A Flexible Microcontroller-Based Data Acquisition Device
Contents
1. 4 An Example of an Application Board A SimpleDAQ chip and module can be used on the breadboard or in combination with a custom developed application board Signals from the sensors often require signal conditioning such as amplification and filtering This signal conditioning could be achieved using a custom application board In addition to signal conditioning circuitry an application board can also contain other external components such as SPI chips An example of a custom application board is shown in Figure 13a This board contains a low power programmable waveform generator 28 that is capable of producing sine triangular and square wave outputs Waveform generation is required in various types of sensing and actuation applications This chip has two 28 bit frequency registers and two 12 bit phase registers the values of which define the frequency and phase of an output waveform The output frequency can be set from 1 Hz and to 1 MHz The generator is connected to the SimpleDAQ module using a 3 wire SPI interface In addition this application board contains a screw terminal LED indicator analogue temperature sensor and a heating element resistor that is placed above the sensor The temperature sensor is the MCP9701A 29 from Microchip Technology This sensor can accurately measure temperature from 40 C to 125 C The output of the MCP970I1A is calibrated to a slope of 19 3 mV C and has a DC offset of 400 mV The offset a2. 8 DO 1 C T Data Translation DT9812 10V 20 12 bit 50 kS s 8 AI 2 AO 16 DIO 1 C T National Instruments USB 6008 21 12 bit 10 kS s 8 AI 2 AO 12 DIO 1 C T LabJack U12 22 12 bit 8 kS s 8 AI 20 DIO 1 C T Measurement Computing USB 1208FS Plus 23 12 bit 50 kS s 8 AI 16 DIO Notes AI analogue input AO analogue output DI digital input DO digital output DIO digital input output kS s kilo samples per second C T counter timer This paper presents a flexible custom made microcontroller based data acquisition device The key component of the presented device named as SimpleDAQ is a microcontroller with an integrated USB transceiver and 12 bit ADC This SimpleDAQ has been developed as a USB human interface Sensors 2014 14 9758 device HID because most of operating systems natively support this type of USB class and consequently any installation of additional USB drivers is unnecessary The SimpleDAQ contains a preloaded program firmware that enables easy acquisition and the generation of analogue digital signals and data transfer between the device and the application running on a PC via USB Unlike many of the above mentioned solutions where the microcontroller peripheral is fixed in the presented solution the peripheral is flexible and can be easily configured to customised needs without reprogramming the microcontroller When using the developed configuration utility a majority of device
3. SimpleDAQ palette in LabVIEW LOSE l Close vi ga LITILITY Utility Data READ DIGITAL Read Digital READ AHALOG Read Analog Sac M BoARo aie SDAQ board 4 Configure WRITE DIGITAL Write Digital Conf Trigger song M EJARD Fwhd lt gt Write PWM wi SDAQ board ai 4 Utility Write SPLvi EDAG A MoouULE aie Sensors 2014 14 9769 Table 5 The meanings of the more important SimpleDAQ virtual instruments Icon Name Palette Meaning Init vi Function establishes USB connection between the SimpleDAQ chip module and the PC Close vi Function closes the USB connection between the SimpleDAQ chip module and the PC Read Digital vi Data Polymorphic VI obtains the state of one or more digital input line s aca Write Digital vi Data Polymorphic VI sets the state of one or more digital output line s Read Analog vi Data Polymorphic VI e Acquires one sampled value of the signal connected to the selected analogue input e Acquires the signal connected to the selected analogue input Configure Trigger v1 Configure VI sets the trigger settings Write PWM vi Data Polymorphic VI sets PWM frequency and PWM duty cycle Write SPI vi Data VI sends the input message to the SimpleDAQ SPI bus Example of Using the LabVIEW Driver SimpleDAQ enables the acquisitions of analogue input signals using software or hardware clocks An algorithm for the first option is shown in Figure 11 Functions nit and Cl
