The phenomenal improvement in motion-control electronics is both a blessing and a curse for designers of control systems. It enables the designers to develop more sophisticated motion-control systems with higher levels of performance. But, it also expands the number of potential solutions that must be evaluated. However, a software design package that simulates the control system operation can both improve its performance and do it with less effort.

At a major elevator-door manufacturer in Connecticut, engineers wanted to lower development costs, raise product quality, optimize motion-control electronics, and speed the time to market of their door systems, Figure 1. Using a software simulation and design package, called VisSim, from Visual Solutions, Inc., Westford, Mass., they designed, prototyped, and optimized a door system in roughly half the time and cost of the previous trial-and-error method. And, this was only their first time using the program.

Door-system makeup

The elevator-door system consisted of the following five components, of which the first three are similar to those used in previous designs. Only the digital controller and encoder (last two items) were new:
• A ¹/₄-hp, armature-controlled dc motor with a rated speed of 85 rad/sec at 110 V. The motor drive controller regulates the armature voltage and current and provides a fixed dc supply for the field windings. This constant field excitation produces a fixed-field flux so the motor torque depends only on the armature current. Modulating the voltage to the armature controls the speed and the voltage polarity determines the direction of rotation.
• A 10:1 ratio gearbox to reduce the motor speed applied to the door. A standard bevel-gear arrangement was found to minimize system backlash at a reasonable cost. The gearbox is used in conjunction with a belt drive (see following item) to provide a combined speed-reduction ratio of 23:1.
• A linkage mechanism to transfer rotary to linear motion. The mechanism consists of a cogged belt drive and connecting rods between gearbox and door. Designers modeled the door and transfer mechanism as a mass-spring-damper combination. They used a nonlinear relationship between rotary and linear motion of the mechanism to convert beltsheave rotation to an equivalent door displacement.
• An incremental encoder, having a resolution of 1,024 parts per revolution, to sense the belt sheave rotation (up to 180 deg). An incremental encoder was selected, rather than an absolute type, because of its lower cost. The software converted the quadrature output signal from the encoder to a relative angle. And, the encoder was initialized when the door reached either the “full open” or “full closed” position.
• A microprocessor-based digital controller. Engineers first modeled this controller, which contains the control algorithm, as an analog computer that sends a continuous signal (zero time delay between signals). They also evaluated Intel 80386, 80286, and 8088 microprocessor chips, each at its respective operating speed. DSP chips were ruled out because of higher cost.

Performance specifications for the elevator door system were:
• Maximum door open and close times = 1.3 sec.
• Maximum motor armature current = 6 A.
• No perceptible door vibration allowed.
• One feedback sensor available, a digital encoder on the motor output shaft.
• At 1.3 sec, the door must be within 0.1 in. of full-open or full-closed position.

Simulating the door system

Because the cost of microelectronics varies widely, the designers concentrated on evaluating different options in this area.

The simulation software enabled the designers to create the door operating system, beginning with a system block diagram, Figure 2. Each of the major component blocks contains a simulation model. Standard models for an encoder, dc motor, gearbox, and digital controller were used to accelerate the design process. The software, running on a PC with Windows, enabled the design team to make changes and see the results interactively.

Control algorithm

Initially, the simulation was used to operate the motor at its rated speed while time, position, and speed data for a door-open cycle were recorded. Then the digital controller algorithm (model) used the recorded data to translate position error into a motor-velocity command. In effect, the controller compares two inputs — a door-position command and sensed door position — and sends one voltage output, a velocity command to the motor. The maximum speed recorded was 15.5 in./sec, which corresponds to a 110-V command signal, or 100% of the motor voltage.

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