Presentations by three electrical engineers at the Vail Tramway Seminar gave a fascinating look at various approaches to lift controls, their philosophies and dreams of the future. Also brought out in these presentations was a comparison of the state of the art in electrical controls for tramways in North America and Europe.
Basic Controls
Phil Berger of Enduratek stressed the basics of controls and circuitry emphasizing simplicity, safety and serviceability. Basic control in its simplist form is a stop/start switch near the motor. Complicating this basic control is “first” remoteness of various portions of the lift which preclude direct wiring of power to that point, therefore, remote signaling devices were introduced.
Berger introduced another most important variable to the control system: “that nebulous area called safety.” Too often it can and has “lead to a proliferation of switches and controls that have taken our simple tramways into the realm of complex control systems.”
What about serviceability for the end user? Berger pointed out that a simple on/off switch arrangement with a single motor is easily serviced. If the tramway failed to run, it would be easy to locate the fault. Each time a switch is added, one more possibility for failure is also added, which also adds to the time required to pinpoint the fault. It is quite simple for a designer to add switches and relays to accomplish whatever he desires, but the designer should also consider the ramifications of its potential for failure and ease of pinpointing the failure. Berger said designers seldom consider these ramifications. Various authorities, codes and standards also add to the requirements for additional switches, safety devices and circuitry, again adding to the potential for failure and longer downtime in pinpointing the fault. Controls can be built to meet operating requirements, but only occasionally are they designed with ultimate serviceability in mind.
Control monitoring or fault locating circuitry is then introduced to expedite finding problems which can vary from simple wheatstone bridge detection locating to sophisticated digital arrays, computors, transponders and microprocessors. Actually, there are three options for circuit monitors: a single pair of wires to each switch, a pair to a number of switches in a zone (i.e., all top terminal safeties) and a complex monitoring system with microprocessor. This introduces the nebulous term “fail-safe.” Monitoring switches is easily accomplished with electronic devices across them, but bringing in the fail-safe aspects, most resistors, tranducers and diodes fail to create a non-fail-safe situation. Static electricity and lightning generally cause such failures. Standards also require detection of open or shorted circuits which complicate it more. Most designers fall back to what Berger calls the “resistance or current loop” method for monitoring, using a simple two-wire system. This is accomplished by having a separate operating and running circuit, with another circuit for troubleshooting.
Summarizing, Berger emphasized that designer consideration for electrical controls must include simplicity, safety and serviceability. “Designs look great on paper and even work great under laboratory conditions,” he said. “Time alone will determine the ultimate reliability and value of all designs.”
Computer Controls
James K. Bunch, P.E. of the Steamboat Ski Corporation, expanded upon Berger’s presentation with the “state of the art” and “dreams” of present and future tramway controls. Included with the controls of today are the computer processed monitoring and fault locating systems relative to modern tramway designs. Berger refers to these more modern control systems as “intelligent control systems” and points out that electronic technology has made great advances allowing computers with speed and accuracy to come into the industry environment at affordable prices. The computer used in the control systems he discussed is the microprocessor type.
In 1977 the Steamboat Ski Corporation made a decision to introduce this “state of the art” electronic microprocessor technology into the ski lift controls under the direction of their electrical staff which included engineers Jon Banks, Dave Herman and James Bunch. First, a cost analysis was done which indicated costs would be comparable to the “relay systems” as we know them today. Therefore, management made the decision to proceed into the realm of computer operated tramways.
The typical intelligent control system can be broken down into three basic components. First is the central processing unit or MPU which sends and receives data and instruction and performs arithmatic functions. Second is the memory component consisting of a series of locations that contain binary coded words or bytes, each having its own exclusive sequentially address. (Recent technology has developed permanent memory which remains unchanged even when the system’s power is turned “off” and many are now available to control system designers.) The third major component is an input/output section and related devices which are the system’s communications link to the real world. These I/O units are the analog or digital types. The latter is single or digital bytes of binary data represented by high or low voltage, or open or closed contacts. Analog, which requires digital-to-analog converters, measures analog signals such as lift speed or motor current. The entire system is driven and coordinated by a system clock which causes all devices to respond to system commands in synchronism.
A single microprocessor can process and perform instructions at an average rate of 200,000 operations per second. Even though some of the monitoring and control logic may require thousands of steps, it can be completed extremely fast compared to the human processor. Bunch said that the Steamboat control MPU is in wait condition 50 percent to 70 percent of the time. The particular system chosen by Steamboat can run on the lift’s 24 VDC battery supply because it requires only 5 VDC to operate fully and the unit can handle a lift up to 6000 feet in length. The basic monitoring system used is the basic constant voltage current loop system. The safety system is installed so that it can stand independent of the control system in the event of a computer failure. Also, as with many lifts, further bypasses are available for evacuation and maintenance service.
A basic two-wire control circuit with a separate monitoring circuit under the direction of a computer can go beyond just locating faults and malfunctions. It can monitor, time, check and doublecheck the control system at extremely fast speeds and if something does fault or fail it displays and records what, where and when. Operators and attendants are continuously given screen readouts showing what the lift is doing, why it stopped and when and how to restart it.
Because of speed, low cost and dependability, redundant controls are now a consideration. Recent code requirements have moved the industry further in this direction. Such microprocessors allow us to do this and monitor it quite easily with dependability. The systems will work on lifts up to 10,000 feet long. Steamboat’s average system cost is in the order of $20,000. Recorders are attached which then record all stops, starts, current loads, etc. which give management another tool to evaluate the efficiency of their lift systems.
Bunch said he was dreaming of the microprocessor computer-run lift only a few years ago and now it’s reality. What about the future? Video image processing? This is a technique now being used in special military and industrial applications, which digitizes video images into binary bytes allowing a processor to “see” the image and analyze it. An image processor may be a more attentive and reliable observer of various lift functions and operations.
European Controls
The last presentation, written by Alex Kaufmann and presented by Erwin Amstad (both with Frey A.G. Stans, Switzerland), was a comparison of the “state of the art” electric and electronic controls in the USA versus those in Europe. This was an excellent update on how various codes and standards in European countries have influenced safety devices, controls and electric drives. Most advances in technology over there have been guided toward the large gondola and tram systems. In this area, “the state of the art” is quite comparable. Of interest, was that some government regulations in Europe will not allow the use of SCR drives on people-moving equipment, such as tramways and elevators. As explained in Berger’s talk, the question of allowing monitoring devices in parallel across safety device contacts is also heavily debated in Europe.
It appears the electronic industry is moving at about the same rate in the USA, but apparently some European governments are less receptive to such advances being adopted for tramway applications which holds the “state of the art” somewhat behind. Basic drives, controls and circuitry are very similar to that used here.
Of interest in the discussion period following these three fine presentations were comments regarding the “state of the art” for tower derailment detection devices. All present seem to agree a better device is necessary and this “state of the art” is much behind control “state of the art.”
