Material handling, in most cases, refers to operations where components are moved between processes with little or no value added during the transition. In other words, there are no changes to the form, shape, or content of the materials being moved. As a result, material-handling interval times should be kept to a minimum. Other important considerations for material-handling operations include product stability (how the products are maneuvered or gripped), accuracy (how accurately the are positioned), and environment (clean room, wash-down, high heat, etc.)
How does a typical material-handling operation work?
There are three main components in a material-handling (MH) system: a controller, a motor or engine that converts energy into motion, and a mechanical system that employs the energy to perform a useful function. The controller, power device, and mechanical system could be any of a number of combinations.
|CONTROLLER||POWER DEVICE||MECHANICAL SYSTEM|
|CPU||Electric motor||Ballscrew |
|MH power devices generate the motion commanded by feedback-adjusted cpu signals.|
A MH system's CPU or controller monitors operator input and feedback signals, using the information to control the power devices that generate the motion. Feedback signals typically include the output from discrete sensors on the ends of the travel, as well as position (from proximity sensors and encoders), force, torque, and time measurements.
What are the primary types of motion encountered in material-handling applications?
Material-handling systems involve all sorts of motion. Some applications require the motion axes to operate independently, achieving point-to-point motion. Typically, the motion profile (velocity vs. time) is trapezoidal or triangular.
The acceleration profile for the standard trapezoidal move (the derivative of velocity vs. time) is usually characterized by abrupt step changes. One way to smooth the reactions from acceleration is S-curving. S-curve acceleration can accomplish a move in the same amount of time as trapezoidal, but without the jerking associated with the step changes. This allows products to move more smoothly, which is easier on the mechanics of the machine and sometimes critical for successful material handling.
In other MH applications, the axes are coordinated with each other in either linear or circular movements. In coordinated motion, all of the axes start and stop at the same instant, typically on the same interrupt or within microseconds of one another. The resulting ‘path’ of motion traces out a straight line (for a linear move) or an arc (for a circular move).
In a circular coordinated move, the individual axes move in sinusoidal relationship. For example, the X-axis motion may be a sine wave, and the Y-axis a cosine wave. The resulting path is an arc. Sometimes velocity is S-curved or ramped to its final value for smoother operation.
Yet another type of motion encountered in material handling is master-slave profiling, including velocity ratioing, gearing, and camming. On a gantry, for example, the motion on one side would be “geared” to the motion on the other to prevent twisting or skewing.
Some material-handling applications involve a mix of independent, geared, and linear/circular axes all on the same machine.
What are the main challenges when it comes to implementing motion in a material-handling process?
Speed — Typically the material needs to move as quickly as possible.
Accuracy — Some applications have very tight accuracy requirements and sometimes the parts being moved are very small or delicate.
Flexibility — If the process changes (say, if different sizes of products are being handled on the same machine) then the control system must be flexible enough to provide quick changeover.
Value added — Here, products are not just moving from one place to another; there are being processed along the way.
Safety — Sometimes the product being moved is a hazardous material; other times are there areas of movement where an operator may reach in or walk through and get hurt.
Where can motion technology make the biggest difference in material handling?
Motion technology allows designers to not only simplify a mechanical system, but also make it more flexible. Because a MH system can sometimes have severe time constraints, speed is at a premium.
Algorithms such as “S-curve” acceleration greatly reduce the amount of force not only on the material being moved, but also the mechanics of the system itself. These algorithms usually can be changed on-the-fly. This can be helpful when moving bowls of soup, for example. Empty bowls can tolerate faster accelerations than full bowls, and the time savings can be significant.
Advanced system controllers also allow some processes to be accomplished during the MH phase of an application. While monitoring the position of a move, discrete I/O can be manipulated while the motion is occurring. Sometimes motions are complex and interdependent. These can involve linear and circular movements between several axis, or master-slave arrangements in which one servo axis follows the motion of another device as is the case on a gantry.
Some controllers also let users compensate for system errors. Features such as registration or backlash compensation for lost mechanical motion can net big gains. Same is true for controllers that can monitor torque or tension and then compensate the end motion to stay within applications limits.
For more information on motion in material handling contact the authors. Sue Dorscheid can be reached at (920) 906-7804 or via e-mail at [email protected]; Ole Olson may be reached at (608)222-9000 as well as [email protected]