The control project replicates a dumbwaiter system from a known restaurant. The inspected system is antiquated with an approximated manufacturer’s year in the 1930s. A scaled-down replica was modeled and built. A control system was integrated to – both – improve and modernize the dumbwaiter. Improvements were based on the largest concerns with the vintage waiter.

1. Control System
Dumbwaiter 2.0
“The dumb turned smart”
Presented by: Anthony Nguyen
Ovidio Pérez
Roderik Nelson Saleh Alvarado
MCEN 4043
System Dynamics
Prof. D. Reamon
University of Colorado, Boulder
Friday, August 5th 2011
Design Implementation Analysis

3. MCEN 4043 Dumbwaiter 2011 Summer 2011
Table of Contents
Introduction ................................................................................................................................ 1
Background................................................................................................................................ 1
Mechanism & Functionality ........................................................................................................ 2
Initial design ........................................................................................................................... 2
Iterations ................................................................................................................................ 2
Final Design ........................................................................................................................... 4
Control system ........................................................................................................................... 7
Overview ................................................................................................................................ 7
Programming .......................................................................................................................... 7
Front Panel ............................................................................................................................. 9
Analytical Model ........................................................................................................................10
Equations of motion ...............................................................................................................10
Simulink Model ......................................................................................................................11
Results ..................................................................................................................................11
Conclusion & Commentary........................................................................................................13
Appendix A: Bill of Materials....................................................................................................... A
Table of Figures
Figure 1: Initial dumbwaiter design ............................................................................................. 2
Figure 2: The IR range finder that monitored the bus height....................................................... 3
Figure 3: Final design of the dumbwaiter.................................................................................... 4
Figure 4: Photo image of op-amp ............................................................................................... 5
Figure 5: Schematic of op-amp for motor operation ................................................................... 5
Figure 6: Reduction gearbox that increases available torque of the DC motor ........................... 6
Figure 7: Block diagram of the control code ............................................................................... 7
Figure 8: Sub VI that translates voltage readings to distance in inches ...................................... 7
Figure 9: Sub VI that programs floors into the control system .................................................... 8
Figure 10: Front panel of the control system program ................................................................ 9
Figure 11: Block diagram for analytical model of system ...........................................................11
Figure 12: Simulink response – rise from cellar to terrace .........................................................11
Figure 13: Acquired measurements – rise from cellar to terrace ...............................................12
Figure 14: Simulink response – lower from terrace to cellar ......................................................12
Figure 15: Acquired measurements – lower from terrace to cellar .............................................13
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4. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Introduction
The control project replicates a dumbwaiter system from a known restaurant. The inspected
system is antiquated with an approximated manufacturer’s year in the 1930s. A scaled-down
replica was modeled and built. A control system was integrated to – both – improve and
modernize the dumbwaiter. Improvements were based on the largest concerns with the vintage
waiter.
Background
The project was inspired by a restaurant familiar to the project team. It’s a two story
establishment whose kitchen and bar are located on the first floor. Food and drinks are taken to
the upper level where patrons are accommodated. A dumbwaiter is used for the purpose of
transporting foods and dishes between floors. The function may be compared to that of a
person elevator.
A 1930s vintage mechanism lifts and releases the dumbwaiter. There is a single toggle switch
that turns on an electric motor, which in turn drives a power screw. The load surface measures
20 in. by 18 in.
Operators toggle the switch remotely with a dog leash when not on the same floor as the
interface. (There is only one control interface, which is located on the kitchen floor.) There are
several concerns attached to the system operation. The wait staff commands the lift to stop
based on visual estimates and experience. The system hasn’t been configured for bounds or
recognition of limits. Other concerns include a continuing motion of the dumbwaiter even after
the switch has been toggled off.
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5. MCEN 4043 Dumbwaiter 2011 Summer 2011
Mechanism & Functionality
Initial design
Early designs sought out to replicate the dumbwaiter at the restaurant. Serious consideration
was lent to the inclusion of a power screw. It would serve to support the bus and drive the same
bus up and down the screw. The screw itself was to be powered by a DC motor via a belt, as
depicted in figure 1. The bus, which was modeled as a flat plate or tray, would have been
stabilized by two guide rails. The control box was designed to travel with the bus and, itself,
would have had a load cell to determine the payload. The payload information would be diverted
to control box for computations and response such that the bus would accelerate accordingly.
However, it was identified that the infrared (IR) range finder played a more important role to the
control system than the load cell.
DC motor
Guide rails
Power screw
Bus with load cell
Control box
IR range finder
Figure 1: Initial dumbwaiter design
Iterations
An actual modernization of the dumbwaiter required an update of the operational parts. The
power screw was indeed a vintage machine and has been replaced by more efficient systems
that also provided quicker actions. Fast motions and appreciable accelerations were needed to
demonstrate the effectiveness of the control system. The power screw became unsuitable and
its implementation was scrapped.