4. 12 the same authors presented a modified solution for an indoor environmental monitoring system that is based on the IEEE 1451 standard In both cases the developed device was based on PICI8F4550 whilst the communication and the GUI were realised using LabVIEW In 13 a microcontroller based data acquisition system is presented for slowly varying signals This DAQ 1s based on PIC12F675 and enables acquisition of four analogue inputs with 10 bit resolution DAQ 1s connected to a PC using a serial RS 232 bus An application program was developed using Visual Basic and MSComm components Over recent times usage of the popular Arduino platform has been increasing in DAQ applications The Arduino Uno development board which is based on an 8 bit Atmel microcontroller has been successfully used in many applications such as low cost platforms for studying lymphatic biomechanics in vitro 14 a LED stimulator for vision research 15 an open source colorimeter 16 an educational system for teaching the fundamentals of photovoltaic cells 17 electrochemical etching 18 and others In addition to the stated references there are also some off the shelf USB DAQ devices available on the market The key features of those devices that are comparable with the presented solution USB interface 12 bit ADC are summarised in Table 1 Table 1 Off the shelf USB DAQ devices Company Model Key Features Advantech USB 4702 AE 19 12 bit 10 kS s 8 AI 2 AO 8 DI
5. 186 Teikari P Najjar R P Malkki H Knoblauch K Dumortier D Gronfier C Cooper H M An inexpensive Arduino based LED stimulator system for vision research J Neurosci Methods 2012 2 1 227 236 Anzalone G C Glover A G Pearce J M Open Source Colorimeter Sensors 2013 13 5338 5346 Zachariadou K Yiasemides K Trougkakos N A low cost computer controlled Arduino based educational laboratory system for teaching the fundamentals of photovoltaic cells Eur J Phy 2012 33 1599 1610 Jobbins M M Raigoza A F Kandel S A Note Circuit design for direct current and alternating current electrochemical etching of scanning probe microscopy tips Rev Sci Instrum 2012 83 doi 10 1063 1 3695001 Advantech Co Ltd USB 4702 User Manual Advantech Co Ltd Milpitas CA USA 2011 Data Translation Inc DT9812 DT9813 and DT9814 Series User s Manual Data Translation Inc Malboro MA USA 2014 National Instruments Inc NI USB 6008 6009 User Guide and Specifications National Instruments Inc Austin TX USA 2012 LabJack Corporation U12 User s Guide Available online http labjack com support ul2 users guide accessed on 9 May 2014 Sensors 2014 14 9775 23 24 25 26 UA 28 29 Measurement Computing Coorporation USB 1208FS Plus User s Guide Measurement Computing Coorporation Norton MA USA 2014 Microchip Technology Inc PIC18F47J53 Family Data Sheet Availab
6. Design and Development of Low Cost Multi Channel USB Data Acquisition System for the Measurement of Physical Parameters Int J Comput Appl 2012 48 47 51 Datta J Chowdhuri S Bera J Sarkar G Remote monitoring of different electrical parameters of multi machine system using PC Measurement 2012 45 118 125 Gupta R Bera J N Mitra M Development of an embedded system and MATLAB based GUI for online acquisition and analysis of ECG signal Measurement 2010 43 1119 1126 Zoric A C Perisic D Obradovic S Spalevic P PC Based Virtual DTA Recording System Design Prz Elektrotech 2011 87 156 160 Yavuz C Yilmaz S Kaya M The Design of Computer Controlled Cold and Hot Therapy Device with Thermoelectric Module J Med Imaging Health Inform 2013 3 221 226 Kumar A Singh I P Sud S K Energy Efficient Air Quality Monitoring System In the 10th IEEE Conference on Sensors Limerick Ireland 28 31 October 2011 pp 1562 1566 Kumar A Singh I P Sud S K Energy Efficient and Low Cost Indoor Environment Monitoring System Based on the IEEE 1451 Standard IEEE Sens J 2011 71 2598 2610 Singh N M Sarma K Design of PIC12F675 Microcontroller Based Data Acquisition System for Slowly Varying Signals J Instrum Soc India 2012 40 15 17 Kornuta J A Nipper M E Brandon Dixon J Low cost microcontroller platform for studying lymphatic biomechanics in vitro J Biomech 2013 46 183
7. pins can be configured as analogue input digital input output PWM output or one of serial peripheral interface SPI lines In this way the device can be easily tailored to the needs of a final DAQ application In addition LabVIEW drivers have been developed for the presented device LabVIEW is a well known industry proven block programming environment commonly used for data acquisition instrument control and industrial automation on a variety of platforms Microsoft Windows Linux and Mac When using the developed drives data acquisition and data analysis algorithms as well as graphical user interface GUI can be easily developed using LabVIEW The remainder of this paper is organised as follows Section 2 contains a short description of the developed device s hardware In Section 3 the firmware configuration utility and LabVIEW driver for the developed device are briefly described Section 4 contains a short description of those applications that are based on the presented platform Finally the conclusions are stated in Section 5 2 DAQ Hardware Overview This section contains a short hardware overview of developed DAQ device The section begins with microcontroller selection requirements and continues with a short description of the selected microcontroller and developed DAQ hardware 2 1 Microcontroller Selection Selection of an appropriate microcontroller for our DAQ device was taken according to the following four main ini
