Linear Actuators Bring Flexibility to Packaging

Cost and regulatory requirements are two primary concerns for today’s machine tool designers.  For cost control, the current buzzword is “lean,” which for the packaging industry heavily favors processes that require minimal setup time.  One of the best ways to reduce setup time is through the use of flexible components and equipment.

Environmental concerns place new emphasis on emissions, which puts pneumatics under scrutiny because lubricated air is often exhausted to the atmosphere.  In addition, the demand for quieter operation is driving the change to alternate technology and components.

Whether specifying, designing, or selecting machinery, answers to several questions will determine which technology is most appropriate:
– Will increased flexibility actually be exploited? 
– Are there environmental issues to be addressed? 
– Are the necessary resources and skills available to implement a new technology?
– What are the costs for each approach, what is the return on investment for each, and what are the overall benefits?

Haydon Non-captive Hybrid Linear Actuator provides varying stroke dependent only on the length of lead screw passing through the motor. Photo Courtesy of Haydon Kerk Motion Solutions.

Pneumatics — A proven technology
A pneumatic cylinder uses pressure differential to move a piston.  That differential is either air versus air that is typically supplied as pressure versus atmospheric pressure, or air versus mechanism, such as supplied pressure opposed by a mechanical spring.

With sufficient pressure and volume, pneumatics can move exceptionally fast and supply high force.  If the actuator uses a spring return, the actuator moves when air pressure is applied.  When the pressure is removed, a spring pushes the actuator back to its original position while exhausting the previously supplied air.  When air is used for the return force, it offers a greater degree of control over the motion, but because of the larger volume of air required, it must be exhausted in both directions.

Most often, a pneumatic actuator moves to a hard stop, either the limit of the cylinder stroke or an external stop.  Positional accuracy is determined by the tolerances of the stop location.  Changing the travel requires either replacing the cylinder or relocating the external stop. 

The benefits of pneumatics include:
Speed — pneumatics react quickly
Simple — the mechanisms themselves are not complex, although the applied technologies are cutting edge
Reliable — although simple, these devices are robust
Extensive knowledge and supply bases in the packaging industry
The limitations of pneumatics include:
Inflexible — changeover is expensive, which is the biggest reason for the move to electrical actuation.
Environmental contamination — lubrication is required for maximum life and performance.  Lubrication is often added to the air supply.  Therefore, unless special components or filtration is used, lubricant can enter the atmosphere wherever it is exhausted.
Noise — a pneumatic actuator’s exhaust air can be forceful and loud. 
High Energy Consumption — pneumatics require compressors to create an ongoing supply of pressurized air. Compressing air is a less efficient method of converting electricity into motion.  Air pressure must be maintained if sustained force or position is required, making it necessary to run the compressors when pressure must be maintained. 

Electromechanical actuation — a rising star
Electromechanical actuation with a rod-type actuator uses a rotary motor turning a screw to move a nut to extend a telescopic rod.  Physically and functionally a rod-type actuator is very similar to a pneumatic cylinder.  The big difference is that the motion is not “on-off” like pneumatics.  Through motor control, the position of the actuator rod can move to any position throughout its stroke, limited only by the resolution of the motor/screw combination.  Velocity and acceleration can be similarly controlled.

Haydon Captive Hybrid Linear Actuator provides precise positioning using step-motor technology.  Photo Courtesy of Haydon Kerk Motions Solutions.

Though an actuator may have a stroke of 12 in. (300 mm), it can easily be instructed to move to any distance within that range, with an accuracy of ± 0.025 in. (0.64 mm) or better.  An encoder on the motor or in the actuator provides position feedback.  Alternately, motion control can come through a stepmotor that counts steps of rotation.  A homing routine establishes position.

