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Mechatronics Learning Studio

 

Mechatronics Crane Project (MCP)

 

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.

 

Components List

(1) 9V battery

 (1) Battery clip

(3) DPDT knife switches

 (3) 9V Motors (models  #43362, and 47154)

 (1) L.E.D.

  (1) 12V DC Magnetic reed switch

 (lots) Lego (gears, winches, shafts, pulleys, structural)

 (lots) Sewing thread

 (1) Breadboard

 (1) Sliding switch

              (lots) of wiring

 

Figure 1:  The dashboard structure and components.

Circuit Diagram

 

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

http://philohome.com/towercrane/tc.htm

“Philo” (2009) Lego 9V motors compared characteristics: retrieved on October 10, 2009 from

http://www.philohome.com/motors/motorcomp.htm

“Northwestern University” (2008) Northwestern University Mechatronic Project examples: retrieved on October 10, 2009 from

http://lims.mech.northwestern.edu/~design/mechatronics/

“Northwestern University” (2009) Northwestern University Mechatronic Resource Wikisite: retrieved on October 10, 2009 from

http://hades.mech.northwestern.edu/index.php/Main_Page