Most high-volume manufacturing involves some form of linear motion and often robots. Recently, manufacturing engineers have been transforming linear-motion modules into Cartesian robots with three or more axes. Doing so builds high-throughput automation equipment and at lower costs than the alternatives.

But in a rush to automate, designers occasionally overlook critical requirements when selecting components. The oversights lead to equipment that needs costly upgrades almost immediately. And over-engineered designs are more expensive and probably less effective than needed.

Manufacturing engineers must also decide which type of robot to use. Three styles dominate such considerations. Scara (selective compliant assembly robot arm) robots usually combine two rotary joints and a Z-axis. Six-axis robots include a couple more rotary joints making them more nimble than Scaras. And Cartesian designs, such as x-y-z gantry robots, have at least three linear axes but are made of modules that allow a greater reach than six-axis robots and in more configurations. A myriad of possibilities and conditions can easily overwhelm a person tasked with automating a critical process. Where is a good place to start?

Think “Lostped”

Motion-system engineers at my company have developed a mnemonic to help provide the best possible advice to those considering linear motion or robotic equipment. Application factors such as load, speed, and travel can be summed into the acronym Lostped. The simple term can remind engineers and designers who are gathering information to specify appropriate components or modules in any given application. Although we use it primarily to help people build Cartesian robotic systems from linear modules, the acronym has proven useful determining which type of robot to choose.

The acronym stands for Load, Orientation, Speed, Travel, Precision, Environment and Duty cycle — Lostped. Each of these factors must be considered when sizing and selecting a linear-motion system.

The best way to approach specification and system development is to work through each factor in the acronym, keeping in mind the special considerations for medical applications. For instance:

Load, of course, refers to the weight or force applied to the system. All linear motion and robots encounter some type of load, such as downward forces from lifting materials, or thrust loads in drilling, pressing, and driving screws. Indeed, any of these could be found in medical assembly or packaging tasks. Also consider varying loads as would occur in dispensing. For example, a robot deposits a reagent in a series of pipettes, thereby lightening its load at each step. Most medical applications involve relatively light loads that can be handled easily by any of the three types of robots. However, a few medical devices, such as beds, require high-load capacities. Linear modules usually provide enough load capacity and stiffness for these devices.

Key load questions include:

• How much weight or force must be managed?

• What is the source of the load and how is it oriented?

• Are there special handling considerations?

While considering load, it's also worth taking a look at the end-of-the-arm tool that will pick up the load. Although not related to load, mistakes here can be costly. For example, if a sensitive component will be grabbed by a pick-and-place application, the wrong gripper can crush it.

Orientation is often overlooked. It's the relative load direction. Because we are positioning something at the end of a robot arm, orientation is critical, along with the precision required. Obviously, technicians can position items anywhere within a given space, but may not be able to do so with predictable precision. Six-axis robots are perhaps the most flexible within their work envelope. They can use the space in any direction with the exception of the area occupied by their pedestal base. Cartesian or gantry robotics constructed from individual actuators, however, can use the frequently rectangular space exactly as the application needs.

Some modules and actuators handle greater up and down loads than side loads, because of the linear guide in the module. Other modules using different guides, handle the same load in all directions. One compact module, the Rexroth CKK, has a dual ball rails for guidance and is frequently used to handle applications requiring side-mounted and axial loads. It is especially useful in gantry robots used in drug discovery where modules are mounted upside down or sideways to protect work materials (reagents and wells) against contamination.

Key orientation questions include:

• Is there a preferred orientation for the robot?

• If so, is it vertical or upside down?

• If using a Cartesian system, where is the load oriented relative to the linear modules?

• Will the load cause a roll or pitch moment on the linear module?

Speed and acceleration in Cartesian and gantry robots rival those of articulated (6-axis) robots and are higher than Scara robots. Speed is a critical consideration because companies typically automate processes to increase throughputs. High speed, high stiffness, and short settling periods make gantry systems ideal for high-throughput screening.

