This article discusses about engineers who are relying on mechanical simulations to improve their understanding of loading and operating conditions with high-rise escalators gain in popularity. To model an escalator design’s steps for dynamic analysis, LG engineers simplified the mechanical system by applying the drive directly to the upper terminal gear and the driving pulley without the motor. To model an escalator’s continuous elastic handrail, LG engineers divided it into 127 discrete rigid bodies, resulting in a total of 381 independent degrees of freedom. Two friction contact elements for each handrail body are applied to prevent rotational motion of the handrail. LG Industrial Systems managers decided to expand the number of engineers at the company with access to the simulation model. They assigned LG engineers to work with Computer Aided Design Software, Inc. (CADSI), of Coralville, Iowa analysts to develop a front end to DADS that simplifies the process of developing a custom escalator model.
With Escalators that provide longer and higher rise becoming increasingly popular in new building construction, engineers who design such systems. face significant new challenges. Traditionally, escalators have been designed based on static loads and torques, using physical experiments that are very expensive and time-consuming to perform. Another problem with the experimental approach is that it is very difficult to capture the actual dynamic operating conditions of an escalator in simplified static tests. Data collected during experiments are limited to specific locations where sensors can be located easily, raising the risk that they may fail to capture important loading conditions.
In an effort to overcome these problems, engineers at LG Industrial Systems in Changwon, Korea, considered the alternative of simulating escalator design on a computer. They investigated several major commercial software packages that are used by engineers in the automotive, off-road equipment, and aerospace industries to simulate and animate mechanical systems. Subsequently, they selected DADS (Dynamic Analysis and Design System) mechanical system simulation software from Computer Aided Design Software, Inc. (CADSI), of Coralville, Iowa. (CADSI can be reached on the World Wide Web at www.cadsi.com.) LG engineers chose this program because, in a number of trial problems they evaluated with the software, it demonstrated a superior ability to analyze the extremely complex, multibody mechanical systems involved in escalator design.
LG Industries was founded in 1968 to manufacture elevators and escalators. It has since grown to a $2 billion firm with nearly 10,000 employees and a wide range of products. Building systems, including elevators and escalators, are manufactured at a plant in Changwon, Korea, which also produces service equipment. Power transmission and distribution switchgear and automation systems are produced in Osan, while the Cheongju and Cheonan plants make control apparatus and components. The company also has an R&D complex in Anyang.
Simulations for a Higher Rise
To model an escalator design's steps for dynamic analysis, LG engineers simplified the mechanical system by applying the drive directly to the upper terminal gear and the driving pulley without the motor. Textbook equations for normal and tangential force were used to model the rolling friction contact and impact of rollers on rails and terminal gears, and sliding friction contact of the handrail on guides and the driving pulley. Material properties, such as the mass, center of gravity, moment of inertia, multiple nonlinear friction coefficient, stiffness, and damping coefficient of bodies, were determined either by three-dimensional computer- aided-design (CAD) software or experimental methods.
The model of the steps is composed of 58 step bodies and 116 roller bodies, and has 523 independent degrees of freedom. To consider the elastic effect of chains, rollers are connected by 174 spring and damper elements. All bodies •are assumed to be rigid, since they do not deform under operating conditions. The frame is fixed to the ground. The upper terminal gear is constrained to the frame by a revolute joint, while the lower terminal gear is set to the frame by a translational joint and a spring element to provide proper chain tension force. The upper terminal gear is driven at the angular velocity corresponding to the step velocity, 0.500 meters/ sec.
To model an escalator's continuous elastic handrail, LG engineers divided it into 127 discrete rigid bodies, resulting in a total of 381 independent degrees of freedom. Two friction contact elements for each handrail body are applied to prevent rotational motion of the handrail. To consider the elastic effect of the handrail, each handrail body is connected by 127 spring and damper elements, which are connected with spring elements. The rigid bodies used to model the handrail are as small as possible, in order to have at least one body in contact at the bending region. The driving pulley is constrained to the frame by a revolute joint and driven at the angular velocity corresponding to handrail velocity of 0.510 m/ so The lower tensioner and the belt tensioner are constrained to make proper tension force on the handrail. To consider the bending resistance of the handrail, bending resistance coefficients corresponding to the relative bending angle of each handrail body were determined by experiment and applied to the model.
The DADS model simulates the operation of the escalator. Output of the simulation includes such critical dynamic characteristics as acceleration, the reaction force of a roller, tension force of a chain, and torque of the upper terminal gear and the driving pulley. Before using the model for real-world design, LG engineers compared its accuracy to physical experiments. They measured the acceleration of an actual elevator step, using a low-frequency accelerometer and frequency analyzer. Force measurements were made with an experimental setup comprised of two load cells, jigs to connect chains on both sides of the step, an amplifier, and an analyzer. Experimental torque measurements were taken with a strain gauge-type torque sensor, an amplifier, and an analyzer. The torque sensor is set between the motor and. the reducer to measure the driving torque more precisely. All of the results tested were measured as the escalator was rising. Each experiment was performed several times in succession to ensure that measurement variations were "less than 10 percent.
The simulation results matched the experimental results very closely. The step acceleration in the simulation and experiment were 40 (cm/ sec2 ) and 45 gallons, respectively. The 10-percent error in magnitude could have arisen from excluding the motor and the reducer in the model. The reaction force of a roller rising near the upper terminal gear also agreed very well with the experimental result within 6-percent error. The difference of the reaction forces between simulation and experiment (in the location where a roller is on the rail) was caused by the weight of jigs used in the experimental setup. The a maximum tension force and the slope of the chain's tension force in the simulation matched those in the experiment within 3-percent error.
To correlate motor torque, the step and handrails were considered separately. The torque of the upper terminal gear and the driving pulley in the simulation was multiplied by the 24: 1 reduction ratio. Torque at steady state agrees well with that of the experiment within 9 percent.
Since it was validated, the model has dramatically improved the design process. Load time history data is used to evaluate the reliability of the base structure. With the computer model, the design can be examined in many ways and other design alternatives can be tried if the initial design is not satisfactory. Since the design can be validated even before the prototype is built, total design time can be substantially reduced. Even more important is the ability to tune and optimize the design without the cost of building hardware prototypes.
In short, engineers are able to develop a more durable structure with lower materials and labor cost, reduced energy requirements, and reduced noise and vibration levels. The model is particularly useful when simulating high-rise escalators, which are especially difficult design because the unique nature of that design makes it impossible to borrow from existing escalators.
Based on this success, LG Industrial Systems managers decided to expand the number of engineers at the company with access to the simulation model. They assigned LG engineers to work with CADSI analysts to develop a front end to DADS that simplifies the process of developing a custom escalator model. Once this front end is completed, engineers will merely have to enter key design parameters to create the model for analysis. This will make it possible for engineers without any experience in using DADS to create and evaluate their own models. Bringing the benefits of simulation to the company's entire engineering workforce will make it possible to improve and optimize every escalator the company designs and delivers to its customers.