EMG Sensor Shirt. Senior Project Written Hardware Description April 28, 2015 ETEC 474. By: Dylan Kleist Joshua Goertz

Transcription

1 EMG Sensor Shirt Senior Project Written Hardware Description April 28, 2015 ETEC 474 By: Dylan Kleist Joshua Goertz

2 Table of Contents Introduction... 3 User Interface Board... 3 Bluetooth... 3 Keypad... 4 LCD... 4 Pre-Amp Boards... 5 Electrode Inputs and Fixed Front end Gain Stage... 5 Primary Band-pass Filter... 6 Secondary Bandpass Filter... 6 Power Components... 7 MCU/Board Inputs... 7 MCU... 8 Signal Inputs... 8 UI Board Connector... 8 Power Connections... 8 EMG Main Board... 9 Programmable Gain/Multiplexing Stage... 9 Mode selector Stage Signal Rectification Multiplexing Switching for Output... 11

3 Introduction This project is an EMG Sensor Shirt. This shirt contains several components, which allow it to sense the contraction of multiple sets of the wearer s muscles. Signals received from the muscles are sent through a preliminary amplification and filter stage close the specified muscle. Followed by a signal rectification and multiplexing to capture every signal in the ADC from the microcontroller to determine the magnitude of muscle contraction. The magnitude value of each measured muscle contraction can then be sent to an external device via Bluetooth link. The Bluetooth connection, calibration and real-time results are all set up using a user interface board. This shirt will effectively be an intermediary device for applications which require access to a user s muscle contractions, such as neuro-muscular health assessments, Exo-skeletal control devices, and physical therapy resistance devices. User Interface Board Bluetooth The data communication that is necessary between the EMG Sensor Shirt and the user s external device, will be done via RN-42 Bluetooth communication link over UART or SPI depending on the necessary speed of transmission between modules. Using the GPIO connected in with the Bluetooth will be what I use to make sure the Baud Rate is set correctly. As the shirt will be the controller for the any system that it is a part of, making the Bluetooth module attached to the User Interface Board the Master in all cases.

4 Keypad Within the User Interface the keypad is made up of eight tactile switches, put into four column by 2 row matrix. Each row and column is using a singular GPIO from the MCU rows as outputs and inputs tied with internal pull-up resistors (10k) for columns. This way the keypad will be scanned one row at a time sending a 0 down that specified row. Knowing a switch is pressed because the column will be brought low making that switch a logic 00. The keypad will be used for the User to navigate through the Bluetooth settings, calibration and real-time results menus. Along with added in silk screening on the top of the board to help the user understand what each button press will do in that present state of the menu. LCD The visual aspect for the user is going to be crucial because without it the Menus, and keypad presses will be useless. For that reason we will be using this character LCD module seen below. I will be using in its four bit mode, using a fewer number of GPIO pins for necessary data transfer than that of the eight bit mode. That being said for this application we are using 6 different GPIO for this display. It is a 4 row by 20 column display giving us a wider range of characters we can display at one time. Since this device will always be written too and not ever read we have grounded the R/W pin. In order to give optimal contrast voltage, at pin 3, between the characters and the LED backlight we used a voltage divider to give Vo = 33 mv.

5 Pre-Amp Boards Electrode Inputs and Fixed Front end Gain Stage The electrodes for the EMG shirt are composed of silver weave flexible conductive fabric, which is attached to a wire through a brass button. There are three electrodes for one EMG signal, at signal input where the wire connects 100k Ohm resistors are placed in series in order to keep maximum electrical discharge in case that full board voltage of 4.8V is shorted to the electrodes down to 48uA. A number of skin effects require that a 10Mohm resistor connect the two electrode inputs together before they are fed to an AD8293G160 fixed gain instrumentation amplifier. This amplifier has a built in filter cutoff of 500 Hz with the application of a 1.2nF capacitor on pin 5 and 6. The amplifier is working with a single supply so a reference voltage is applied at 1.65V to bias the signal at the mid-voltage point to accommodate maximum signal swing. From here the signal is AC coupled and fed to an inverting operational amplifier with a gain of ten. This allows a high front end gain before

6 sending the signal to a band pass filter. The signal must be AC coupled in order to remove a substantial amount of low frequency DC noise at the skin. Primary Band-pass Filter The primary band-pass filter for the pre-amp is constructed so that is will cause a -24dB/octave (4 th Order Band-pass). The cutoffs for the band-pass filter must be set at 100Hz and 250Hz. The 100Hz cutoff must be set to reduce 60Hz frequency noise from electrical power grid and the 250Hz cutoff must be set to reduce movement noise from the fabric electrodes sliding over the skin. This filter stage should have no gain, as gain in the four-pole setup will cause instability. The primary band-pass filter for the pre-amp board is composed of two amplifiers of a quad opamp IC chip. The MCP6044T is used for this application because it has a low bandwidth gain product of 14kHz which further reduces any high frequency noise. These operational amplifiers use a single supply so the non-inverting input must be biased with 1.65V DC in order to accommodate the AC signal. Secondary Bandpass Filter This filter is composed of the final remaining opamp of the MCP6044 IC. It is used to continue filtering the signal but also add gain back into the system before the signal is sent to the main EMG board. Prior to arriving at the secondary band-pass filter the signal underwent attenuation through the 4 pole band-pass filter and this filter will increase the remaining signal amplitude at a gain of 1.2V/V. This small gain will make up for loss from the previous stage but also ensure the signal does not get too large before going the main EMG adjustable gain stage.

