the blade. The 1 ms loop time in the motion controller running the advanced gear move instruction enables repeatable and accurate slice thickness.

The advanced gear move is just one of a number of gearing functions supported by the motion controller. Gearing is used when one axis (the slave axis) must move incrementally and proportionately in relation to a master, which is typically the position or velocity of another axis. Whereas simple linear relationships can be set up between master and slave (using the motion controller’s gear absolute function), an advanced gear move operation lets the user program a nonlinear relationship between master and slave. For example, Figure 4 shows an advanced gear move operation where the motion of the slave is dictated using a fifth-order polynomial equation. Most of the motion controller’s simpler gearing commands allow the designer to choose the exact gear ratio to be used by specifying a numerator and denominator when the motion step is programmed. This avoids the need for complex software that changes gear ratios on the fly.

Figure 4. A plot generated by Delta Computer Systems’ Plot Manager software shows the operation of the advanced gear move instruction in a complex slicing application (not the bacon slicer described in this article). In the plot, the yellow line shows the

“To be clear, achieving a uniform bacon slice width in the middle of the pack wouldn’t be a problem for a PLC-controlled solution,” said Malcolm Jones. The motion controller “pays for itself…in the control of the first slice to meet our critical design goal.”

Besides the machine with single cutting blade, Titan also offers a higher-speed two-blade version, where the blades spin in opposite directions. The reason for the twin orbital drive is to allow very fragile or warm products to be sliced quickly. In this slicer version, each motor drive is given a 0-10 V speed command and separate head encoders provide feedback to indicate the position of each blade. As with the single-blade version, the bacon slab is advanced or pulled back as the blades turn to prevent incomplete slices.

To program and tune the motion, Malcolm Jones used motion control software, which includes a plot manager that displays trend or capture plots for key motion parameters versus time. Figure 4 is a plot produced by the plot manager that shows the operation of the advanced gear move instruction. The motion control software also includes a tuning wizard, which enables quick and easy setting of control loop parameter gains for optimizing motion.

Using the software tools, “I was able to prove that the system had enough timing margin to handle the higher cutting speeds,” said Jones.

The bacon slicing machine is a typical example of a design that requires close attention to cycle timing to ensure the desired results. In developing such a machine, it pays to select components that are optimized for performing closed-loop control with determinism.

- Brad Smith is regional applications specialist, Delta Computer Systems Inc. Edited by Mark T. Hoske, content manager CFE Media, Control Engineering and Plant Engineering, mhoske@cfemedia.com.

www.deltamotion.com  



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Improving flowmeter calibration; Selecting flowmeters for natural gas; Case study: Streamlining assembly systems using PC-based control; CLPM: Improving process efficiency, throughput
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me