It's also worth considering that applied loads create different forces during acceleration and deceleration than they do at constant speeds. Trapezoidal or triangular motion profiles must also be considered because acceleration needed to meet required speeds or cycle times will be determined by the type of move. (A trapezoidal move profile means the load accelerates quickly at first, moves at relatively constant speed, then slows. A triangular move profile means the load accelerates and decelerates quickly, as in point-to-point pick-up and drop-off applications.) With linear modules and gantry systems, speed and acceleration are also critical factors in determining what drive to use, which is typically a ball screw, a belt, or a linear motor.

Questions regarding speed should include:

• Is it constant or variable?

• What is the target speed or cycle time?

• How will the load affect acceleration and deceleration?

• Is the move profile trapezoidal or triangular?

• Which linear drive best addresses the requirements for speed and acceleration?

Travel refers to distance or reach. Gantry systems can use standard modules that come in lengths to 20 meters. Scara and articulated robots, however, offer strokes of only a meter or less, with the exception of massive 6-axis robots used in heavy automotive applications. The long lengths possible with module-based systems makes them ideal for dispensing blood or adding liquid pharmaceuticals to test tubes or bottles in long rows.

Key travel questions include:

• What is the distance or range of motion?

• Is overtravel necessary in case of an emergency stop?

• If so, how much overtravel?

Precision defines either travel accuracy (how the system behaves while moving from point A to B), or positioning accuracy (how closely the system reaches a target position). It can also refer to repeatability, or how well a system moves to the same position at the end of each stroke.

Each type of robot offers different performance characteristics for each type of precision. For example, continuously smooth transfers at moderate speeds may be preferable for sensitive components because high-speed transfers can be jerky. Understanding the difference between the three terms- travel accuracy, positioning accuracy, and repeatability-is often critical to ensuring the overall system meets its specs.

The drive mechanism is the main reason to think through precision requirements for linear-module designs. Will it use a belt drive, ball screw, or linear motor? Each offers trade-offs between precision, speed, and load-carrying capacity. The best choice is dictated mostly by the application.

Questions regarding precision include:

• How important are travel accuracy, positioning accuracy, and repeatability?

• Is precision more important than speed or other Lostped factors?

Environment refers to the surrounding conditions in which the system will operate. For example, extreme temperatures affect the performance of plastic components and lubrication, while dirt, liquids, and other contaminants damage bearing raceways and load-carrying elements. In a laboratory, it may be necessary to test the effect of certain chemicals on anticipated materials to ensure they will not damage anything if they are inadvertently splashed. Cartesian systems are easily customized to withstand a range of environments by adding specially plated components to combat rust, or coatings to protect from chemical splashes and vapors.

The work environment is often overlooked, but it greatly influences the application's overall success. Options such as special lubrication and positive air pressure in Cartesian and gantry systems, for example, let designers certify them for class 10 clean-rooms, thereby reducing or eliminating risks of contamination by mechanical systems.

Environmental questionsinclude:

• What types of hazards or contaminants are present?

• In the opposite case, is the linear-motion system a potential source of contaminants through electrostatic discharge, lubricants, or particulates?

Duty cycle is how long it takes to complete one operating cycle, but the idea includes machine life as well. Internal components in linear actuators generally determine the life of the final system. Bearing life, for example, is directly affected by applied loads along with duty cycle. A robot may meet the previous six factors, as determined by the application, but if it runs 24/7, it will reach the end of its life much sooner than if it runs only 8 hr/day, 5 day/week. In addition, the amount of in-use time versus rest time influences heat build-up and directly impacts life and cost of ownership. Clarifying these issues in advance saves time and aggravation later, because wear parts such as belts are easily stocked for rapid replacement. High-throughput screening and pharmaceutical manufacturing typically require 24/7 duty cycles, so it's important their systems have long lube intervals and low maintenance requirements.

Questions regarding the duty cycle include:

• For what length of time is the system in continuous use, including dwell time between strokes or moves?

• What is the required service life of the equipment?

Some final advice

While Lostped provides seven basic factors and key questions to guide the sizing and selecting of linear-motion systems, best results come from consulting a trusted distributor, or the manufacturer's application engineering department. They have been exposed to hundreds of applications, and possibly several dozen similar to yours. Factory experts may also be able to steer you to a machine builder with special expertise in medical applications. With all this help, modular manufacturers may cut substantial time and costs by anticipating potential problems and considering special requirements. The goal is to get the best system possible at the lowest cost of ownership. Lostped can assist getting just that.