Semi-Automatic Cable Car Turntable: How It Works
Introduction  -   Project Pictures  -   Movies  -   More Information
  We have developed a smaller scale turntable to demonstrate our design which can easily be implemented on the actual turntables. Our solution involves creating a motor-driven mechatronics system, which is actuated by an optical encoder. All components of this design are hidden beneath the deck of the turntable, giving the appearance of manual operation and therefore preserving the authenticity of the cable car spectacle.

 		 
 

Powell St. Turntable

  

Our design doesn't use any switches to operate the turntable because we wanted a system that is totally hidden and would emulate manual operation. Therefore, with our system, operators would still use the same operating procedure for the turntable. However, with the computerized encoder-motor system, the force needed to move the turntable would be greatly reduced. This highly responsive system senses the amount of force the operator is pushing and supplies a proportional amount of power to the motor to keep the amount force they are exerting to a constant low level. This means that the operator can accelerate the turntable up to high speeds with the same amount of force needed to rotate it at low speeds.

  
         
  TURNTABLE HARDWARE

The items colored in pink are items that would be needed to upgrade the actual turntables.

  
 

PART

DSP
Motor
Motor Wheel
Encoder
H-bridge
Power Supply
Support Wheels
4' x 8' x 0.5" Wooden board
Miniature Cable Car
1-inch diameter metal rod
2-inch diameter metal rod
2.5" x 2.5" x 3.25" Metal Block
4" x 4" Wood Post
1" x 1" x 8" Square Metal Bar
Nuts Screws and Bolts
Wiring

QUANTITY

1
1
1
1
1
1
3
1
1
1
1
1
1
4
Multiple
Multiple
 
COMPONENT LIST

1. Base
2. Base shaft support
3. Shaft
4. Table shaft support
5. Table
6. Pushbars
7. Support wheels
8. Spacer blocks
9. Motor
10. Drive wheel
11. Encoder
12. Encoder belt and pulley system
13. DSP
 
         
 

Turntable Schematic

  
         
  CRITICAL PART DESCRIPTIONS

      
  Digital Signal Processor (DSP)


  

The DSP is the computer of our system and can be mounted anywhere underneath the turntable, although it requires a stardard AC wall outlet. We use the digital input on the DSP for our encoder and the program we have loaded on the DSP calculates the output and sends out a pulse-width modulation (PWM) signal.

Connected at the top of the bread board in the picture is the H-bridge. It takes the PWM signal, amplifies it and powers the motor. The H-bridge is hooked up to an external power supply (blue-black wrapped wire) set at 23 volts and regulates the voltage fed to the motor (red-black wrapped wire).

 

 
  Encoder


   The encoder (blue item in the picture) is the sensor in our mechatronics system. It is coupled to the shaft of the turntable by way of a pulley and belt system. As it rotates, the encoder sends a pulse to the DSP for every 0.9 degrees that it has rotated. The DSP counts the pulses which we use to track the movement of the turntable. The encoder has 400 pulses per rotation and with the gear ratio that we have setup, we get approximately 1,750 pulses per revolution of the turntable.  
  Motor


   The motor that we use is also used in power windows of Porsche automobiles. It is connected to the H-bridge powered at a constant voltage of 23 volts and draws current between 0 and 3 amps. For simplicity, we decided to use a wheel to propel turntable but for the actual turntable we believe that the static friction force between the wheel and table will not be large enough for proper operation.  
  Wiring Setup
   This is a diagram of the basic wiring setup we have for our system. We also used buffer chips for our input and outputs with the DSP.  
     
 

TURNTABLE SOFTWARE

The combined speed of the digital signal processor and the 0.2 degree/count sensitivity of our encoder setup allows the implementation of a software program that can actuate the motor in real-time. Our program fluidly mimics the behavior of manual operation by amplifying rather than supplanting the force applied by the human operator. The instant an acceleration is imparted by the human, a proportional motor force is calculated and applied. When the human's contributed acceleration goes to zero, because he or she has stopped pushing or desires the turntable to go slower, the motor ramps down its applied force as if the table were experiencing a high friction force. Control occurs so quickly that the turntable instantaneously obeys the natural quirks and nonlinearity of the force applied by the human. This operation appears completely manual and only a small fraction of the force need be applied.

System Flow Chart


  
 

Introduction  -   Project Pictures  -   Movies  -   More Information