Steven Chen, Darius
Wajda, Oscar Wasilik, Department of Mechanical Engineering, University
of Ottawa
Description of
the Project
•
The project consists of a Hammerhead crane 0.60m
high with three degrees of freedom for lifting objects and
positioning them accurately. All crane motions and a master
safety switch can be controlled from a dashboard (see Figure 2 for
the dashboard).
•
Although there are many crane ‘toy-sets’, this
crane is unique: it includes electronic sensors for safe
operation. Sensors include a magnetic switch to prevent
over-retraction of the hoist cable; a thermistor and a voltage
suppressor diode for power surge protection; and a ‘master kill
switch’.
Introduction
Electronics provide a
useful approach to controlling heavy and dangerous machinery.
Although our crane is by no means heavy or dangerous it does
replicate the dangers that a life-sized crane and crane operator
would experience.
In this way, our little crane acted as a
‘danger laboratory’ in which we could test out various safety
sensors.
The circuit is all integrated
on the dashboard. We control the crane using three `knife
switches`. Each three-position switch may provide a positive,
negative, or zero voltage to the LEGO motors. We control the
polarity of the voltage in the motor, and so we also control the
direction of the motor. When using the NI Multisim simulation
software we represented one knife switch as a `box`of four
switches.
Figure 2: The realized circuit.
Safety Features
The mechatronics crane
employs four safety features:
A master kill switch
A magnetic overhoist sensor
Power surge protection
Overcurrent protection
Master
Kill Switch
This switch corresponds to the ‘big red button’
found on most heavy machinery and equipment. Using this switch
will sever the power source from every component. The crane
stops all motions in case of an emergency
(see Figure 2 to view the switch on the
dashboard).
Magnetic Overhoist Sensor
This sensor prevents the hoist cable from winding
up too tight. A magnet on the load hook activates the sensor
when the load hook is too high. The hoist motor then stops, but
it can still reverse to lower the load safely. One of the first
electronic overhoist sensors was patented in 1990 by George
Coull (http://www.freepatentsonline.com/4905849.html).
See Figure
3 for the location of the sensor on the crane. See Fig. 4 for a
close-up of the sensor system.
Figure 3: Mechatronic crane
and magnetic safety sensor.
Figure 4: Magnetic safety
sensor system.
Overcurrent Protection
Our LEGO motors came
prefabricated with a 1.7
Ω PTC thermistor-resistor. The thermistor
increases its resistance with increasing temperature. The
temperature of the thermistor increases with high currents.
Therefore, at high currents, the thermistor has high resistance
to subdue the current. The thermistor protects the motor
components from damage by high current levels. The following
chart uses a simplified linear model to predict the increase in
resistance with increasing temperature and the resistance’s
effect on the current.
Surge
Protection
Our LEGO motors also
include a bidirectional transient voltage suppressor diode.
Essentially, this diode allows current peaks to flow through (to
the thermistor) but clips the waveform of any surges or ‘voltage
peaks’ (see Fig. 6).
Figure 6: Current and voltage
activity for a bidirectional transient voltage supressor diode
(inset)
Mechanical
Design
All gearsets were made to suit the output torques
and speeds for each crane function. For example, to rotate the
boom took a gear reduction of 200:1. To hoist the cable only
took a gear reduction of 4:1. The motor input speed for the boom
motor is about one third slower than that of the hoist motor at
the same voltage. The hoist motor and trolley motor were the
same model (43362). Different gear ratios were used for each
motor to perform hoisting and trolley motion at different speeds
and torques.
Simulation
and Validation
The crane components were tested using a circuit
modeling software: NI Multisim. We could not find any knife
switch simulations so we used ‘boxes’ of four switches with the
same effect. Just as expected, when a forward switch was
selected the motor read a positive voltage of 9V. Naturally,
when a reverse switch was pulled the motor read a negative
voltage of -9V. Following are the results of our simulations,
validated by actual tests:
Issues
All components
behaved as expected. We attribute this success to our
straightforward design and our rigorous testing:
·
Circuit
testing using Multisim software.
·
Testing of
individual physical components and of each electronic part
independently.
·
Combining
all electronic and physical components only after testing.
Next Steps
In the future we could
implement digital motor control for the trolley with the
following design:
Five switches
control the position of the motor. A counter and pushbutton
represent an optical encoder on the motor. The counter notes the
trolley location based on the number of spins from its motor.
The comparator compares the difference between the desired
trolley location (the user button) and the actual trolley
location (counter value). This circuit allows for forward and
reverse control of the motor. The motor stops at the desired
location. If the user chooses no switches (shown), the motor
will reverse back to the reset position, selecting the “R” reset
switch, and the counter resets.
Figure 7: Digital trolley motor control circuit.
The digital motor control circuit can be
explained using six switches, a counter, and a comparator. (See
below.) The sixth switch below does not represent a switch in
Figure 7. Instead ‘reset’ represents having no switches
selected. See the explanatory figure below for the logic
functions of how a comparator and counter work to position the
trolley.
Concluding Remarks
·
The crane
works as expected.
·
The sensor
systems provide safety ‘feedback’ control.
·
Mechatronics system
integration was achieved through combining electronics, control,
actuators and sensors, and mechanicse.
Resources used and References
”Philo” (2009) Philohome tower crane project:
retrieved on October 10, 2009 from