Although a DC motor was still desired, it would power a shaft with a spool at the tip
instead. A cable system fed into the spool and was also attached, at the other end, to the bus.
This interface was designed on the ceiling of the bus, which was transformed into a boxed
structure. The bus was designed to suspend by the cable within a towered structure. This
iterative step came closer to modern dumbwaiters, which closely resemble person elevators.
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6. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Research into person elevators provided major advances for the dumbwaiter. The end-on-end
cable system was modified and a counterweight was introduced. The system was designed
such that, at any height, the bus would suspend in equilibrium due to the presence of the
counterweight. The counterweight was placed to move external to the support structure. This
system implied further that much less motor output was required for motion and that the system
could act and response fast.
Across redesigns it became evident that the load cell would complicate the system
unnecessarily. The IR range finder alone was sufficient to accomplish the sought control
system. As such, the implementation of the load cell was disregarded. The range finder is
shown below.
Emitter
Input / Output Receiver
Figure 2: The IR range finder that monitored the bus height
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7. MCEN 4043 Dumbwaiter 2011 Summer 2011
Final Design
The design of the dumbwaiter was practically an elevator at the point of final design. It consisted
of a bus travelling up and down a towered structure, as shown in figure 3.
DC motor & gear box
Chains
Guide wheels
Bus
Support structure
Counterweight
IR Range finder
Figure 3: Final design of the dumbwaiter
The cables were switched out for chains as illustrated. Accordingly the spool was replaced by a
toothed wheel. These replacements resolved concerns of cable slip with the spool and
transverse stability of the bus in the shaft.
An amplifier, operational amplifier specifically, was needed to operate the motor. The op-amp
schematic is shown in figure 4. The circuit was made based on the schematic and is also
shown.
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8. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Figure 5: Schematic of op-amp for motor operation
Diode - FR104
TIP31C
Resistors – 1.0 KΩ
IC – LM358N
TIP32C Diode - FR104
Figure 4: Photo image of op-amp
Insufficient torque was delivered by the DC motor. The increased weight of the bus demanded
more torque, which was unavailable even at maximum voltage. A gear box, of two-step
reduction, was developed to overcome this issue and became very effective. In fact the required
torque was being delivered with no apparent trade-off in speed. A series of compound gears
gave a gear ratio of 1 to 25 (input to output) and is shown in figure 6.
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9. MCEN 4043 Dumbwaiter 2011 Summer 2011
Two-step
reduction
DC motor
Chains
Figure 6: Reduction gearbox that increases available torque of the DC motor
Bus speeds were greater than expected which led to stability considerations. The chains offered
transverse stability as aforementioned. Secondly, guide wheels were brought onto the bus.
They tracked the sides of the structure, which was especially designed to encompass this
feature.
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10. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Control system
Overview
A LabView program VI set was created as the interaction and command port for the dumbwaiter
build. The main VI shown in figure 7 provided a closed loop control system that converts voltage
readings from the IR range finder to an end result desired distance.
Figure 7: Block diagram of the control code
Programming
The main VI used a data acquisition (DAQ) input function to bring in a voltage reading from the
Sharp IR range finder positioned beneath the bus. The raw voltage reading was then filtered
using a high-pass signal filter set to 10 Hz. The voltage signal was further filtered using a mean
function to average the continuous readings of the DAQ input. A sub VI (figure 8) was employed
to convert the filtered IR data to a distance in inches values using the LabView – mathscript
function that allowed script style equation editing.
Figure 8: Sub VI that translates voltage readings to distance in
inches
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11. MCEN 4043 Dumbwaiter 2011 Summer 2011
Actual distance values were then subtracted from a user defined desired position values. This
value was considered the error function value in the control system. This value was sent
through a while-loop shift register and subtracted to update the error every loop.
Updated error values were then manipulated to produce a PD controller. This was
accomplished by taking the sum of the error value divided by the loop iteration time multiplied by
a Kd value, and the error multiplied by a Kp value. This signal is then multiplied to provide a
sufficient acceleration and running voltage. The amplified signal was passed into a case
structure that contains a control to shut off all voltage when the program is stopped and limit the
motor voltage to ± 10 volts while the system runs. The entire program is contained within a while
loop that allows the user to change the position of the bus without restarting the main VI.
A timing function was contained within the main VI while loop that measures the time
required for the bus to move from one position to the final desired position. Along with the
position reading for each loop, the measured time values are written to a spreadsheet file for
later analysis and comparison to Simulink model results.
