One of the most critical steps in the conceptual design of the stable platform is determining the scale to which the system will be designed and built. Initially, the goal of the project was to produce a stable platform that could stabilize a mono- wheel vehicle. After concept generation it was decided that the manufacture of the system was more complex than initially expected. Because of this, the motivation for the build of the project shifted from an application specific design and build to a broader proof of concept approach.
This shift increased the range of the sizes that the prototype stable platform could be built at. The availability of electric motors was established as the governing factor in determining the size of the prototype.
The flywheel electric motors were chosen to define the scale of the stable platform because:
the size of the motor casing and shaft will govern the size of the flywheels that can be used. The inertia of the flywheel determines the size of the restoring torque the platform can produce.
for this application the torque that the motor is able to produce is not critical. As the flywheels will not be under any load, they will be aided by momentum once they reach their desired speed.
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6.4.1 Electric motor selection
Four different types of electric motors were considered for the flywheel drive motor. These were: induction motors; wound field “universal” motors; permanent magnet motors and brushless permanent magnet motors. Each of these motors are summarised in Table 6.4.
Table 6.4 - Types of Electric Motors
Motor Type DC or AC Applications
1. Induction AC Mains electric power applications
2. Wound Field “universal” AC and DC Power tools, and domestic appliances 3. Permanent Magnet DC Air pumps, golf carts, wheelchairs
4. Brushless Permanent Magnet DC Segway, model planes/helicopters, portable power tools
For the stable platform design, it is desirable that the flywheels run on a separate power supply. This will make the system applicable to situations where mains power is unavailable. Because of this, induction motors can be eliminated as a drive option. The remaining motors are evaluated in Table 6.5.
Table 6.5 - Motor evaluation chart
Motor Type
Functional (geometry, control, load paths, motion) /5 Information (cooperation, expertise, experience) /5 Manufacturing, quality (production, ease of purchase) /5 Can be made to work (potential, confidence) /5
Comments Score /20
2 2 4 5 2 Pros – most common type of motor, cheap, constant speed under load
Cons – poor efficiency at high speeds, speed not easily controlled
13
3 3 3 5 4
Pros – solid construction, high starting torque, sealed bearings
Cons - poor ability to accelerate inertial loads, high voltage sensitivity 15
4 4 5 4 5 Pros – low maintenance, high operating speeds, easy to set up
168 Table 6.5 shows that of the three available electric motor options for driving the gyroscope flywheels, brushless DC motors were found to be best suited to this application.
6.4.2 Brushless DC motors
Brushless DC motors come in a range sizes. They are commonly used in model aircraft applications to drive helicopter and plane propellers. The main advantages of using these types of motors to drive the flywheels in the stable platform are:
The external casing of the motor rotates as well as the shaft. This means that the flywheel can be mounted over the casing aligning the motor and flywheels centre of mass.
The motors have a large number of mount points on them. These are usually used for attaching aircraft propellers yet this will help aid in assembling the motors into the gyroscope assembly.
Due to the use of brushless DC motors in the model industry, there is a vast amount of information available regarding setting up the motors and controlling their speed. Because most of the consumers who use the motors are hobbyists, the motors and their associated control systems are very simple to use.
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6.4.3 Selection of motor/scale of stable platform
The main governing factor that will determine that size of the brushless motors used will be the load the bearings and main shaft are subjected to. To accommodate for the largest possible load the largest available motor shaft diameter was selected (Ø10mm).
To satisfy the conditions established in the design specification requirements (Table 6.1), the motor must have a rotational speed of at least 5000rpm and be as lightweight as possible.
Research into brushless motors and their associated specifications revealed an Exceed RC Brushless DC Motor (MP160) as the most appropriate motor to drive the gyroscopes. A summary of the MP160 motors specifications is available in the CD insert associated with this thesis under “Purchased Project Components”.
Brushless DC motors maximum speed is related to the associated Kv rating (not to be confused with kilovolts) and the voltage that it receives from the batteries. In this case, the motor has a 245Kv rating. This results in the motor rotating at 245 rpm per 1 volt it receives from a battery. By connecting this motor up to a 22.2V Li-Po battery, the top speed this motor is able to achieve is approximately 5390rpm.
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