Intelligent Motion Drives Faster Packaging Machines
By Daniel Jones, President, American Institute of Motion Engineers
If one wants to tie, seal, snap, strap, cap, bundle, box, tag, mark, label, wrap, fill, unfill, feed in, feed out or case products, a packaging machine is required. A broad range of products from diverse markets, such as drugs and pharmaceuticals, food products, bakery products, paper products, novelties, industrial products, appliances and many others require special packaging and labeling. Packagers want more product types to be run on the same packaging machine at faster rates.
Packaging today is a high technology motion business, challenging today's motion control supplier to provide an intelligent system that offers increased throughput, flexibility and accuracy to meet the needs of the packaging industry. The arrival of the digital signal processor (DSP) supplied the control flexibility, accuracy and repeatability needed to make on-the-fly electronic cam (motion profile) changes and provided the packaging machine manufacturer with reduced wastage, quick changeover capability, greater productivity and higher reliability.
Servos and Steppers in Packaging
Most packaging applications require high peak torque capabilities to achieve the high cycle rates necessary for packaging machines that fill and close bags, bottles or boxes. The servomotor, best represented today by the PM brushless motors, offers peak torques up to 5 times larger than the continuous torque valves. The servo controller or drive combines both power and control capabilities that can shape the motion profile into machine vibration, inhibiting motion by using S-curve velocity profiling, providing motorload inertia matching and motion system fault/error diagnostics capabilities. AC induction motors using the latest vector control techniques are becoming very popular in continuous velocity applications such as conveyor or belt motion applications. The variable speed capabilities of today's vector controlled AC motor-driven systems are important to the overall throughput and synchronization of a packaging machine.
The PM brushless motor (also called the brushless DC motor) is combined with its servo amplifier or servo controller and a position feedback device, such as an optical encoder or an electromagnetic resolver, to provide a complete closed-loop electric motion system. This closed-loop motion system is a servo system whereby the actual motor shaft position is measured by the position feedback device (encoder or resolver) and compared to the commanded signal from the controller to create an error signal. The error signal is continuously monitored and minimized by the servo controller.
An encoder or resolver provides servo system resolution and accuracy. For example, an encoder with 1000 counts per revolution provides 4000 separate and different positions per revolution (5.4 arc-minutes). This encoder is equivalent to a 12 bit resolver. However, accuracy and revolution are not the same. Typical accuracy is at least twice and could be five times as much as normal feedback device resolution.
The servomotor is selected by its torque, speed and thermal dissipation capabilities. Typical PM brushless motor shaft speeds routinely reach 6000 rpm. The PM brushless DC motor outperforms all other motor types. Packaging application friction torques, mechanism inertias, machine speed requirements, etc. are carefully matched to the servomotor in order to optimize motion performance versus cost tradeoffs.
As mentioned earlier, the DSP provides the computing power to provide digital signals to control sinusoidal current waveforms for smooth low speed operation and servo loop gain levels (for inertia matching). Today's servo controls provide digital signals to all servo system control loops. DSPs from Texas Instruments, Analog Devices and others can provide the necessary control engines at OEM prices below $10. The motion control supplier, systems integrator or packaging machine manufacturer will need to program the DSP for the necessary motion profiles to achieve the desired packaging machine operation. Boxing, filling, strapping, feeding and other small and large packaging activities use servomotors and closed loop control systems.
The step motor, best represented by the high-torque hybrid step motor, develops lower peak torque capabilities, but higher continuous torques for equivalent sizes than the brushless DC servomotor. The step motor operates in an open loop format by receiving a pulse train at a number of different pulse rates from the step motor driver. Microstepping drivers are combined with velocity profile controllers to provide the necessary low vibration motion control. Step motors and their associated step motor drives use lower cost open-loop control. Driven by torque margin control techniques, step motors are used to move the packaging machine application load along the desired motion profile. The DSP also provides the step motor with a sensorless torque-vector control strategy that maximizes step motor torque capabilities at speeds up to 2000 rpm. Labeling, document handling and small package sealing are typical examples of packaging machines using step motors.
Synchronizing the System
Typically, each packaging machine has a number of motion axes that must be coordinated or synchronized. There are two primary synchronization methods, high-speed registration and line-shaft. In high-speed registration synchronization, sensors locate the registration mark on the product which triggers the start of the operation after a predetermined time interval. Each detection of the registration mark resets the process operation. An example of high speed synchronization involves finding the registration mark on wrapping material as it spools off the roll to initiate the form, fill and seal packaging process. Line-shaft synchronization can be seen in a box folding operation. The product is moving on a conveyor belt. If the conveyor slows, the box folding process must slow with the belt.
Multi-axis controllers, both PC- and PLC-based types, are used to supply these synchronized, multi-tasked interactive motion profiles. These controllers provide greater flexibility in synchronization since they usually let any axis link to any other axis, then let the user modify that link during program execution. Open architecture control systems are emerging that combine the dedicated, hard-wired, real time machine control of the PLC with the superior data processing and manipulating of the PC's capability. The trend is toward using the PC-based motion controller with embedded microprocessors or DSPs, plus better software created to safeguard against machine interface failures. Still, PLCs are the choice of many packaging manufacturers because of their history of success on the factory floor.
While the cost of servo components is usually higher than the mechanical components they replace, the elimination of complex mechanical devices and the reduction in overall machine size can help reduce total system cost and improve machine reliability. The added flexibility may reduce the number of packaging machine models or the manufacturing processes needed to build these machines. Future packaging machines will continue to use programmable intelligent motion components and systems to provide the basis for faster, smaller, smoother and cheaper operation.
Daniel Jones can be reached at firstname.lastname@example.org. 509 Marin St., Suite 221, Thousand Oaks, CA 91360. 805-496-2621.