In almost every segment of engineering, Moore's Law has ushered in new eras of programming, and electronic camming is no exception. Cam operation is a mode in which controls follow a cam profile curve to derive slave-axis displacement from master-axis position, whether linear or rotary.
Applications that use coordinated master and slave axes (and the increasingly malleable relationship between their positions) can now be defined by hundreds of tables and thousands of curve points. Atef Massoud, motion and drives engineer at Omron Industrial Automation, Schaumburg, Ill., says that he's seen electronic cams evolve in a number of ways over the last decade.
“The number of applications utilizing electronic cams has greatly increased due to the availability of the technology. In addition, the design tools that assist engineers with optimizing the cam profile and offering multiple interpolation algorithms are spreading among automation suppliers. What's more, servomotors and drives are constantly achieving higher performance with faster sampling rates and advanced algorithms. Finally, tools for programming and implementation are constantly improving.”
Electronic camming background
Electronic cams were originally designed to displace their mechanical counterparts — lineshafts that distribute power from a prime mover to machine subelements, and cams on the lineshaft that output inherently synchronized linear motion with each shaft rotation. Here, the resultant motion can usually be described in three to five polynomials.
“Camming is nonlinear coordinated motion between two axes, and normally, one axis is linear while the other is rotational,” says Craig Dahlquist, engineer at Lenze Americas Corp., Uxbridge, Mass. “Before processor-based servo controllers, camming was done mechanically. Even today, mechanical cams can be found in myriad applications. Consider the most common, the camshaft in an automobile engine. The motor cam opens and closes the exhaust and intake valves at precisely the correct angle of the motor crankshaft rotation.”
This coordinated movement allows for each of the engine's cylinder to operate in optimum power output.
In contrast, electronic cam designs use servo drives with sufficient digital-microprocessor computing power to electronically coordinate axes with other machine functions. What dictates a mechanical-cam design (system dynamics and manufacturing constraints) differs from the chief consideration of electronic cams — system characteristics. “The servo controller will take the master rotational or linear information from one axis and then follow the master to a predefined nonlinear path. The operation is cyclical and both axes will start each cycle at the same 0 position reference,” adds Dahlquist.
In some cases where a design's required speed is being increased, mechanical cams are initially integrated or retrofitted with feedback components (typically encoders) and servo controls for closed-loop functionality. The mechanical issues of wear and tolerance control are typically irrelevant. Where the issues are a concern, however, electronic camming offers a suitable alternative, and the added benefit of economy of scale, in the form of motion profiles that can be entered into multiple machines. Here, machine builders can also protect proprietary design elements and load a system with only functionalities required by a given application.
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According to Massoud, although the concept of replacing mechanical cams with electronic cams is universal, the design, programming, implementation, and performance of electronic camming products are anything but. He underscores that engineers should consider the following issues regarding controllers and universal software for configuration, programming, and monitoring.
- The design tool
Some universal software packages with electronic-cam programming capabilities provide powerful and flexible graphical design tools. With these, the motion engineer can start a cam profile by:
Typing in profile coordinates, importing from a .CSV file, or
Visually, by way of mouse clicks
After this initial entry, the engineer can move on to other tasks required for electronic-cam programming:
Adjust cam points, choose resolution and spacing of points, choose and verify limits, and choose cycle times
Work with interpolation algorithms to meet application needs
Optimize the cam by moving points to ensure continuity and meeting boundary conditions, and evaluate not only position, but also velocity, acceleration, and jerk. Note: The extreme changes in acceleration known as jerk must be minimized in all systems; however, some power-transmission elements with inherent elasticity, such as belt drives, can absorb jerk better than machinery that allows for instances of metal-to-metal contact.
- The programming
To meet shifting production requirements, scaling an electronic system up or down is simply a matter of changing settings. Doubling a motion, for example, merely involves dialing in a multiplier. This not only reduces the time required for configuration, but also eliminates the requirement to stock a variety of cams for differing requirements.
“After a cam profile has been created, the engineer can use built-in PLCopen function blocks (FBs) for programming — with certain software packages,” explains Massoud. “The FBs include applying the cam to a slave axis and cancelling the cam operation. Here, there are FBs for scaling and shifting the cam's path definition if the definitions are adapted to new operations or products — which allows an engineer to manipulate the cams.” Some controllers and software also allow switching of cam profiles on the fly without any discontinuity.
Finally, an engineer can use cam equations in the program to create a profile after an initial dummy one has been created, or read points from a .CSV file loaded on an SD memory card — inserted in an SD slot on the controller, for example.
One unique feature of some controllers with electronic-camming capabilities is that the user application, motion engine, and EtherCAT control are performed at the same time in exactly the same period (scan cycle) for perfect synchronization.
One criteria for designing an electronic cam is how the servo processes a point. “Some controllers can perform 32 axes in 1 msec,” explains Massoud. “When combined with a high-performance servo drive and motor, they deliver the fast dynamic response needed to accurately execute cam profiles.” On the other hand, if that value is a few msec, for example, the designer should choose an algorithm with higher fidelity.
