S15: Wireless Power Transfer System

Wireless Power Transfer System

Abstract

Wireless power transfer or wireless energy transmission is the transmission of electrical power from a power source to a consuming device without using solid wires or conductors. The technology dates back when Tesla first created his coil and the project is based on that idea. As the world is gaining momentum on wireless communication technology, there is a niche market for wireless power transfer for low power devices. This project explores one such idea and implementation for wireless power transfer from source to destination. The project is limited to near field application of such technology. The project does not explore the EMI/EMC aspects of the design.

Objectives & Introduction

Introduction

The project implements the principle of an inductively coupled power transfer system. A coil, attached to an AC power supply acts as a transmitter by the virtue of its time varying magnetic field and on the other end a receiver coil with magnetically induced current, drives a load motor attached to it. The two coils are aligned in such a way that the magnetic field interaction, and hence the power transfer is maximum. For this purpose, the receiver coil, mounted on a DC motor, makes a complete 360 degrees turn and the ADC continually monitors the voltage induced at each angle. The DC motor automatically goes to the position of the maximum sensed voltage and the coil starts driving the load motor attached to it.

Objectives

The main object of the project is to drive a motor wirelessly and making sure that the circuit is designed in such a way as to minimize the transmission losses and the load at the receiving end maintains a constant voltage without any fluctuations.

Team Members & Responsibilities

Team Member 1
- Haroldo Filho - Lead Software Engineer

    - software architecture - define application abstraction layers, data flow.
    - motion control - develop means to implement motion including hardware specification and software interface.
    - mechanical fixturing - solve hardware mounting problem by fabricating holding fixtures with wood.

Team Member 2
- Hemant Raman - Lead Hardware Engineer

    - investigate the physics behind wireless power transfer and fabricate copper transmission loops
    - design and implement signal conditioning and amplifier circuits for receiver and transmitter
    - design and implement ADC signal conditioning

Team Member 3
- Hassan Naveed - Software Implementation

    - Learn wireless mesh driver and develop wireless mesh interface to send information between transmitter and receiver
    - Develop command parsing for master/slave

Team Member 4
- Guangyu Chen - Software Implementation and integration

    - integrate motion control and communication layers into multi tasking application
    - Develop command parser for master/slave

Schedule

Week#	Date	Task	Actual
1	4/14	-Light an LED wirelessly -Test ADC: read 0~3.3V range -Test Nordic wireless: send/receive ADC value	Completed. Initially there was an issue that the master node was missing some packets sent by the slave node. It was resolved by defining the two tasks using class method and then adding them to the task scheduler. Also there was a debate on the number of ADC samples to be taken in the ADC task. But after some deliberation, the number of samples was set to be 100.
2	4/21	-Find channel frequency: 7.7MHz -Define wireless package structure: status and commands -Spec/order motor	Completed
3	4/28	-Implement wireless communication -Define software architecture layers -Define error code and handler -Implement motor control	Completed, See software design section below for details
3	5/5	-Evaluate ADC reading dynamics and design LPF -Implement software architecture layers	Completed, uneven voltage waveform was giving inconsistent readings, therefore, the numbe of samples in the ADC task were increased to 1000 and their average value was used to calculate the voltage at the receiver end.
3	5/12	-SW/HW Integration and testing.	Completed, there were issues with finding the optimum spot for the receiver in the initial scan, but it was resolved.

Parts List & Cost

Item#	Qty	Reference	Part Name	Manufacturer	Description
1	4	C1-4	CAPMICA1,10pF	IPC SM-782 STD.	MICA TANTALUM CAPACITOR 5.69 X 10.9 MM
2	2	D2-3	DIO-MLL41	IPC SM-782	SURFACE MOUNT DIODE, MELF
3	1	L3	IND-MOLDED,849nH	Regular Copper Pipe 15 inches Dia Loop	MOLDED INDUCTOR, 0.5" PIN SPACING
4	2	L1-2	IND-UPRIGHT,100uH		UPRIGHT INDUCTOR WITH CORE
5	1	D1	MLED76	MOTOROLA	INFRARED LED
6	1	R6	RES-1/4W,100 ohms		RES BODY:100 CENTERS:500
7	1	R7	RES-1/4W,10K		RES BODY:100 CENTERS:500
8	1	R5	RES-1/4W,4.7K		RES BODY:100 CENTERS:500
9	1	R1	RES-1/4W,5.6K		RES BODY:100 CENTERS:500
10	2	R3-4	RES-1/4W,68 ohms		RES BODY:100 CENTERS:500
11	1	R2	RES-1/4W_SMALL,5.6K		RES BODY:100 CENTERS:500
12	1	S1	SW-NKK-JF15	NKK JF15SP3C	NKK SPST N/O MOMENTARY PUSHBUTTON SWITCH
13	3	U1-3	TO220-4_MOSFET

Design & Implementation

The design section can go over your hardware and software design. Organize this section using sub-sections that go over your design and implementation. The sections below provide a descriptive overview of the hardware block and flow diagram.

Hardware Design

The Hardware design consists of major blocks an shown below. For the transmitter section, there are two methods of turning the system ON/OFF. One is a physical switch which maybe used as an override and the other is a software way of turning it ON/OFF. This switches the 60 Watts power supply module which is transmitted to the Copper loop (ref Pictures). The system runs at about 1.7MHz which is clear of the wireless signal transfer frequency of the Nordic wireless system (2.4GHz). For the Receiver, the loop which is tuned to the resonant frequency of the transmitter has very little losses and picks up about 24V 1.7MHz signal. The Power section has diodes which outputs RMS voltage of 1.41 x 24V AC= 33.9Vrms. Since all logics that the SJSU one board uses 3.3V a zener diode voltage regulator topology is implemented. A 1 ohm power resistor is used to sense the voltage on High Side and a current amplifier is used to set the gain of the system. The amplifier has common mode rejection ration of Bandwidth 500KHz and the signal is very noisy. A low pass filter was implemented in hardware so that the ADC could read stable values. Furthermore the software has a digital filter implemented to further stabilize the readings.

