Motion System Design

Productivity Forum: Linear Actuators

Careful planning early in the design phase is key to linear actuator system performance

When using linear actuators, what applications present the most challenges in terms of machine productivity and why?

John/Exlar: Applications that require high force and speed are challenging because high speed requires low mechanical advantage, and high force is best achieved with high mechanical advantage. Applications with extremely high acceleration are also difficult in rotary screw design actuators. Actuators that lend themselves to very high acceleration are also typically limited in force capability.
Any application requiring high speed, high throughput, high accuracy, and/or low heat generation, such as electronics assembly pick and place or PCB drilling, or semiconductor die bonding and metrology, presents challenges.
Applications which require custom engineering, exotic materials, or special manufacturing will consume the most time and manpower. These applications can be found within many industries, from medical to industrial and military.
Existing machine or device modifications often create the largest challenges. Unlike fresh applications, modifications and mid-production upgrades force engineers to fit ideas and concepts into physical packages that were probably not designed for them. If not thought out correctly, the solutions may work, but may not add real value to the system.
Danielle/Bosch Rexroth:
Processes that require downtime to change out tools or to perform maintenance and repairs are the biggest deterrent to optimum productivity. Harsh environments, such as woodworking, metal cutting, and water jet applications, create special challenges to increasing productivity because chips or other contaminants can enter the linear bearing systems and cause failure. Protective sealing strips help prevent contamination and reduce downtime.

What are the worst cases of improper design and implementation you’ve seen? Describe what went wrong and how these and similar problems can be prevented.

Danielle/Bosch Rexroth: The worst situations occur when the application or machine is under-designed at the outset or later changed because the application was not well understood during the design phase. I recall one application in which a customer had to purchase all new linear actuators due to a poor understanding of the real application requirements. Once they had built the machine, they discovered it needed to move twice as fast and carry double the load that it was designed for.

The following acronym can serve as a reminder of the major factors in system design: LOSTPED stands for Load, Orientation, Speed, Travel, Precision, Environment, and Duty Cycle. Each factor needs to be considered as early as possible during the design phase to ensure proper actuator specification.
The main cause of problems in linear motion systems is incomplete specifications. In one of the worst situations we have seen, the designer acquired numerous single-axis stages designed with a tight bend radius for design compactness. He then designed his machine around a performance level that exceeded cable and velocity limitations, resulting in many hours of rework and redesign. This could have been avoided if proper information and full specs were available up front.
Application implementations that do not prevent high-speed end crashes are regularly seen. When a large mass is stopped in a crash situation with close to no deceleration time, the energy absorbed by the actuator is extremely high. Eventually, repeatedly stopping an actuator operating near its rated load in a crash situation will usually result in actuator damage.
Al/Danaher LMS:
Bent actuators are one of the worst cases of improper design. The cover/extension tube can get bent by offset load or by extending the actuator around a pivot point. To avoid bending the tube, keep the load along the extension tube axis, keep the mounting pins parallel, and make sure there is clearance when the actuator is fully extended and retracted.
I have seen quite a few improper designs. I tend to separate them into two types: when the design is functional but not the best way to accomplish the designer’s intent; the second is when the design is a poor concept that is poorly engineered. The second type is much worse and is often times downright dangerous. Many improper designs are simply caused by the engineer living in “magical micron tolerance” land while the assemblers work in the real world. These designs have unbelievably tight, unnecessary tolerances in every aspect of the project — tolerances that are not needed for the project and only add time and expense during assembly.
Some examples of real-life failures: a machine had a very compliant structure and caused control issues; failure to follow the manufacturer’s thermal protection recommendation resulted in a fried motor; and drives not matched to motors caused overheating or underperformance.
Proper specification and implementation all comes back to one, all-important thing — knowing the application.

Describe best practices in designing with linear actuators. Can you offer any application-specific tips to other engineers?

Dave/Kollmorgen: Take the time to adequately define the machine structure and motor performance based on mass, force, and motion profiles. Realize that the machine (especially in terms of rigidity) may not be able to handle the improved motor/system performance. Use proper shielding and grounding techniques. Gain an understanding of all system components’ new technology and how to use it.
Knowing the application involves a number of factors, including load, weight of the object being moved, the speed required for movement, acceleration and deceleration rates, time from cycle start to stop, the environment, and rate of use (continuous versus cyclical). The actuator does not operate alone; the whole system should affect actuator selection.
To properly design in a linear actuator, you must have realistic, clear goals for the linear system. How heavy is the moving package? What are its dimensions? How fast do you need to move? Are there accelerations? What sort of lifetime is required? All of these questions, and others, affect the actuator choice. Also, we suggest running the calculations with the real loads, then, based on the other factors, choosing the desired safety factor and then the correct system.
When designing with linear actuators, there are a few simple tips. The system is only as accurate as the bearings and supporting structure. Typically the maximum acceleration or velocity of the system will be limited by the bearings (5 g), encoders (<1 m/sec at 0.1 μm), cables (bend radius 7 x diameter), or available bus voltages.

