Linear Motion Selection Criteria: POSTLUDES

POSTLUDES
Linear Guides, Linear Shafting, Plain Bearings

There are many factors to consider when selecting the correct linear motion system for your specific application. P.O.S.T.L.U.D.E.S is an acronym which will quickly guide you to asking the right questions to determine the best product for the job.

You may be familiar with the acronyms LOSTPED and LOPSTED, which contain letters representing Load, Orientation, Speed, Travel, Precision, Environment, and Duty Cycle. These are seven of the most basic key elements to consider when selecting and sizing a robotic or motion system. Each should be considered independently and jointly, as the combination of requirements will sometimes lead to the qualification or disqualification of a potential solution. These acronyms have been in use for some time now, albeit with two omissions: a “U” for Unknown and an “S” for Safety.

To remedy this omission, we can add a “U” and an “S” to the end of the two existing acronyms to form LOSTPEDUS and LOPSTEDUS, or we can create a new acronym: POSTLUDES. Many of us are familiar with the word prelude, meaning an opening performance, action, or event. Postlude is simply the opposite, meaning a closing performance, action, or event. Any of these three helper acronyms will do, so choose the one that’s easiest to remember.

P — Precision
  • What is more important, accuracy or repeatability?
  • What is the accuracy/repeatability requirement?
  • Are these values realistic, based upon the desired motion profile?

Precision is an often misunderstood and misapplied element. Instead of the word precision, let’s use the words accuracy and repeatability, although people often confuse the two. In general, repeatability is more important than accuracy. That said, customers often request an accurate system. Accuracy can be defined as the difference in position between where a system actually is, and where the controller thinks it is. Repeatability is defined as the difference in position when a system returns to a location under the same circumstances (i.e., same direction and motion profile). Because of backlash and “slop” within the mechanics, there is often a big difference between single directional repeatability and bidirectional repeatability. Most systems have a bidirectional repeatability rating that is much worse than the single directional repeatability rating.

When combined with the speed element, things can get complicated. Most systems with high dynamic performance levels (high velocity and acceleration) tend to have difficulties with high accuracy requirements. These systems often tend to overshoot their target location, then reverse direction to “hunt” for the intended position. This can be problematic for some applications. Note: Linear motor based systems do not typically have as extreme of a problem as rotary servo motors due to the larger force they typically exert on the system; however, they are much more expensive than other comparable systems and if the application’s force requirement is near that of the peak force of the linear motor, it will take longer to settle.

O — Orientation
  • How will the system be mounted? (“normal”, on its side, inverted, vertical, at an angle, etc.)
  • How will this affect the load requirements?

The orientation of a system is often taken for granted, the assumption being that the normal (horizontal) orientation will be utilized. This is a faulty assumption, because a system that works in the horizontal orientation may not work if it is inverted, and likely will not work as intended if the system is vertical. When orientation is combined with load, there can be a dramatic impact on the performance of a system. For example, not all systems can support the same payload in the normal (horizontal) orientation as they can when the payload is inverted or mounted on its side. A system that works fine in a horizontal orientation may not work properly if moved to a vertical orientation, as the motor must now directly overcome the force of gravity.

S — Speed
  • What is the maximum speed and acceleration required?
  • What is the maximum jerk allowable?
  • What motion profile (shape) is desired?

The speed of a system is much more than the maximum velocity. If you were to ask a customer to define maximum velocity, maximum acceleration, or desired motion profile, they might find it difficult; but they could easily explain how they need their equipment to produce X parts per hour or move from A to B in so many seconds. Once this requirement is established, simple mathematical calculations will reveal the maximum velocity, acceleration, and motion profile. If a customer needs to move a distance of two meters, there’s a significant difference between doing it in, say, ten seconds, versus a single second.

An often overlooked factor is “jerk”, which is the rate of change of acceleration. For example, slowly depressing a vehicle’s accelerator results in a low jerk value, quickly depressing the accelerator yields a medium jerk value, and smashing into a brick wall would produce a very high jerk value indeed.

Motion profile is a term used by control engineers to describe the motion of a system by defining position, velocity, and acceleration (sometimes jerk) versus time. The two most common motion profiles are triangular and trapezoidal.

T — Travel
  • What is the required travel (stroke)?
  • What is the overall envelope allowed?
  • How much over-travel (safety zone) is required?

Travel is one of the easiest elements to define; however, there are elements to this variable that are often overlooked. The three elements that need to be defined are stroke, overtravel, and overall envelope. Stroke is the distance the system needs to travel. Overtravel is additional travel used to compensate for errors during installation (e.g., misalignment) and extra travel during an emergency stop situation. Overall envelope is the total space available to the motion system. Some systems have very little additional space around the motion system, which can make component selection difficult.

L — Load
  • What is End of Arm Tooling (EOAT) and where is it located?
  • What additional forces are seen by the system during use (e.g., cutting or pushing forces)?
  • What do the static and dynamic free body diagrams look like? Have all loads been accounted for?
  • What is expected of the system after an impact?