8. Figure 2 SimpleDAQ module TT TTT TP PP ay ae i d i es WEEEREREE SEE Ss ae T ees EET Sensors 2014 14 9761 3 DAQ Software Overview The SimpleDAQ software includes the following parts 1 the firmware that 1s deployed on the microcontroller 2 SimpleDAQ configuration utility that enables customisation of microcontroller s peripheral to the customised needs and 3 a LabVIEW driver that enables easy creation of data acquisition generation data analysis and data storage algorithms using a LabVIEW development environment 3 1 The Firmware SimpleDAQ firmware has been developed using Microchip MPLAB X Integrated Development Environment IDE MPLAB X IDE 1s a free software program that runs on a PC Windows Mac OS Linux and enables the development of embedded applications on Microchip s PIC microcontrollers The firmware has been written in ANSI C language and compiled into the binary code using an MPLAB C Compiler for the PIC18 family of 8 bit MCUs MPLAB C18 Figure 3 SimpleDAQ firmware flowchart Main loop Bootloader wee Firmware upgrade button pressed mode No Init variables and constants Configure chip Pin config I O pins data l l l l l l l l l l l l l l l l l l l l l l l l l l l l l m Initialize USB l l l l l l l l l l l l l l l l l l l l l l l l l l Process USB
9. Sensors 2014 14 9755 9775 do1 10 3390 s140609755 Sensors ISSN 1424 8220 www indpi com journal sensors Article A Flexible Microcontroller Based Data Acquisition Device Darko Hercog and Bojan Gergi Institute of Automation Faculty of Electrical Engineering and Computer Science University of Maribor Smetanova ulica 17 Maribor 2000 Slovenia E Mail bojan gergic um s1 Author to whom correspondence should be addressed E Mail darko hercog um 3s1 Tel 386 2 220 7309 Fax 386 2 220 73 15 Received 25 March 2014 in revised form 23 May 2014 Accepted 27 May 2014 Published 2 June 2014 Abstract This paper presents a low cost microcontroller based data acquisition device The key component of the presented solution is a configurable microcontroller based device with an integrated USB transceiver and a 12 bit analogue to digital converter ADC The presented embedded DAQ device contains a preloaded program firmware that enables easy acquisition and generation of analogue and digital signals and data transfer between the device and the application running on a PC via USB bus This device has been developed as a USB human interface device HID This USB class is natively supported by most of the operating systems and therefore any installation of additional USB drivers is unnecessary The input output peripheral of the presented device is not static but rather flexible and could be easily configured to customised needs w
10. ants whilst in 2 for thermoelectric property measurements In the latter case the DAQ is based on a PIC 8F4550 microcontroller The resolution of the microcontroller s internal ADC 10 bit has been improved by using external 22 bit delta sigma ADC with a performance of 60 samples per second S s The DAQ GUI was developed in Delphi In 3 a data acquisition and control system for high speed gamma ray tomography based on the USB and Ethernet communication protocols has been designed This system is based on Microchip s PIC18F4550 and PICI8F4620 whilst the DAQ software is realised using LabVIEW In 4 the authors present a microcontroller based USB DAQ device for radiation detection and environmental monitoring purposes The developed DAQ which is based on an 8 bit AVR microcontroller has 8 analogue input channels 10 bit ADC and enables acquisition of analogue input with a maximum sampling rate of 50 kS s In 5 the authors present a low cost USB DAQ based on the PIC18C442 microcontroller with integrated 10 bit ADC 12 bit A D and D A resolutions have been achieved using external A D TI ADS803 and D A TI DAC7545 converters A USB data acquisition system for humidity and temperature measurements is presented in 6 This DAQ device is based on PIC18F4550 and enables acquisition of 8 analogue inputs with 10 bit resolution Humidity measurement is achieved using an HIH 4000 humidity sensor whilst for the temperature using an LM35 temperature sen
