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The world has changed dramatically over the last few years where there has been social distancing, conflict, and economic hardship in addition to the accelerating effects of climate change. These issues present a new set of challenges for the global community but also an opportunity to plan for a brighter, more resilient future.
Certainly, it is an exciting time to be an engineer, as the hastening development of renewables, alternative fuels, and electric vehicles encourages further innovation within sectors like energy and transport. This momentum has already boosted the popularity and accessibility of smaller e-mobility platforms like electric pushbikes and scooters and we are now seeing much broader industry support for light-duty, high-efficiency electric drivetrains, such as the latest offering from Microchip.
To this end, this article aims to explore the features of the new e-mobility reference design from Microchip, in the interest of building a contemporary control system for a three-wheel electric skateboard.
Key features of the e-mobility reference design
The e-mobility reference design board is a brushless DC motor driver circuit that makes efficient use of existing Microchip hardware including the dsPIC33CK digital signal processor, MCP16331 & MCP1754S DC-DC converters, and MIC4104 MOSFET drivers, within an intuitive circuit design and an impressively compact form-factor.
The board supports an input voltage range of 18v to 42v which falls flexibly within the operating capacity of both 24v and 36v lithium-ion battery packs and is also rated for motors up to 350w. The board firmware is capable of monitoring fault conditions like overvoltage and short-circuits which can be configured further using the PICkit-4 programmer and Microchip’s open-source code.
The board can operate in both open and closed loop modes, with a choice to use the hall effect inputs and current sense capabilities for more precise BLDC motor control. The circuit hardware also features support for regenerative braking thereby extending the potential range of any e-mobility platform.
Lastly, the board features a potentiometer input for proportional speed control which keeps the motor output precise, smooth, and exceptionally quiet under operating conditions.
A comprehensive test of the three-wheel electric skateboard.
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- Microchip e-mobility dev board
- 36v 12Ah Lithium-ion battery (Link)
- 36v 350w BLDC Hub Motor (Link)
- 36v 350w BLDC stock driver (Link)
Design mentality and bench test
The Microchip e-mobility reference design is a single brushless DC motor driver circuit that presents a perfect opportunity to make some much-needed upgrades to an old three-wheel electric skateboard prototype that was completed a few years ago.
The previous three-wheel electric skateboard design
In the previous design, we made use of a crude combination of lead-acid batteries and a brushed DC motor in a single chain-driven rear-wheel drive design to promote the feasibility of alternative electric transport. Now that we have our proof of concept and some additional hardware including a compact brushless DC hub motor and a lightweight lithium-ion battery pack, we can look at making some seriously impressive updates to the design.
The Microchip e-mobility reference design board layout
The new system will consist of a 36v 350w brushless DC hub motor with positional feedback that can be plugged into the reference design board via its respective three-phase motor output terminals and hall-effect input headers. The motor speed is controlled with a simple potentiometer input which is plugged into a separate header with space for further user-defined inputs. Lastly, the system is powered with a 36v 12Ah lithium-ion battery pack that for the bench test is stepped down to 24v to accommodate the default overvoltage settings of the device.
Testing the Microchip hardware on the bench
The intuitive design of the circuit and documentation made it straightforward to get the control system up and running on the bench with the hub motor turning smoothly. The reference design also features an array of protection circuits and fault flags that are indicated with a red on-board LED and were of great help during debugging, although it was not always clear what fault was being triggered.
Building the skateboard
In the interest of promoting alternative electric vehicles, the skateboard was designed to be as simple as possible while exhibiting good operating performance as a practical mode of transport. To achieve this, the chassis of the board was originally constructed out of a plank of wood and two strips of mild steel, a design that will be kept but cut down to half the size for added practicality and storage.
The chassis of the board is built for simplicity
As with any skateboard, the vehicle can be steered using the metal truck that is attached to the wooden deck of the board, using the user’s weight to steer it left or right. The front wheels of the board have been uprated from solid polyurethane to pneumatic tyres that were borrowed from my kiteboard and will allow the new design to be driven in less accommodating environments.
Upgraded tyres for ground clearance and traction off-road
The brushless hub motor represents the most significant improvement in terms of space efficiency where the whole drivetrain is integrated into the wheel itself, rather than relying on an exterior chain drive as in the original prototype. The addition of feedback sensors also enables the efficient allocation of speed and torque by the control system which will greatly affect handling and range. The wheel is mounted to the metal chassis using two keyed holes on either side of the skateboard with the control cable exiting the axle on one side and fed to the control board mounted near the front wheels.
The brushless hub motor is more compact and capable
The skateboard makes use of an underslung lithium-ion e-bike battery to minimise its footprint and overall weight while maximising the available power and range. Careful consideration was given to ground clearance and lean angle while designing the board, which resulted in the use of larger wheels. The battery was mounted to the deck of the board using four countersunk machine screws.
The underslung lithium-ion battery is lighter and has a higher power density
The Microchip e-mobility reference design uses a potentiometer input to give the user proportional control over the speed of the motor. This is usually in the form of a twist throttle but as the skateboard has no handlebars, we will instead use a tethered remote control that the user holds.
Ergonomic tether with variable speed knob and go button
The tether design consists of a potentiometer that sets a consistent speed for the skateboard, while a push button is held down in order to move. This setup is designed for safety, ease of use and to stop the skateboard from running away if the user drops the controller or falls off. The tether enclosure was modelled in DesignSpark Mechanical, 3D printed, assembled and then wired to the controller input with some three-core cable for the 5v power, control signal and ground wires.
Exploded CAD render of tether
The new skateboard prototype was built to test the rated limits of the Microchip e-mobility reference design which has a lithium-ion input voltage range of 18v to 42v and a maximum motor power output of 350w. Unfortunately, while the Microchip e-mobility hardware did work exceptionally well during the bench test, the default firmware limited the input voltage to 24v which, where I currently lack the debug tools to update it, affects the ability of the skateboard to use its 36v motor under load.
However, the Microchip hardware is designed as a drop-in replacement for any e-mobility application, therefore the skateboard can easily be rewired to work with the stock speed controller that came with the hub motor in order to test the skateboard at full power.
BLDC speed controller mounted with 3D printed brackets
The speed controller takes the input from the tether potentiometer as part of a closed loop system that compares the input from the hall-effect position sensors on the brushless DC motor to generate a three-phase output back to the motor at an appropriate speed. This is a vast improvement over the go/stop button of the previous prototype that connected two lead-acid batteries to a DC motor with no intermediate control circuitry whatsoever.
The speed controller was mounted to the underside of the skateboard behind the truck using two 3D printed brackets and four self-tapping wood screws. The various cables were then tucked in behind the battery so as not to catch any moving parts.
Results and conclusions
The three-wheel electric skateboard was designed to promote the feasibility of alternative electric transport, and now in its second generation, it far exceeds all original expectations. Combined with off-grid renewables, this project even hints at what zero-carbon ecosystems may look like in the future.
It is fantastic to see an attempt from Microchip to make these e-mobility solutions more accessible to a broader market, as a lot of these products are still heavily dependent on overseas suppliers. The technology behind brushless DC driver circuits has advanced to allow for efficient speed and power delivery in a far smaller form factor, in addition to some cutting-edge automotive features including lightweight lithium-ion storage and regenerative braking.
The Microchip e-mobility reference design has great potential to simplify the engineering workflow when designing contemporary light transport applications, it was therefore frustrating that, due to some legacy firmware issues, we could not test it at its full potential. However, this is still one of the most exciting projects I have worked on and given the potential of the skateboard as a developing platform, I see no reason why this reference design would not be extremely useful to the engineering community.