When designing a system to achieve improved performance through higher accelerations, remember that F = ma. For any given acceleration you command, the reaction forces into your mechanical structure are proportional. Machinery has been known to “walk across the floor” after redesigning the motion system but not the supporting structure. This also applies to mechanical resonance set up by reducing cycle times to milliseconds rather than tenths of seconds.
Al/Danaher LMS:
It’s critical that the actual load on the actuator – both in terms of weight and direction – be determined. Mechanical disadvantages and spring loading can often increase loading on the actuator. Just use the actuator to position the load and other bearings to support the load and keep it parallel to the actuator axis.
Danielle/Bosch Rexroth:
The machine’s final location, oddly enough, is sometimes not even considered until machine installation. Designers and users need to consider the end application and the requirements that will be placed on the machine daily.
Pay special attention to atypical operating modes. For example, what happens when the operator triggers an E-stop with the actuator moving at high speed? Oftentimes this will result in a freewheeling mode and crash if not dealt with in the machine operation logic.

What can linear actuator manufacturers do to improve productivity?

John/Norco: Manufacturers should never assume that the customer has chosen precisely the right actuator, even if they order by part number. In some cases, though, time or confidentiality prohibits customers from being open and specific about an application. John/Exlar: Assuming that the actuator is sized to properly perform the application as requested by the customer at the desired rates, expected productivity will naturally result.

Maximum up time means maximum productivity. Reviewing every damaged actuator allows design changes that prevent damage caused by the atypical operation, even if that operation is outside the actuator specifications.
Danielle/Bosch Rexroth: Productivity depends on the machine design, its full use, and downtime minimization. So, linear actuator manufacturers should provide quality modules that are easy to install, maintain, and service. By reducing actuator maintenance, you increase the machine’s overall productivity. And, when a manufacturer produces the linear motion components inside the actuators, the same design principles should be applied to guide rails, screws, bushings, and shafts.
Manufacturers should aim to provide more force per motor size with lower heat generation.
Users do not want to lubricate and maintain the units. Manufacturers should provide maintenance-free products when possible and design units that last longer but still cost less.
John/Baldor: To improve productivity, throughput is not only enhanced by increasing the acceleration, but also by improving peak velocities and settling times. By switching to a linear servosystem, a customer could see potential improvements in settling times from 100 msec to less than 10 msec. Linear motors can achieve peak velocities well in excess of 4 m/sec easily (faster than ball screw speeds), while improving the positioning repeatability and mean time between failures of the system as well.

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What can designers do (when selecting and applying linear actuators) to improve productivity?

John/Exlar: Don’t size an actuator to the edge of its ratings. Leaving some overhead will usually allow for design oversights, and also most often means longer life in a mechanical product.
Al/Danaher LMS:
With many variations available, from rod style or rodless, ac, dc, programmable, and so on, be sure to select the right actuator for the application.
Optimize the actuator/machine combination, for example, machine rigidity and mass relative to actuator performance, for the desired machine output.
When selecting and applying linear actuators to systems, designers do best to consider where actuator use can increase productivity by removing an element of manual labor or another less-reliable form of motion control. For example, reel manufacturers can use a ball reverser actuator in an automatic, level rewind mechanism, thereby eliminating the possibility of human error or injury and making the rewind function faster and uniform for every use.
There are few downfalls to using linear actuators. The benefits of using a linear motor are tremendous, but are not all tangible, up-front costs. The benefits of ease of assembly, reduced maintenance, and flexibility of operation all increase productivity. For an OEM customer assembling multiple machines in a day, the time saved in assembly is directly related to the sale price of the equipment. In a well-designed system, the only linear motor maintenance should be bearing lubrication and semiannual cable replacement. Because linear motor actuators allow not only high speeds, but also high repeatability, more flexible motion profiles can be written. For example, in board-level manufacturing, an odd form placement head must be extremely repeatable (optimized with a ball screw), while a screw insertion head needs to be high speed (optimized with a belt drive). Because the machine needs to be built from the same motion platform to achieve cost reductions and both levels of performance, performance in both respects is compromised. A linear motor-driven system allows for better repeatability than the ball screw and faster moves than the belt drive, increasing productivity literally across the board.
Andrew/Rollon: Designers can provide manufacturers with application data, giving them an extra sounding board where they can compare and check their calculations and decisions.

What should end users do (in terms of care and upkeep) to maximize life and productivity?

Dave/Kollmorgen: Follow the manufacturer’s recommended maintenance schedules.
Danielle/Bosch Rexroth: Establish a preventive maintenance schedule and follow it. Also, choose the right product for the application. Production needs might change on the shop floor, and it’s important for the end user to understand the implications of changing the operating parameters. Additionally, stocking critical replacement parts, such as belts, ball screw assemblies, carriages, and so on; or even having the proper lubricant in ready supply, are ways to minimize downtime and ensure continuous production.
Most end users are not assembling many products in an assembly line or cell type environment. Therefore, replacing standard OEM equipment with the same is not often an option. They need products that meet or surpass the original specs and are also easy and quick to mount and use.

