Virtually all manufacturing processes incorporate some type of linear motion. A common mistake that designers make when sizing and selecting linear motion systems is to overlook critical application requirements in the final system. This can lead to redesigns, and may also result in an over-engineered system that is costlier and less effective than desired. “LOSTPED” is a simple acronym that guides the designer in gathering the information needed to specify the appropriate linear motion components or modules in any given application.
What Is LOSTPED?
LOSTPED stands for Load, Orientation, Speed, Travel, Precision, Environment, and Duty cycle. Each letter represents one factor that must be considered when sizing and selecting a linear motion system. Each factor must be considered individually as well as in conjunction with the others to ensure the best overall system performance. For example, the load imposes different demands on the bearing system during acceleration and deceleration than during constant speed movements. As more linear motion solutions move from individual components to complete linear module or Cartesian systems, the interactions between system components become more complex, and designing the right system becomes more challenging.
Following are descriptions of each LOSTPED factor, as well as key questions to ask when determining the criteria to size and select a linear motion system.
Load refers to the weight or force applied to the system. All linear motion systems encounter some type of load, such as downward forces in material handling applications or thrust loads in drilling, pressing, or screw-driving applications. Other applications encounter a constant load, such as a semiconductor wafer-handling application, in which a FOUP (Front-Opening Unified Pod) is carried from bay to bay for drop-off and pick-up. A third type is defined by varying loads, such as a medical dispensing application, where reagent is deposited in a series of pipettes one after another, resulting in a lighter load at each step.
- What is the source of the load and how is it oriented?
- Are there special handling considerations?
- How much weight or force must be managed?
- Is the force a downward force, lift-off force, or side force?
The orientation — the relative position or direction in which the force is applied — is also important, but often overlooked. Some types of linear modules or actuators can handle higher downward/upward loading than side loading because of the linear guide system used in the module design. Other modules, using different linear guides, can handle the same loads in all directions (Figure 1).
- How is the linear module or actuator oriented?
- Is it horizontal, vertical, or upside down?
- Where is the load oriented relative to the linear module?
- Will the load cause a roll or pitch moment on the linear module?
Speed and acceleration affect the selection of a linear motion system. An applied load creates far different forces on the system during acceleration and deceleration than it does during a constant speed movement (Figure 2). The type of move profile — trapezoidal or triangular — must also be considered, as the acceleration required to meet the desired speed or cycle time will be determined by the type of move required. A trapezoidal move profile means that the load accelerates quickly, moves at relatively constant speed for a period of time, and then slows down. A triangular move profile means the load accelerates and decelerates quickly, as in point-to-point pick-up and drop-off applications. Speed and acceleration are also critical factors in determining the appropriate linear drive, which is typically a ball screw, a belt, or a linear motor.
- What speed or cycle time must be achieved?
- Is it a constant speed or variable speed?
- How will the load impact acceleration and deceleration?
- Is the move profile trapezoidal or triangular?
- Which linear drive will best address the speed and acceleration needs?
Travel refers to the distance or range of motion. Not only must the travel distance be considered, but also overtravel (Figure 3). Allowing some amount of “safety travel,” or additional space, at the end of the stroke ensures the safety of the system in case of an emergency stop.
- What is the distance or range of motion?
- How much overtravel may be required in an emergency stop?
Precision is a broad term that is often used to define either travel accuracy (how the system behaves while moving from point A to point B), or positioning accuracy (how closely the system reaches the target position). It can also refer to repeatability, or how well the system moves back to the same position at the end of each stroke. Understanding the difference between these three terms can be critical to meeting performance specifications and not overcompensating for a high degree of accuracy that may be unnecessary.
But the main reason to think through the precision requirements is drive-mechanism selection: belt drive, ball screw, or linear motor. Each type offers trade-offs between precision, speed, and load-carrying capacity, and the best choice is dictated mostly by the application.
- How important are travel accuracy, positioning accuracy, and repeatability in the application?
- Is precision more important than speed or other LOSTPED factors?