The benefits of electromechanical actuators include:
Flexible, programmable positioning
Clean — there are no emissions
Maintenance-free — can outlast the machine in which installed
Quiet
Lower energy consumption — power need only be applied to make a move and when turned off will maintain position and force
Some drawbacks of electromechanical actuators include:
Culture change — new skills and tools are needed within packaging to operate
Indirect setup — changes are accomplished by running a motor.  This requires input to a control
system.
Speed — pneumatic cylinders can respond faster
Initial cost — in many applications, electro-mechanical actuators will cost more than a pneumatic
cylinder of equivalent stroke and force

The impact of sensors
The changes in packaging operations are built upon the availability of low cost, high speed, reliable computing capability and sensor technology.  Operators no longer need to make decisions.  All the knowledge is pre-programmed, sensors know what is coming, and the line “knows” what to do.  Even motor technology has the intelligence to receive a command, take action, and confirm whether the action was successful. 

The same sensing and controls can be applied to pneumatics, but pneumatics remain an on-off technology.  The motor and screw combination in a linear actuator can take the information and use it. 

Alternatives to pneumatic cylinders
Rod-type linear actuators are obvious candidates to replace pneumatic cylinders due to their physical similarities.  There are other forms of linear actuators that look different but offer the same opportunities with alternative forms. 

Rodless actuators take a form that might be classified as a compact slide, with the motion generated by a motor turning a screw or a belt.  These are not telescopic but instead are more of a transport mechanism positioned above or below the workspace or pathway. 

The non-captive linear actuator uses a non-rotating screw driven through an internally threaded hollow rotor motor.  These compact units produce surprisingly high actuation force.

The common element of all these options is easily programmable motion.  All can be used in an intelligent process that detects the product requirements and adjusts automatically to the situation.  Lot sizes of one can be accommodated without a changeover or additional set up.  All can be pre-programmed for a variety of configurations so that changeover requires the press of a button.

Points to consider
When comparing the technologies, there are few absolutes.  Generally, pneumatics has the speed advantage.  Force capability comes down to size with pneumatics, where motor driven actuators have the additional elements of electrical and mechanical efficiencies.  Linear actuators are the clear leaders for accuracy, especially if stopping short of full stroke.  Travel life or cycle life varies widely with both technologies; analyze closely as the cost implications can be great.

Familiarity with a technology strongly influences its perceived complexity.  If designing from the ground up, the tasks should be similar.  A redesign to switch from pneumatics will usually be straightforward and involve different controls and the removal of some manual stops and adjustments.  Increased electrical resources will be required but this should be offset by reduced air requirements.  Initial set up will likely take longer for programming an electromechanical process. 

ET Series Electrothrust Cylinder servo-motor driven actuator provides high thrust loads in a conventional telescopic configuration.  Photo courtesy of Parker Hannifin Corporation.
 
But once the system is complete and a good user interface established, process changes or additions are easy.

The big question mark comes when deciding how much of this new flexibility will be used.  It is wonderful to have the flexibility provided by automation.  But without the appropriate feeding processes, output processes, and a manufacturing mix that can exploit the capability, the benefits are few.  Providing the sophisticated input and output may be the greatest challenge of enhanced production flexibility.

ER Series Electric Rodless Cylinder has a guided carriage that is particularly well suited to transport mechanisms.  Photo courtesy of Parker Hannifin Corporation.

Cost
Purchase price is unlikely to be the determining factor, although pneumatics is likely to cost less based on hardware.  The simplicity of a pneumatic actuator gives it a cost advantage.

First time implementation of an electromechanical system can be costly because of the need for new resources, new knowledge, and the learning curve. The process starts at the design phase and continues through the supply chain, operations, maintenance, and customer training.  But once the investment is complete, it should be less costly to operate from both a labor and energy standpoint. 

Electromechanical systems reduce or eliminate downtime for additional savings.  As product mix increases and inventory levels drop, productivity measurements rise and cost savings grow.

Other demands such as environmental requirements still might justify a change of strategy.  Building a new line using electromechanical actuation will be a good choice because the initial investment should be comparable and the future capability will be more flexible and versatile.

Specification
The application will set the direction from the outset.  It will lay out all the performance and operational goals and should include information that defines the associated input and output processes and materials.  This is the point to consider future possibilities, as it will naturally define a technology path.  Prospective suppliers will critique the specification and offer options that may not have been considered.  Their responses will alert you to risks and opportunities and will give you insight to the suppliers’ capabilities, creativity, and willingness to provide the best product. 

