02-06-2012, 04:28 PM
DESIGN OF DUAL ELEVATOR CONTROLLER
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INTRODUCTION
An elevator or lift is a kind of transport device used to move people between building floors. Whenever a passenger presses the call button for an elevator, a computer receives the request and logs it for future reference. There are actually two sets of doors, which allow passengers safely to exit and enter the lift. One set of doors remains shut until an elevator car's presence is detected and the elevator’s computer controls the other door. Once both doors are open, passengers should leave quickly to allow new passengers to board and more calls to be answered. Elevator doors also contain motion detectors and other presence-sensing devices to keep doors from trapping passengers. Another issue want to be considered is the weight capacity. Overload in elevator will lead to accident and using the load sensor can control it.
In modern life, elevators have become an integral part of any public or commercial complex. It does not only ease the faster movement between any two floors and provide a way for movement of disabled, but has also become a status symbol.
Elevators work on a gearless traction system in which the movement of the elevator is controlled by several steel hoist ropes and a counter-weight. The weight of the car and counterweight provides sufficient traction between the sheaves and the hoist ropes so that the sheaves can grip the hoist ropes and move and hold the car without excessive slipping. The machinery to drive the elevator is located in a machine room usually directly above the elevator hoist way. To feed electricity to the car and receive electrical signals from it, a multi-wire electrical cable connects the machine room to the car.
In this chapter, design of dual elevator controller is made using Verilog HDL and is verified by using the MODEL SIM and XILINX ISE tool.
BASIC PRINCIPLE
Elevators themselves are simple devices, and the basic lifting systems have not changed much in over 50 years. The control systems, however, have changed sub-stantially to improve safety and speed of operation. Elevators are designed for a specific building, taking into account such factors as the height of the building, the number of people traveling to each floor, and the expected periods of high usage.
Controllers can also be programmed to respond differently at different times of the day. For example, the elevator controller in a busy office building will receive a preponderance of calls from the ground floor in the morning, when workers are arriving and need to go to their workplaces on the upper floors. In that case, the controller will be programmed to send all unassigned cars to the ground floor, rather than have them return to a home floor in their sector. Later in the day, a different set of instructions can be used to send unassigned elevators to different sectors, since passengers leaving the building will be much more evenly distributed among the floors than in the morning.
Elevator Controller & it’s Working
Elevator controller controls the entire operation of the Dual elevator system. The proximity sensors located to sense the positions of the cars, provide the current state storing it in register. The obstruction sensors provide the status of obstruction. The elevator controller also reads the requests, if any, from any of the request positions through the flip-flops.
Introduction
The dual elevator controller designing in HDL has been a great challenge. The Moore model did simplify the approach, but in order to operate the system in optimized way and accept user request by the nearest elevator, highlighted the complexities involved in it. The elevator algorithm respects the constraints defined and works in alignment to the assumptions made.
The timer implementation as a part of the code remained an unresolved issue, as the coding should also have been synthesizable. The other issues that could not be addressed were to store the request and provide to the elevator at a later stage if the elevator is in busy state at that time. As the elevator system in for only two floors, the same could also be automated that once user gets into the lift, he does not need to request for the destination.
In general, the elevator controller has to control a larger group in a multi-floor building. Then, the capabilities of designing the algorithm of the designer decide the functionality of the elevator system.
Assumptions for the Elevator System
The Dual Elevator System does face some conflict in the operation that which car should take the request when both are at the same positions. So, some assumptions are implemented in the elevator algorithm. The assumptions considered are :
Elevator Priority: Elevators are prioritized for the requests. The elevator1 has a priority for the first floor request and the elevator2 for the request from second floor.
Default State: Elevator1 on first floor with closed door and the elevator2 at second floor with closed door. This default position provides quick response to the request coming at any of the two floors.
Closing the Elevator Door: Door of the elevator close after some time duration, defined by the timer. By default the timer should be 0 and when to close the door, it should be 1. The system also checks for any obstruction if present between the doors. Both, the timer and obstructions, are implemented using switch.
Thus, to close the door the door of an elevator, the respective timer needs to be triggered to high state and the obstruction should be 0.
Finite State Machine
A finite-state machine (FSM) is a mathematical abstraction sometimes used to design digital logic or computer programs. It is a behavior model composed of a finite number of states, transitions between those states, and actions. The finite-state machine is similar to a flow graph in which one can inspect the way logic runs when certain conditions are met. It has finite internal memory, an input feature (reading symbols in a sequence, one at a time without going backward), and an output feature, which may be in the form of a user interface, once the model is implemented. The number and names of the states typically depend on the different possible states of the memory, e.g. if the memory is three bits long, there are 8 possible states. The state that reads the first symbol of the input sequence is called the start state and the state which signifies the successful operation of the machine is called the accept state. Every state may lead to a different one depending on the next input symbol and output can be provided during the transition. State machines can also have probabilistic transitions, fuzzy states and other oddities.