11. ata to SPI bus Set chip select pin to high state Create response message Read digital inputs Create response message Read acquire analog inputs Store DAQ parameters ouro waveror Create response messages SSS Soe oe 3 1 1 Digital Inputs Outputs All the SimpleDAQ flexible pins can be configured as digital input or digital output pins If a pin is configured as digital output an initial value 0 or 1 can be additionally defined 3 1 2 Analogue Inputs The SimpleDAQ chip has up to 10 analogue inputs connected to a 12 bit ADC Analogue inputs can be acquired as a single sample or finite samples waveform The latter option refers to the acquisition using predetermined sampling frequency and a predetermined number of data samples In the finite sampling mode the samples are initially stored within the SimpleDAQ buffer Once the specified number of samples has been acquired the acquisition stops and the sampled data are transferred from the microcontroller buffer to the client application on the PC via USB Figure 5 SimpleDAQ has the following limitations in the finite sampling mode 1 the maximum sampling frequency is 100 kHz and 2 the maximum number of samples in the buffer is limited to 1024 These Sensors 2014 14 9764 limitations are the consequence of microcontroller RAM size and the performance of the internal ADC The ADC includes a self calibration feature which compensates for any offset generated
12. bootloader button pressed on the SimpleDAQ module then the bootloader forces itself into the firmware upgrade mode Figure 3 Otherwise the firmware application code starts executing In firmware upgrade mode the bootloader communicates with the PC host application Figure 7 that is used to perform erase and programming operations Bootloader waits for new data new firmware and loads it into the remaining part of the flash memory Figure 7 SimpleDAQ USB HID Bootloader PC application a SimpleDAQ US3 HID Bootloader ss fos Open HecFie _fraseDevice Read Device EmotHex Program Venty veiy Oe Reset Device a Allow Corfiguretion Word Programming Erase Started no status update until complete may take several seconds Erase Complete Programmng Started Programing Complete Verity Bunning Erase Progran Verify Completed Successfully 3 2 Configuration Utility SimpleDAQ Configuration Utility Figure 8 is a PC program that enables changing the functionalities of microcontroller pins without reprogramming When using this utility the microcontroller s peripheral can be easily tailored to the needs of a customised application In total 18 out of 28 pins are configurable The configuration utility Figure 8 contains the Current pin assignment table with a list of all the SimpleDAQ pins Modification of an individual pin can be achieved by selecting the desired pin within the Current pin assignment table a
13. d analog input channel VISA resource name A a B Tonia n c M a A D ae ic a time out ms i p of a and Time The output of the signal generator 1s connected to pin 3 of the SimpleDAQ module and the output of the temperature sensor to pin 1 respectively This application board was developed especially for educational processes A SimpleDAQ module in combination with this application board Figure 13b represents a simple and low cost platform for signal generation signal acquisition and also for simple temperature control Figures 14 and 15 present the LabVIEW application front panel and block diagram that enables signal generation and acquisition using this platform The output of the signal generator is set at 100 Hz sine wave whilst an acquisition of this signal is performed using a hardware timer having a sampling rate of 100 kS s 5 Conclusions This paper presented a flexible microcontroller based DAQ device The presented hardware and software solutions enable easy realisations of custom data acquisition and signal processing algorithms using microcontroller I O peripheral and LabVIEW development environment This approach eliminates tedious and time consuming low level microcontroller programming and enables developers to focus their energy onto custom analogue front end development if necessary and data analysis algorithms developments The presented solution is also highly appropriate for other technical fields such a
14. d to the ADC sampling rate SimpleDAQ 1s one of the best but it has a limited on chip buffer SimpleDAQ supports only single ended analogue inputs whilst some commercial DAQs also support differential ones SimpleDAQ does not contain any analogue output as the used microcontroller does not include a digital to analogue converter DAC However analogue outputs can be realized using external SPI DAC chip on a custom application board SimpleDAQ also includes some peripheral which could not be found on commercial devices such as SPI bus and PWMs The SPI bus in particular is very usefully hence the variety of different SPI chips could be easily connected to the SimpleDAQ device The presented solution has many potential uses It is relatively powerful inexpensive and flexible The presented DAQ could also be used as a platform for custom smart sensor development When using the presented solution the prototype of a sensor application can be developed without microcontroller programming Data acquisition data analysis and decision making algorithms can be developed on the PC Once the developed algorithms have been proven and provide satisfactory results they can be forwarded to a software developer group that writes optimised codes for this microcontroller To date this solution has been successfully tested on the Windows operating system However most operating systems Windows Linux Android etc natively support the USB HID class In our future