To guarantee the longest lifetime, proper mounting can oftentimes be more important than regular maintenance. Poor alignment or mounting of linear bearings and actuators can have a tremendous effect on system life span. Choosing actuators and bearings with selfalignment capabilities can be a huge help.
If the actuator is specified properly in the first place, it will prove the most durable option for the customer. Once the actuator is in place, however, the customer can’t forget about it. Periodic maintenance truly extends the life of the product. Also, users should keep the environment in mind. The actuator’s life decreases in harsh conditions, so use a bellow (or protective boot) when warranted.
Al/Danaher LMS:
Don’t exceed the duty cycle of the actuator, keep the load along the axis of the extension tube, and don’t ratchet the clutch at the end of stroke for extended periods.
Pay attention to how the equipment operates when it is new, and look for any changes in that operation as time passes. Follow all of the OEM’s recommendations for leading causes of premature mechanical failure. Another bit of advice: Have a spare! Even the most well-maintained actuator can be victim to a catastrophic situation, such as a collision with a forklift. John/Baldor: Linear motors can add great benefits, but only if properly used. Because there is no “NEMA” rating system, it is imperative that linear motor specs be fully understood. One manufacturer could rate a motor at 100 N continuous force, while another might claim an identical motor gets 140 N. Only evaluating these motors in a prototype provides the truth.

The two other main issues relating to reliability are drives and cabling. All servo and stepper drives are not created equal, and the differences from one manufacturer to the next are amazing, raising the importance of the actuator manufacturer’s recommendation when specifying linear motion. An inductance mismatch or bandwidth problem can play havoc in getting a system running right. Also, the technical help you will get from the vendor will be tenfold better if he is familiar with and understands the entire system.

Always use connectors on the moving portions of a linear motor actuator. Because of the high accelerations and velocities, cables wear at an accelerated rate in these systems. Angular accelerations in a small-radius cable track can tear apart the shielding and reduce cable life.

How do choices involving linear actuators affect other areas of the machine or system? Where do performance trade-offs arise and what other design considerations come into play?

Dave/Kollmorgen: The type of linear actuator selected can drastically affect machine performance and size. Trade-offs arise relative to productivity and cost expectations/limits.
Danielle/Bosch Rexroth: There may be performance trade-offs, for example, speed versus load, versus positioning accuracy. Machine footprint is one area that may not be obvious. You may actually be able to cut the process space requirements substantially by adding a third axis to the machine. Or, maybe you’re building the machine to accommodate all possible parts and thus need a large footprint. In this case, it may actually make more sense to build two machines. Designers should also remember that actuator selection influences the motor, drive, and control product choices.
There are indeed performance tradeoffs to consider when using linear actuators. For example, heavy loads need larger actuators, increasing the system size. They also mean slower cycle times. Thus, one small part can affect the entire system.
Al/Danaher LMS:
Compatibility with process controls may require position feedback or end-ofstroke limit switches for interlocks. Speed and load are also inversely related. The higher the load, the slower the movement. Lower loads may also permit higher duty cycles than full rated loads. Other necessary design considerations include envelope, restraining torque on the extension tube, and available voltage. John/Exlar: For pressing applications requiring high force and high precision, the actuator must have high stiffness, but the machine too must have adequate stiffness. Deflections in mounting surfaces are a common point of error in these applications. Applications requiring high speed and high force will require a power level proportional to speed x force. Slight compromises in top-end speed, or in high-end force can result in better machine performance, based on settling time in positioning applications, or required acceleration and deceleration torque in high cycle rate applications. John/Baldor: Linear motor actuators as a rule use linear encoders. These encoders have high output frequency -1 to 2 MHz. Most modern controllers have no problem with these encoder input frequencies, but older PLCs are not capable of processing information fast enough.

Also, a fully programmable PID loop tuning is required. Due to the direct-drive nature of linear motor-driven actuators, there is little dampening, so this must be done electronically in a PID controller. Mechanically, a machine must be capable of absorbing the extra loads incurred by increased accelerations. This could mean extra framework, or simply additional smarts in the control, such as implementing “S-curve” acceleration profiles. The other loads that must be accounted for are the attractive loads found in high-force iron-core motors. These loads range between 400 N and 30 kN, for very high forces. This means better bearings are not only for carrying motor loads, but also for keeping the encoders properly aligned. With linear motor actuators, iron or magnetic particles are taboo. Despite most attempts, if steel particles have access to the actuator, they will find a way inside. When using linear motors with magnetic materials, special care must be taken to keep the motors physically as far as possible from the contaminants. This also applies for encoders, as most popular optical encoders would fail quickly if exposed to dirt or fluids, and, while magnetic encoders would be more robust, they are not as accurate. This would imply that linear motor actuators should not be used in environments that a ball screw or belt would be used in; actually, dirt in the gears or nuts will quickly destroy these devices too.

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