Design Considerations

As an engineering project in the final year of the Bachelor of Engineering Degree, the author built a Six-Step Motor Controller along with a ‘mountain board’ style electric powered skateboard. This LEV prototype was built for the purposes of allowing extreme sport enthusiasts to take the sport of mountain boarding to flat or even uphill slopes, rather than being limited to downhill only. A photograph of the electric powered mountain board is given in Figure 3.1.

Figure 3.1 - Electric mountain board prototype built as part of final year engineering project.

A paper was written on this project titled “Instrumentation and Control of a High Power BLDC Motor for Small Vehicle Applications”. This was published in the IEEE International Instrumentation and Measurement Technology Conference (I2MTC 2012):

40 Alexander Rowe, Gourab Sen Gupta, Serge Demidenko, “Instrumentation and Control of a High Power BLDC Motor for Small Vehicle Applications”, Proceedings of the IEEE

International Instrumentation and Measurement Technology Conference (I2MTC 2012), Graz, Austria, May 14–16, 2012, pp.559-564

Abstract:

Brushless DC (BLDC) motors are becoming an increasingly popular motor of choice for low powered vehicles such as mopeds, power assisted bicycles, mobility scooters, and in this reported application, motorised mountain boards. With rapid developments in technology, high energy density batteries such as Lithium-ion Polymer batteries are becoming more affordable and highly suitable for such vehicles due to the superior charge rate and light weight of the lithium chemistry batteries. This combined with the high power, light weight, and cheap BLDC motor results in the BLDC motor being a very favourable solution over an internal combustion (IC) engine for low power vehicles with power requirements of up to 7kW. A BLDC motor controller was developed specifically for the motorised mountain board application. The motor controller is a ‘sensored’ BLDC motor controller which takes inputs from Hall Effect sensors installed inside the motor to determine the motor position. Many other sensors are used to monitor the variables that are critical to the operation of the motor controller such as the motor phase current, battery voltage, motor temperature, and transistor temperature. The reported system is further enhanced by several additional features such as output for an LCD screen, regenerative braking, timing advance, cruise control, and soft start functions. These topics are discussed briefly in this paper.

The full journal article is provided in Appendix A.

Following from the electric mountain board prototype, a second prototype electric skateboard was designed and built as part of this Masters project. This second prototype was designed to address the key issues identified in the first prototype phase.

3.1.1.

Motor Controller

The motor controller would frequently blow transistors due to limitations of the chosen

microcontroller causing delays in commutations. The motor controller was also large in size as there had been limited efforts to design for minimal size of the circuit board.

The second prototype incorporates a completely re-designed motor controller with a high performance 32-bit microcontroller. This allowed for the study of advanced motor control techniques as discussed in section 2.3.

3.1.2.

Turning Radius

The mountain board was designed for off road use, on surfaces such as grass, gravel, dirt, etc. and therefore a two wheel drive design was chosen for the purposes of increased traction on loose surfaces such as gravel, and to eliminate the unwanted torque-steer that is an inherent property of one wheel drive terrains. The electric mountain board uses a single motor to drive both of the rear wheels. It does not allow for the rear wheels to rotate at differential speeds; the two rear wheels are indirectly coupled together through the motor shaft.

41 While the electric mountain board handles well for its intended purpose of off road use on surfaces that allow for some wheel slip while cornering, the design falls short in on-road use. On high traction surfaces such as sealed roads or footpaths there is no wheel slip while cornering and therefore makes the handling on such surfaces difficult as there is a very large turning radius.

A major consideration for this project was to eliminate the issue outlined above. A simple and cost effective solution was to use two separate, smaller motors to drive each rear wheel separately. The decision to keep the skateboard as two-wheel drive comes from the fact that a one-wheel drive system would suffer from torque-steer; when accelerating or breaking, the force on the wheel due to motor torque would cause the skateboard to turn and therefore the handling of the electric skateboard would suffer.

3.1.3.

Size and Weight

The electric mountain board was heavy and bulky. The second prototype was designed to be smaller and lighter in order to make it more practical for everyday use such as short commutes. With this in mind, the decision was made to use a ‘longboard’ style of skateboard as the basis for second prototype of the electric skateboard for this Masters project.

The longboard is physically smaller in overall dimensions and significantly lighter which makes it easier to carry, making it more practical as a mode of transportation. In contrast, the electric mountain board which was found to be useful for recreational use only.

3.1.4.

Hub Motor Design

A typical electric skateboard uses a belt or chain drive to transfer power from the motor to the wheel. This allows for an appropriate gear ratio to be selected in order to increase the torque and reduce the speed from the motor to the wheel.

In an attempt to simplify the design of the electric longboard, an innovative hub motor design was chosen. This eliminates the need for a belt or chain reduction drive which reduces cost, weight, maintenance, and complexity of manufacture and assembly. A hub motor is simply an “outer-rotor” motor with the wheel attached to the outside of the rotor, and the stator fixed in the centre of the wheel.

A direct drive hub motor offers numerous advantages over the commonly used belt or chain reduction drive as well as some disadvantages, as summarised in Table 3.1.

42

Advantages Disadvantages

Reduces complexity – No motor mounts, no tensioning system

More limitations on motor selection – must be “outer rotor” and must produce sufficient torque to drive the wheel directly.

Reduces cost – less components to purchase and manufacture

Limits wheel size – no speed reduction through gearing

Reduced weight – less components Limits Battery voltage – no speed reduction through gearing

Reduced maintenance – no belt to tension and occasionally replace, no periodic chain lubrication

The motor spins at a lower speed therefore the motor and motor controller combination inherently are a lower voltage, higher current system – resistive power loss is greater

No power loss in power transmission.

Table 3.1 - Comparison of the advantages and disadvantages of a hub motor setup over a reduction drive setup.

The disadvantages of the hub motor drive are largely to do with creating more limitations on

component selection. However, if a suitable combination of motor, battery configuration, and motor controller is available at a similar cost to that of the components required for a belt or chain drive setup, then the hub motor system could prove advantageous.

3.1.5.

Physical Layout of Equipment

The electric mountain board prototype was designed for off road use and therefore the additional equipment was placed so that it would not compromise the overall clearance. As a result, the batteries and motor controller were placed on the top side of the skateboard deck, placed in between where the user’s feet would usually be positioned. The motor was mounted behind the rear wheels.

The difficulty with this layout is that the user must place their feet in fixed positions on the

skateboard deck, and would often trip on the equipment when unexpectedly having to dismount the electric mountain board.

The longboard style of skateboard is designed for on road use (sealed surfaces) due to its smaller, solid rubber wheels. The electric longboard prototype adopts an equipment layout that places all of the electrical equipment on the underside of the deck therefore eliminates the tripping hazard. The layout was also selected to be as flat as possible so that it does not have a large effect on the clearance of the longboard.

3.1.6.

Handheld Controller

The handheld controller from the electric mountain board prototype used a cable connection to communicate with the motor controller. Having this cable connection was fond to be cumbersome as it restricted movement of the user.

43 The electric longboard prototype addressed this issue by using a wireless handheld controller. For simplicity, a RC transmitter and receiver was used. The transmitter takes an analogue input from the ‘throttle’ potentiometer and transmits the throttle value via digital radio on the 2.4GHz band. The receiver detects this signal and outputs a digital PWM signal to the motor controller.

The disadvantage of this ‘off the shelf’ handheld controller is that there is no customisable display of data as there was on the original handheld controller LCD. Instead, a power meter was installed in the electric longboard to display and record data such as battery capacity consumed, current, voltage, power, speed, and temperatures.