The entire program was contained within a while loop that allowed the user to change
the position of the bus without restarting the main VI. User-defined position is controlled by a
second case structure within the main VI while loop. To provide preset position values
corresponding to “floors” a second sub VI was implemented. The floor to position sub VI (figure
9) contained an if, else if structure using the same mathscript function as the voltage-to-distance
sub VI.
Figure 9: Sub VI that programs floors into the control
system
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12. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Front Panel
User interface controls are contained within the main VI front panel (figure 10). Settings for Kp,
Kd, and signal magnification as well as diagnostic charts are also found on the front panel. The
following bullet-list breaks down the front panel.
 Pre-run settings
o Kp
o Kd
o Signal magnification
 Run time controls
o Destination
 Terrace
 VIP Lounge
 Bar & Restaurant
 Cellar & Kitchen
o Desired position (functions without presets)
 Indicators
o Target floor
o Bus position
o Motor voltage
 Diagnostics
o Unfiltered IR readings
o Filtered IR data
Figure 10: Front panel of the control system program
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13. MCEN 4043 Dumbwaiter 2011 Summer 2011
Analytical Model
Models of physical systems add to the control system and form part of control theory. A model
was developed for the dumbwaiter and was verified by Simulink. This section of the report is
dedicated to the presentation of the model, the yielded results with Simulink, and the
comparison of the analytical approach to the physical events.
Equations of motion
The pertinent equations of motion for the dumbwaiter system were developed and are shown
below. First, the kinetic energy of the system was modeled.
(1)
Where subscripts 1, 2, and 3 refer to the motor shaft, governor toothed wheel, and guide
toothed wheels, ω is angular speed, MB is the mass of the bus, MCW is the mass of the
counterweight, and v is linear speed.
(2)
Where r denotes the radius of the toothed wheels. Then, formulation of equivalent inertia:
(3)
Applied the following assumption:
(4)
In addition, it was required to add the next two equations for a complete and accurate analytical
approach.
(5)
(6)
Where ω(dot) is the angular acceleration, Tm is the motor torque, FMb is the force applied by the
bus, FMcw is the force applied by the counterweight, and g is the acceleration due to gravity.
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14. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Simulink Model
Previously shown equations were used to code the Simulink block diagram shown in figure 11.
Figure 11: Block diagram for analytical model of system
Results
Plots were drawn to visualize both the analytical and practical results. Upon comparison and
evaluation it was concluded that the analytical model was an acceptable approach to reproduce
the physical system. Figures 12 and 13 show how the results compared when the dumbwaiter
was activated to rise from the cellar & kitchen floor to the terrace.
Position (in.)
Time (s)
Figure 12: Simulink response – rise from cellar to
terrace
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15. MCEN 4043 Dumbwaiter 2011 Summer 2011
Figure 13: Acquired measurements – rise from cellar to terrace
Similarly, Figures 14 and 15 show how the results compared when the dumbwaiter was
activated to lower from the terrace to the cellar & kitchen floor.
Position (in.)
Time (s)
Figure 14: Simulink response – lower from terrace
to cellar
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16. MCEN 4043 Control System Design, Implementation, and Analysis Summer 2011
Position (in.)
Time (s)
Figure 15: Acquired measurements – lower from terrace to cellar
Conclusion & Commentary
The team’s presentation of the dumbwaiter 2.0 demonstrated a successful control system. The
design of the machine was robust, the implementation of the control was crisp, and the analysis
was within bounds. An all-round performance was accomplished thanks to iteration and careful
identification of elements from the system. Truly, this project provided an exemplary experience
of a systems engineering approach.
Much gratitude and appreciation is extended to the engineering facilities. The ITL laboratory
made many of the team’s ideas attainable and easy to execute. Resources available, both
human and material, added even more to the positive, learning, and constructive experience.
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17. MCEN 4043 Dumbwaiter 2011 Summer 2011
Appendix A: Bill of Materials
Part Amount Acquisition Manufacturer
Breadboard 2 ITLL, UCB 3M
Bus – assorted bricks Bulk ITLL, UCB LEGO
Chain links Bulk ITLL, UCB LEGO
DC Motor 1 ITLL, UCB N/A
Diode – FR104 2 ITLL, UCB N/A
Gears / toothed wheels 10 ITLL, UCB LEGO
Integrated circuit – LM358N 1 ITLL, UCB N/A
IR Range finder 1 ITLL, UCB SHARP
Resistor (1.0 KΩ) 2 ITLL, UCB N/A
Support structure – assorted bricks Bulk ITLL, UCB LEGO
Transistor – TIP31C 1 ITLL, UCB N/A
Transistor – TIP32C 1 ITLL, UCB N/A
A