15. ers DAC digital inputs outputs counters computer bus circuits for digital routing and clock generation circuits for automatic offset and gain calibrations etc Such large scale DAQ devices are very powerful and suitable for applications where functionality has precedence over price such as within the research area and industrial applications There are however also several applications where such state of the art solutions are sometimes unnecessary In the education process or simple low cost applications for example less efficient scalable and low cost DAQ devices are welcome Most of DAQ hardware producers also provide low cost DAQ solutions but such devices cost at least 150 and that is still too expensive for low budget applications Nowadays very simple and low cost DAQ devices could be realised using microcontrollers with integrated analogue to digital converters ADC A microcontroller is a small computer on a single integrated circuit containing a processing unit memory and programmable input output peripherals Several microcontroller based data acquisition DAQ systems have been presented by different authors In the majority of the presented solutions the developed DAQ devices are not general purpose DAQs but rather have been specially designed for specific DAQ applications such as temperature or humidity measurements In 1 the authors present a microcontroller based DAQ that is applied to decentralised renewable energy pl
16. frequency 1 byte Request message l 5 Sampling frequency 2 byte 6 Number of samples 1 byte 7 Number of samples 2 byte 8 Trigger timeout in ms 1 byte 9 Trigger timeout in ms 2 byte Command CMD READ ANALOG _ WFM Response message i i i 1 to 2 Number of samples Acquired data The maximum message size for the USB HID class is 64 bytes All the request messages are smaller than 64 bytes whilst the response messages can be larger In this case the firmware splits the response message into several 64 bytes messages The current version of SimpleDAQ firmware supports the following requests from the client 1 read the state of one or more digital inputs 2 set reset one or more digital outputs 3 acquire single value from one or more analogue inputs 4 acquire waveform from analogue input 5 configure the trigger parameters 6 send forward the retrieved data to SPI bus 7 configure the PWM frequency on PWM output 8 configure the duty cycle on PWM output 9 reset SimpleDAQ and 10 configure SimpleDAQ pins according to the input configuration data The flowchart of the request message processing procedure is presented in Figure 4 More than half of the SimpleDAQ pins are flexible which means that the functionalities of these pins can be changed without microcontroller reprogramming For example an individual pin can be configured as digital input digital output analogue input PWM output or one of three SPI
17. ithout changing the firmware When using the developed configuration utility a majority of chip pins can be configured as analogue input digital input output PWM output or one of the SPI lines In addition LabVIEW drivers have been developed for this device When using the developed drivers data acquisition and signal processing algorithms as well as graphical user interface GUI can easily be developed using a well known industry proven block oriented LabVIEW programming environment Keywords data acquisition DAQ microcontroller analogue to digital converter ADC USB HID LabVIEW GUI data logging Sensors 2014 14 9756 1 Introduction Data acquisition DAQ is the process of measuring an electrical or physical phenomenon using a computer The complete DAQ system consists of sensors DAQ hardware and a computer with programmable software The sensor performs a conversion of the physical phenomenon into an electrical signal This signal is further converted into digital numeric values by DAQ hardware which is controlled by a software program developed using various general purpose programming languages C LabVIEW Visual Basic MATLAB In addition to the hardware control such DAQ software usually also includes data analysis data visualisation and data storage algorithms Commercially available DAQ devices range from relatively simple devices up to very sophisticated ones The latter ones contain ADC digital to analogue convert
