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Wireless Power Transfer System for Rehabilitation - 2nd Prototype

This is the article of the second prototype of the wireless power transfer system using the inductive link for the rehabilitation. The article will be divided into 2 parts – hardware and software part.

Overview

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Demonstration Video

This training platform is designed for rehabilitation of stroke patients to train their upper limb motor functions. By using the state-of-art wireless power transfer and communication technologies, the patients can be guided to conduct various exercise therapies.  The platform instructs the patients to follow the movement path on the platform from one target to another.  Apart from the training purpose, the system can also provide therapists with the information to assess the condition of the patients.

The training platform can be divided mainly into two parts, including the transmitter and the receiver. The transmitter is in modular-based design to enhance the extensibility and feasibility of the platform. In this demonstration, the transmitter is formulated by four power modules. Each module consists of four wireless charging coils. Each charging coil is connected to a series resonant capacitor and driven by a full-bridge inverter at 150kHz. When alternating current is passing through the coil, it will generate an alternating magnetic field. If the receiver is near the charging coil, part of the magnetic field is coupled by the receiver to deliver the electrical energy to the load. The module is controlled by a high-performance microcontroller to navigate the position of the receiver and regulate the power transfer. By monitoring the change of the impedance on the charging coils, the position of the receiver can be located. The coil impedance is determined by the ratio of the measured coil voltage and current. The target position is indicated by a green colour LED series. The receiver can be attached to different objects. A plastic cup, made by 3D printing technology, is selected for the realistic training. The receiver is composed of a receiving coil, power and sensing circuits, a communication module and a microcontroller.  

Structure

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The whole prototype is including 4 identical modules, each includes 1 MCU, 2 Drivers, and 4 coils. The above block diagram shows the structure of each module. The structure of each module is basically similar to the first prototype. The STM32F407VG is used instead of the Discovery Board as the MCU. And 2 drivers are used instead of one driver only in the last version to make it more efficient. At this version, a current sensor and a peak detector are added to each coil to get the electrical information of each coil and have some adjustment during the performance.

The MCU is designed to use an independent PCB board. The reasons to separate the MCU board from the main board are that we are trying to make it easy to handle, protect it from accidentally high power and lower the common mode noise.

MCU Board

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Here is the schematic diagram of the MCU board. The schematic diagram shows the basic operation connection of STM32F407. The pin assignment table is also attached below.

Pin Assignment
PA0 TIM 5 PWM PB0   PC0 MS_1_3 PD0 CAN_Rx PE0  
PA1 PD_1 PB1   PC1 MS_1_2 PD1 CAN_Tx PE1  
PA2   PB2   PC2 MS_1_1 PD2   PE2  
PA3 PD_2 PB3 TIM 2 PWM PC3 RESET_CD_1 PD3   PE3  
PA4   PB4 TIM3 PWM PC4   PD4   PE4 RESET_AB_1
PA5 PD_3 PB5 RESET_CD_2 PC5   PD5   PE5 TIM9 PWM
PA6   PB6   PC6 TIM8 PWM PD6   PE6  
PA7 PD_4 PB7   PC7   PD7   PE7  
PA8   PB8 TIM10 PWM PC8   PD8   PE8  
PA9 TIM 1 PWM PB9 TIM11 PWM PC9 RESET_AB_2 PD9   PE9  
PA10 MS_2_3 PB10   PC10 UART_Tx PD10   PE10  
PA11 MS_2_2 PB11   PC11 UART_Rx PD11   PE11  
PA12 MS_2_1 PB12   PC12   PD12   PE12  
PA13   PB13   PC13 OTW_1 PD13   PE13  
PA14   PB14   PC14 FAULT_1 PD14 FAULT_2 PE14  
PA15   PB15   PC15   PD15 OTW_2 PE15  

*MS = Mode Selection (for driver)

RESET_XX_X = Driver Reset Pin

OTW = Over Temperature Warning Pin

PD = Peak Detector

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To assign the pins, there are 2 criteria to consider. The first consideration is the capability of the pins. Some pins in STM32F407 are capable to generate PWM signal, some are not, which is necessary to check the pin description table from the datasheet. The second one is to consider the position of the pin. By considering the pin position, it will be easier to design the PCB board.

As well as the last prototype, each driver needs 4 PWM signals, 3 mode selection pins, 2 error protection pins and 2 reset pins. Since there are 2 drivers in the module, the pins are labelled as XX_1 or XX_2 to identify driver 1 and driver 2.

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There are also protections on the board. A lowpass filter and 2-diode protection are provided to the pins which involve signal input. Since the absolute maximum voltage of the MCU pin is 4V, and the maximum voltage provided on the other board can up to 10V. The 2 diodes are added to prevent damaging the MCU from accidentally high voltage input or short circuit happened. The RC low pass filter is added to better the performance. The values of the resistor and capacitor are not fixed which providing flexibility to change the frequency range of the usage.