Supplier Selection
A supplier’s prior experience, references, and empirical product data are valuable.  Product specifications in electromechanical components can be deceiving.  A good supplier will understand and articulate the features, benefits, and trade-offs of various options.  Training, design support, and service are as important as performance.  Reliability is very important.  Often, equipment is specified at peak capability but should be down-rated for normal use.

Prototype and Testing
With custom equipment, it may not be possible to prototype the system but it is still valuable to test the claims against the specification.  With a well-written specification, it’s possible to simply test critical features.  Isolate those parameters that represent new challenges, such as response time, positional accuracy, force-speed capability, or efficiency.  Work with the supplier to come up with a meaningful way to validate them.  The information may include data from equipment built for other applications.

A cautionary note about reliability and life:  be very careful about accelerated life testing.  Many electromechanical components do not behave in a linear way when speed, load, and duty cycle are varied.  In an accelerated test, friction, heat, and lubrication breakdown can cause premature failure that would never occur in normal use.  If an accelerated test is used, a positive outcome is likely valid but a failure is often inconclusive.

Opportunities
Imagine that each product manufactured carries a bar code or RFID tag that is read as the item travels into the packaging line.  Sensors know when each item reaches a station and have adjusted appropriately.  Conveyer track widths adjust, case packer strokes change, and the output is directed like trains in an automated freight yard, while the customers’ preferred transportation companies pick up the products recently completed.  Product is gone within hours, not warehoused for weeks or months.  Electromechanical actuation has established its value and those companies who have embraced it are already seeing the benefits.

Kerk Motion
www.kerkmotion.com

Pneumatic actuation, and its role in motion control, has generally been quite simplistic but at least one company is working to change that.  Enfield Technologies, in Trumbull Connecticut, enables precise motion control and programmable positioning by applying proportional control to pneumatics.

Proportional control means that the output of a system is directly related to the input. System inputs include both a target value (set-point command), which is the desired result such as position, pressure, flow rate, or force level; and the real-time measurement of actual progress toward that goal (sensor feedback signal).

With proportional control, the output can be any interim value within a fixed range.  For example, an air cylinder can be positioned anywhere between fully retracted and fully extended.  In another example, a tank’s pressure can be maintained anywhere between vacuum and full line pressure.  Proportional control is a cost effective way to create flexible systems with dynamic control.

Pneumatics is viewed as on or off, with positioning characterized by hard stops.  Attempts to provide proportional pneumatic control using solenoid technology have been slow and relatively imprecise according to Enfield.  Thus, Enfield uses fast-response voice-coil technology, which is three- to seven-times quicker than solenoids, along with special valves, drives, and controls to make rapid adjustments to the air pressure for fast, accurate positioning anywhere within the total range of motion.

Voice coil actuators take their name from loudspeaker technology that uses a fluctuating current to cycle an electromagnetic coil (voice coil) back and forth through a flux field created by a permanent magnet. 

The amplitude and frequency are defined by the input signal, thus moving an attached cone to generate sound waves.  Enfield’s voice coil-type actuated valves distribute the air pressure to opposing sides of pneumatic cylinders, rapidly cycling to constantly update the forces necessary to reach or hold the desired position. 

While more common proportional systems rely on two or more “on-off” solenoids to do the switching, voice coil-type actuated valves require only a single element to provide bi-directional and variable volume flow. 

Used with closed-loop feedback, that system can achieve positional accuracies of ±0.032 in. with ±0.005 in. repeatability.  Though not as accurate as electro-mechanical actuation where ±0.001 in. can be readily achieved, this is an exceptional capability for variable position pneumatics.  Force and linear speed are limited only by the amount of air available and the choice of pneumatic cylinder.  It is possible to generate thousands of pounds of thrust with only 50 to 100 psig air pressure.

The air used to provide power must be exhausted.  The location and conditioning of the exhaust may be critical due to noise and environmental constraints.  Like any other prime mover, quick moves demand adequate power.  In the case of pneumatics, that means air pressure and volume.

For more information on Enfield Technologies go to www.enfieldtech.com