7 Power Components Power is regulated on the Pre-Amp board. Due to the fact that the pre-amp board is fed a voltage supply from a battery attached to the main EMG board, the possibility exists that there could be a voltage drop between the two points. In order to get around this, the power is regulated down to 3.3V on the board. The regulated power is then used to power components on the board. In addition to the voltage regulator, a virtual ground reference voltage is created by using a TLE2426 rail-splitter IC. This IC contains precision resistors with a buffer amplifier and is used to form a reference voltage at 1.65V which will be used to bias the EMG signal. MCU/Board Inputs Board inputs are a combination of inputs from the Pre-Amp boards, I/O going back and forth from the UI board. As well as some power components, including the battery, linear regulator and the bypass capacitors necessary for powering the MCU.

8 MCU Our project is going to need a substantial amount of processing power and speed, making us choose the 256 pin K70 Freescale MCU. It is a familiar 3.3V powered board that had the peripherals and processing capabilities that are necessary for our project. Peripherals available to us include Ports A-F with a total of 130 usable pins for GPIO, ADC, UART, SPI and many other alternates available to us. With the combination of all the different aspects of our project this MCU is a great fit for the size of our project. Signal Inputs These two sixteen pin headers are set up to receive the analog muscle signals delivered after they have been amplified and filtered on the Pre-Amp boards. Once the signals reach this point they are able to be fed through the main EMG hardware. UI Board Connector On the main board this 17x2 pin header allows us to connect the ports necessary to run all of the User Interface board devices, such as the Bluetooth, LCD and keypad. Doing this through a simple wire connections from EMG Main board to the User Interface board, permitting us to only connect to the K70 board through one PCI-e connector. This has allowed us to utilize the primary side of the board that contains the all of the necessary ports. Power Connections To start we have the hook up for our 4.8V rechargeable battery that is hooked up to our linear voltage regulator, to bring us down to the necessary 3.3V for our project. But between the powering of the MCU and other portions of the project we have injected the necessary bypass capacitors for power stability. Followed by being fed into our virtual ground/reference voltage IC.

9 EMG Main Board Programmable Gain/Multiplexing Stage The first stage that signals encounter when they enter the EMG main board is the Programmable gain/multiplexing stage. This stage serves two purposes. First it allows the board to accommodate multiple channels of EMG signals with less hardware, second it allows the board to accommodate signal levels of differing levels which may not be high enough to produce good results at later stages within the EMG board. Currently this stage is served by a single MCP6S26 IC OP-Amp. This chip is a non-inverting operational amplifier which has an analog mux that it can use to input different EMG channels. It also has a programmable gain from 1 32 which allows it to increase the signals which will undergo substantial attenuation after the narrow band-pass filtering. The MCP6S26 is programmed via a four wire serial peripheral interface, and can run at a max clock speed of 10MHz. These

10 chips are the only SPI devices on this SPI circuit which means it can be run at full clock speed. Switching which is done for the multiplexed analog signals is done here. This Op- Amp must switch input channels in synchronization with later stages of analog multiplexers. Fifty Ohm resistors are added on the output of each Op-Amp at this stage to accommodate any capacitive loading of the output. Mode selector Stage In order to allow the device to calibrate the signals for maximum amplitude, and two allow two different modes of operation, an intermediate analog mux is needed to allow the device to switch between modes. This stage is filled by a single ADG734BRUZ single pole double throw analog switch. Each switch is tied to a GPIO input, since there is only two modes, this means each input can be tied to a single GPIO bit from the microcontroller. Signals enter from four different EMG channels and are sent to the next stage from here depending on the mode of operation. If the input for each switch is turned high, the signal will be sent to the signal rectifier, then low pass filter. If the input is low, the signal will be sent straight to an analog to digital converter. Signal Rectification If the device is in the low-pass filter mode, the signal will next be passed through an op amp signal rectifier. This utilizes three channels of a quad-operational amplifier. First the signal is AC coupled through a 1uF capacitor and 1.6k Ohm resistor to form a high pass stage of 100Hz, which further attenuates any noise below 100Hz. Once the signal passes the 100Hz filter it loses its DC bias around

11 1.65V and is then applied to an inverting operation amplifier with a gain of 2 and a noninverting amplifier with a gain of 2. The gain must not exceed 2 otherwise the max signal amplitude could exceed the power rails. Once the positive and negative portion of the signal are removed, the resultant signals are pushed into a non-inverting summer amplifier circuit. It is important to understand that the signal which is seen at the input of the summer amplifier in this setup is only half of the signal amplitude. Since a resistor divider is formed at the input. Multiplexing Switching for Output If the mode of operation is set to low-pass filter each analog signal, each channel needs to be passed into its own low pass filter. In order to do this four sets of quad single pole double throw analog switches are used to feed different low-pass filters. This circuit uses one ADG734BRUZ analog switch IC to feed four low-pass filters. The analog signal is first fed to pole A of the switch, one switch at a time is activated on a periodic basis, at the same time the corresponding input for the selected filter is selected at the PROGRAMMABLE GAIN/MULTIPLEXING stage so that the same signal is fed to the same low-pass filter every time. The switches are controlled by four GPIO pins and only one GPIO pins is active at one time. The low pass filter cutoff frequency is set to 4 Hz, it is formed by a resistor and capacitor combination. The output for this filter is fed to an ADC channel on the microcontroller. The low pass filter is connected to the common pin of each single pole double throw switch, when a switch is deactivated, the common pin is then connected to pole B of the switch. This pole is connected to ground through a drain resistor. The time constant of the filter resistor plus the drain resistor with the low pass filter capacitor must be three times longer than the low pass filter time constant. This will ensure the filter capacitor can slew up and down at the same rate over the switching period.