Transmitter Receiver Block Diagram

The hardware block diagram for the transmitter is:

Receiver Block Diagram

The hardware block diagram for the receiver circuit:

Transmitter Block Diagram

Hardware Interface

In this section, you can describe how your hardware communicates, such as which BUSes used. You can discuss your driver implementation here, such that the Software Design section is isolated to talk about high level workings rather than inner working of your project.

Motor Interface Block Diagram

The motor control part is implemented with a step and direction controller driving a bipolar 4 wire stepper motor.

The motor is a STP 3375 Nema 23 200 steps per rev, 166 in-oz. The motor driver takes 3 GPIO lines: enable, direction, and step.

It increments the rotor position by 1.8 degrees at every positive transition of the step input line as long as enable is asserted.

In order to move, a "steps_todo" counter is loaded with the distance (number of toggles of the step line) and a motion task exhausts this counter at a frequency proportional to the desired speed.

Software Design

For the software design, we define a layered architecture where each layer takes its services from the layer directly below it. The integrity of layered system was important as to ensure that no race conditions are generated where two tasks wait indefinitely for each other. The layers are:

WIFI : Receive wireless packet from mesh network and pass the packet to communication layer for decoding.

Communication : Decode the packet and execute corresponding actions.

Motion Control : Control the motor to move to the desired position.

Package structures between three layers:

Wireless Data Package Structure
|| 1 byte   | length - 1 ||
|| commands |    Data    ||

Status Package Structure
||   1 byte   |      1 byte      |      1 byte      |     x byte     ||
|| Error byte | ADC upper 4 bits | ADC lower 8 bits | Motor position ||

Error byte Structure
|| 8th bit  | 7th bit  | 6th bit  | 5th bit  | 4th bit  | 3rd bit  | 2nd bit  | 1st bit  ||
|| reserved | reserved | reserved | reserved | reserved | reserved | reserved | Busy bit ||

Motion Control Package Structure
|| 1 byte  |   1 byte    |   1 byte    |   1 byte    ||
|| Command | Parameter 1 | Parameter 2 | Parameter 3 ||

Implementation

This section includes implementation, but again, not the details, just the high level. For example, you can list the steps it takes to communicate over a sensor, or the steps needed to write a page of memory onto SPI Flash. You can include sub-sections for each of your component implementation.

Testing & Technical Challenges

Describe the challenges of your project. What advise would you give yourself or someone else if your project can be started from scratch again? Make a smooth transition to testing section and described what it took to test your project.

Include sub-sections that list out a problem and solution, such as:

My Issue #1

ISSUE
The current sensing on the receiver through the 1 ohm resistor has a lot of noise riding on the signal. Using a software digital filter was not possible since it would put the software in a blocking mode for the duration of the capture. This would change the topology of the software architecture and deviate from being a good RTOS implementation. A quick alternative option needed to be implemented.

RESOLUTION
The resolution was:
1) to find a component with good common mode rejection ratio. LT1362 device was used, CMMR 500KHz.
2) to implement a Hardware filter before the ADC readings were taken. Even with hardware filter the signal was still noisy. Along with Hardware filter implementation, there was still noise on the signal and showed in terminal window of software. This variability in signal would cause "Hunting" on the transmitter platform where the platform would jitter. A software digital filter was implemented so the signal was smooth and some hysteresis was added to the PID control system of the tuning table.

My Issue #2

ISSUE
The voltage output from the receiving antenna was ~33Vrms and all the electronics for the SJSU board were running at 3.3V logic. Even with voltage dividers it still presented a danger to damage the SJSU board ADC channel in case a large spike lasting long period was generated.

RESOLUTION
A very cheap and simple foldback voltage regulator was implemented which produced 3.3V DC. This ensured that the absolute maximum voltage that the ADC would see was 3.3V. To further protect the SJSU board peripherals, a Instrumentation amplifier block was placed between the 3.3V supply and the ADC. This gave us some confidence that in the worst condition the damage will first be sustained by the Instrumentation amplifier and not the ADC channel.

My Issue #3

When we first implemented the software layers, we handled and decoded all the wireless packages in the top level while the lower two layers also required data feedback to the top level. In this case, the top level needed to listen two queues or to fill feedback to the wireless quese which are both impossible. So we changed the whole architecture and moved the decoding and wireless feedback to the middle layers that made the whole software simpler without any layer violation.

Conclusion

Wireless power transfer was a very unique project since it had not been done before in CmpE 244. We came across really complex hardware problems like noise, signal integrity and signal conditioning cannot be solved by software or hardware alone. It takes a combination of the two to solve it. For the software part, we learned how to build and design a layered architecture of firmware where each layer interacts with the layers adjacent to it. It also posed a lot of problems initially since every new function that we would add or modify would somehow violate the layered architecture. But eventually we got it right and it gave us a great overview as well as insight as to how professionals in the industry actually design and implement there software projects.

Project Video

Youtube video of hardware prototype

Project Source Code

Sourceforge Source Code Link

References

Acknowledgement

We would like to thank our instructor Preet for always inspiring and encouraging us to learn and and explore new projects related to embedded systems. We really appreciate his patience and his guidance and whenever we needed him, he was just an email away always willing to help and point us in the right direction.