Load refers to the forces acting upon a system. The load starts with the End of Arm Tooling (EOAT) and the payload. The EOAT is often permanently attached to the robot or motion system and performs some type of work. Typical examples of EOAT include: grinders, welders, suction cups, grippers, dispensers, vacuums, and spindles. The EOAT usually has an additional load applied to it. This load can come in the form of a weight being transported by the system or forces acting upon the system if the EOAT is a cutting tool or pusher.

There are three components to “load”: static, dynamic, and impact. Important: People often forget about the dynamic condition, and almost everyone forgets about the impact condition. Dynamic and impact load conditions are often forgotten because they cannot be seen on a drawing.

A static load condition occurs when the system is fully loaded and at rest. This is the easiest condition to describe. Additional forces, such as acceleration and deceleration, act upon the system while it’s in motion. There’s also the impact load condition to consider. What will happen when the system crashes? Is it expected to survive? How many crashes is the system expected to survive? Should a “weak link” component be designed in as a failure point?

U — Unknown
  • What are the known unknowns? What is a reasonable value for these unknowns?
  • How will someone misuse this system?
  • What could possibly go wrong?
  • What else could go wrong? (Repeat this question until you can’t think of anything else!)

No matter how much planning goes into a system, there will always be unexpected variables which can negatively affect performance. The most common factors are actually seen by most systems! Two of the most common are extra load caused by misaligning parallel linear guides, and extra drag from cables and cable carriers. Both can be overcome by allowing for an adequate safety factor when sizing system components. These factors are often referred to as the “known unknowns” because you know they exist and will affect the system, but you don’t know to what extent.

In addition, designers need to consider how their system might be misused. Is it likely the end user will make the system go faster or carry a heavier load? Will a maintenance technician stand on an important piece of equipment to gain access to something else? These things do happen, and it’s important to plan for them.

D — Duty Cycle
  • What is the actual duty cycle for the system?
  • What is the expected lifetime?

Duty cycle is an often miscalculated value. For example, a factory runs a robot eight hours a day. During those eight hours the robot is in almost constant motion. The most common mistake people make is to wrongly assume the duty cycle is 33 percent (8/24 hours). The duty cycle is actually 100 percent, because when the system is in use, it’s in constant motion. The easiest way to calculate duty cycle is to look at a single move. How long will it take to make the single move, and how long will the system rest between each move? Many motion control component manufacturers publish a defined maximum allowable duty cycle, and some will even reduce the rated load capacity when their component is used in a high duty cycle application.

The second aspect of duty cycle to consider is how long a customer expects their system to last. They will typically state a certain amount of time, and it will be necessary to work backwards to figure out the total distance traveled in that time, and the total hours of operation. Motion components have a lifetime in meters or inches, and electrical components have a lifetime listed in hours.

E — Environment
  • In what environment will the system be installed?
  • Are there any hazards in the environment?
  • What is the maintenance schedule, and is the system accessible for maintenance/lubrication?
  • Are there contaminants in the environment which can damage the motion system?
  • Will the system disburse contaminants into the environment that could damage other equipment or products

There are two ways to consider the environmental effects for a motion system. First, you must consider how the environment will affect the system; and second, you have to consider how the system will affect the environment. Is the environment at a temperature extreme (i.e., hot or cold)? Are there contaminants in the area, such as dirt, chips, or liquids? Is a vacuum applied? Are there vibrations or other shock loads that could be applied directly to the system or to the area around the system? Will the system be installed in a clean environment where particulate generation could be an issue?

Finally, where and how will the system be installed? What surrounds the system? Can a technician easily gain access to perform preventative maintenance? Does the system require lubrication storage because there will be no maintenance?

S — Safety
  • Are there any safety standards to which this system needs to conform?
  • What could happen if the system fails?
  • Are there safeguards that need to be installed for a system failure?
  • Could people be injured by this system? If so, how will the system be protected?

In today’s “litigation happy” world, addressing safety issues is more important than ever. Nothing can sink a company faster than a lawsuit, and many insurance companies refuse to cover an accident if a company has intentionally neglected to install required safety equipment. Tougher standards have been imposed upon industry by governmental regulation and protection agencies (e.g., OSHA, CCOHS, EU-OSHA) and industry standards (e.g., ANSI Z##, ISO13849, IEC61508, EN61508, EN954, UL, CE). To determine which safeguards to include, it is very important to ask a customer if there are any special requirements to which their system needs to conform, especially if human beings will interact on any level.

Also consider what might happen during unusual and/or unexpected events. What will happen to this system and the payload during a power outage or natural disaster? Will the payload be safe? Will the people around the system be safe? What needs to be done to make the system safe, and how much will it cost?

DETAILS

Components:
RST Simplicity Plain bearings

Application strengths: Simplicity bearings are resistant to many adverse environmental factors, outperforming recirculating ball bearings under these tough conditions