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Motor and drive requirements
The acceleration possible with electronic cams is limited by the capabilities of the servo drive and the motor, as well as the physical limitation of the design's mechanical elements. Motor acceleration is limited by winding characteristics, input-current ratings, and the drive's current capabilities. In fact, legacy electronic-cam designs exhibit torque and speed constraints that can limit their ability to exactly match a mechanical cam's output motion.
To minimize these issues, what kinds of motors are best suited for electronic camming? “Typically, electronic camming requires low-inertia servomotors with fast dynamic response,” explains Massoud. “Dynamic response can be characterized by the mechanical and electrical time constants of the servo system, typically to the order of a few msec. This is because, by definition, electronic camming has a fast dynamic profile compared to gearing.” This fast dynamic profile includes speedy accelerations and decelerations and demanding jerk values.
For safety, electronic cam designs also protect machines against failures. If the servo drive fails, for example, controls can detect the breach, and initiate a shutdown to minimize damage or waste. Even if the failure goes untraced, signals go missing, and a backup circuit will shut down the machine. Servomotors can also be quickly stopped in an emergency: Here, the servo supplies a negative torque to the motor for braking — to initiate a stop in msec.
During normal operation, as the required torque due to inertia is proportional to the acceleration/deceleration times the effective inertial load, faster acceleration/deceleration or shorter cycle times require higher torque. In this case, the significant ratio is the rated or maximum torque to the rotor inertia.
Other considerations include how fast the servo drive controls the current, velocity, and position loops. Note that in a servo drive used for electronic camming, required algorithms include velocity and acceleration feedforward — particularly important in camming, as velocity is constantly changing — plus compensation algorithms, response and vibration filters, and high encoder resolution.
Myriad applications benefit from the use of electronic cams. Generally speaking, these applications fall into two categories. The first type is that in which electronic camming replaces existing mechanical components. With electronic cams, for example, a processor can control the amount of fluid ingredients in a filling operation to output a specified blend — plus the total amount dispensed in each container. Changing these two parameters requires only a slight amount of programming.
“The second type of application is used in devising specific cam profiles to achieve an operation or functionality that requires a non-linear master-slave operation. These could not be implemented previously, by mechanical means or otherwise. Some examples include functions in modern packaging, printing, and metal forming,” says Massoud. “These applications typically incorporate an axis that must execute interactions with a product in processes that include handling, gluing, cutting, bending, and so forth.”
One example involves a rotary knife. In this application, material is running continuously underneath the cutting blade. “Then the blade rotates and cuts the material at predefined lengths,” explains Dahlquist of Lenze Americas. “When the circumference of the rotary knife is equal to the cut length, the knife rotates synchronously to the material speed.”
When the material length is longer than the circumference of the rotary knife, the knife must slow down while it is not in contact with the material. The knife then speeds up to synchronous speed when cutting the material. “Conversely, when the cut length is less than the circumference of the rotary knife, the knife roll must speed up after the cut,” continues Dahlquist. The blade then slows down to synchronous speed when the material is being cut.
In essence, the material line speed is the master reference. The camming drive follows a curve function that maps the position of the material to the angle of the knife. “However, as the cut lengths get smaller and smaller, the dynamics of the knife rotation becomes more demanding,” notes Dahlquist. If the cut lengths get too small for a given line speed, it may be necessary to slow the line speed.
In fact, Massoud has patents on two applications for the printing industry that use advanced mechanical cams and electronic camming to tailor the motion of paper-handling processes. In short, the cam designs aim to help machinery handle paper signatures more gently, but at faster rates for higher throughput. These “signatures” are folded bundles of pages that form multi-page sections of a book or magazine — most commonly grouped into four, eight, 16, or 32 pages. Pairs of rotating drums and conveyers typically handle them during processing.
In one iteration of Massoud's recent mechanical-cam design — and a testament to where the usefulness of mechanical cams endures — twin gripping drums collect and then release a paper signature to drape it over an inverted-V-shaped “saddle” conveyor at its spine. In a concurrent motion, cams for reciprocating briefly translate the drums in the direction of the conveyor receiving the paper — to reduce the sudden velocity changes of transfers. This allows the machine to run faster without damage to the sheets of paper.
In his electronic-cam design, gripper drums rotate at modulated speeds, so printed signatures are gripped from stacks at a lower speed (a slower region of cam rotation) than the speed at which the printed product is released onto a pocket conveyor for distribution (a faster region of rotation); this coordinates drum speeds to pickup and release. These tailored motions reduce problematic tears, misfeeds, insufficient separation and pulling time, and rollover of subsequent items in paper stacks — and allows increased machine speed with otherwise difficult-to-handle signatures.
Quantitatively speaking, without electronic camming, machine speed (and therefore throughput) must slow by 50% when thin or otherwise difficult signatures are handled. Electronic camming, on the other hand, keeps throughput high by tailoring different velocity profiles to different regions — slow when gripping, and faster when delivering.