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The MCU board will be connected using pins header and pins socket. Therefore, the pin header is a customized component. The library of the pin header will be attached for your reference. There are some spare pins designed for further development.

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MCU board component list:

Item Unit RS Stock No.
STMicroelectronics STM32F407VGT7 1 (880-5395)
Yellow Plunger Tact Switch 1 (686-6853)
CSTCE8M00G52A-R0, Ceramic Resonator, 8MHz 1 (721-4827)
Murata Ferrite Bead, 120Ω impedance at 100 MHz 1 (011-0116)
Vishay LL4148-GS08 Switching Diode 4 (700-2886)
Thin Film Surface Mount Resistor 0603 Case 47Ω 1 (866-7197)
Thin Film Surface Mount Resistor 0603 Case 220Ω 1 (866-7103)
Thin Film Surface Mount Fixed Resistor 0603 Case 510Ω 2 (666-2017)
Thin Film Surface Mount Resistor 0603 Case 10kΩ 1 (866-7030)
Thin Film Surface Mount Fixed Resistor 0603 Case 100kΩ 1 (693-4473)
KEMET 100nF Multilayer Ceramic Capacitor MLCC 16V dc 11 (264-4630)
Murata 1μF Multilayer Ceramic Capacitor MLCC 16V dc 3 (790-0562)
KEMET 2.2μF Multilayer Ceramic Capacitor MLCC 16V dc 2 (691-1170)

 

Main Board

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Here are the schematic diagrams of the main board. The board is composed of 2 drivers, the power supply components, the communication components and the feedback circuit.  

The driver circuit is same as the previous prototype. There is not coil-matching capacitor on the board. But if the coil and the matching capacitor is confirmed, putting the capacitor on board will be better.

Another thing to mention, there is a 1Ω resistor between the 2 grounds. The reason to add a resistor between 2 grounds is to lower the noise issue. Since the driver ground, the current sensing ground and the MCU ground are using the same ground, it will be noisy for the MCU to read the signal when there is a switching voltage (±10V in this case). Adding the 1Ω (or 0Ω) resistor to separate the general ground and the MCU ground without having a significant voltage difference, the noise will be reduced.

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This is the current sensing and the peak detector circuit design for each coil. The inputs of the current sensor (I+ & I-) are coming from the 300mΩ sensing resistor which next to the coil and shows in the schematic diagram of the driver. The connection is referenced to the application example of the AD8216 datasheet, the Vref is connected to GND instead of 2.5V in the case since the positive part of the AC current is enough for this application.

As mentioned in the last article, the AC current shape and phase are designed to obtain. However, due to the difficulties, the target is changed to have the amplitude only. AD8216 is more than enough for the goal.

 

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Mainboard component list:

Item Unit RS Stock No.
Texas Instruments DRV8412DDW, Brushed Motor Driver IC 2 (738-5456)
Texas Instruments SN65HVD230D, CAN Transceiver 1 (461-9759)
Analog devises AD8216WYRZ, Differential Amplifier 3MHz 8-Pin SOIC 4 (806-7903)
Microchip MIC39100-5.0WS, LDO Regulator, 1A, 5 V, 1 (910-1834)
Microchip MIC39100-3.3WS, LDO Regulator, 1A, 3.3 V 1 (910-1569)
Vishay LL4148-GS08 Switching Diode 4 (700-2886)
Nichicon 220F 50 V Aluminum Electrolytic Capacitor SMD 4 (843-2171)

Panasonic Aluminum Electrolytic Capacitor 330μF 16 V

2 (708-5837)
TE Connectivity RL73 Series Thick Film Current Sensing Surface Mount Fixed Resistor 2512 Case 330mΩ 4 (223-1048)
Bourns CRL0603 Series Thick Film Fixed Resistor 0603 Case 1Ω 3 (693-4473)
Panasonic ERA Series Thin Film Surface Mount Resistor 0603 Case 240Ω 4 (828-0718)
TE Connectivity CPF Series Precision Thin Film Surface Mount Fixed Resistor 0603 Case 510Ω 8 (666-2017)
RS Pro Thick Film Surface Mount Fixed Resistor 0603 Case 150kΩ 2 (804-9016)
KEMET 100nF Multilayer Ceramic Capacitor MLCC 16 V 24 (264-4630)
Murata 470nF Multilayer Ceramic Capacitor MLCC 16V dc 2 (798-0271)
Murata 1μF Multilayer Ceramic Capacitor MLCC 16V dc 10 (723-5613)
TDK 2.2μF Multilayer Ceramic Capacitor MLCC 16V dc 6 (915-5673)
Murata 47μF Multilayer Ceramic Capacitor MLCC 16V dc 2 (723-6795)

TE Connectivity Buchanan Series 5mm Pitch Straight, PCB Terminal Block

5 (361-7667)
Straight Pin Header 2.54mm Pitch 59  
Straight Pin Socket 2.54mm Pitch 48  

 

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