18. lave modes of operation This family of microcontrollers is ideal for applications requiring cost effective low power USB applications with more code space and peripheral flexibility set within a small package The key features of the selected PIC18F27J53 microcontroller are summarised in Table 2 Table 2 Microchip PIC18F27J53 specifications Parameter Name Value Program Memory Type Flash Program Memory 128 KB RAM Bytes 3 800 CPU Speed 12 MIPS Digital Communication Peripherals 2 A E USART 2 MSSP SPI I2C Capture Compare PWM Peripherals 7 CCP 3 ECCP Timers 4 x 8 bit 4 x 16 bit A D converter 10 ch 12 bit USB ch speed compliance 1 Full Speed USB 2 0 Temperature Range 40 to 85 C Operating Voltage Range 2 to 3 6 V Pin Count 28 Package Type SPDIP 2 2 SimpleDAQ Chip The SimpleDAQ chip is actually a Microchip PICI18F27J53 microcontroller 24 with preloaded custom developed firmware the firmware is described in more detail in Section 3 1 The deployed firmware enables the acquisition and generation of analogue digital signals and data transfer between the chip and the computer using USB serial bus The SimpleDAQ chip includes a built in USB transceiver 12 bit A D converter and 18 flexible I O pins These pins could be configured as analogue inputs digital inputs outputs PWM outputs or one of three SPI Serial Peripheral Interface Bus lines The SimpleDAQ chip is procurable within a 28 pin SPDIP package that can be easily p
19. le online http ww 1 microchip com downloads en DeviceDoc 39964B pdf accessed on 7 March 2014 Wikipedia USB human interface device class Available online http en wikipedia org wiki USB human interface device class accessed on 12 March 2014 IEEE Standard for Terminology and Test Methods for Analog to Digital Converters IEEE Standard 1241 2010 2011 doi 10 1109 IEEESTD 2001 92771 Holcer R Michaeli L Saliga J The Test of the Ad Converters Embedded on Two Microcontrollers Measurement Sci Rev 2001 7 55 58 Analog Devices Inc Programmable Waveform Generator AD9833 Data Sheet Available online http www analog com static imported files data_sheets AD9833 pdf accessed on 16 March 2014 Microchip Technology Inc MCP9700 9700A and MCP9701 9701A Low Power Linear Active Thermistor ICs Data Sheet Available online http ww1 microchip com downloads en DeviceDoc 21942e pdf accessed on 20 March 2014 2014 by the authors licensee MDPI Basel Switzerland This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license http creativecommons org licenses by 3 0
20. lines The pin configuration is achieved using the SimpleDAQ configuration utility as described in the next section This utility creates the configuration data and stores it within the microcontroller flash Sensors 2014 14 9763 memory At the microcontroller power up at the booting stage the SimpleDAQ configures its pins regarding the settings stored within the flash memory Figure 4 SimpleDAQ firmware flowchart processing request message oe e en Process request message page 1 of 2 Extract command Switch command Case No Case No Case No Case No Case No Read Digital Write Digital Read Analog Read Analog Write SPI WFM Yes eos Read and check input parameters channel sampling frequency of samples etc Read and check input parameters chip select pin of bytes to send SPI data Read and check input parameters of pins to read pin numbers l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l Params I OK l l Yes l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l Read and check input parameters of pins to set pin numbers value Read and check input parameters of channels to read channel numbers Set chip select pin to low state Write SPI d
21. llows measurements of the negative temperatures without the need for a negative supply Vow r Ly Voc 4 where T is the ambient temperature V out at 0 C Vc 400 mV and c is the temperature coefficient c 19 3 mV C is the sensor output voltage V is the sensor output voltage Sensors 2014 14 9771 Figure 13 a An application board b An application board with an attached SimpleDAQ module a b Table 6 Key components of an application board Component Label Programmable signal generator AD9833 U2 Temperature sensor MCP9701A Ul Heating element resistor R2 LED D1 Screw terminals J1 and J2 Sockets for SimpleDAQ module J3 and J4 Figure 14 Application front panel 13 Simp eDAQ buerd Signal Gerneraliun and Acquisition nese Signal acquisition Trgger settings Sigral generatior P n rumber Bo mai pin Enable trigger Output frequency Hz Actual recucncy Hz E C w g1 99 9579 Sampling frequency Hz Triqcer slope A 100000 4 Positive slope Trigger Lire vul Type of wave Sint Wave gt l Number cf samples Trigger level Ar i y 1000 Ji 0 3 Tiiyyer Lire vul rms As ACQUIRE SIGNAL g 1000 Waveform of sclected aralog input channel Plot 0 az l 0 8 Vokage V 0 003 0 004 0 c05 Time 5 Sensors 2014 14 9772 Figure 15 Application block diagram ftType of wave gt ftOutput fequency Hz gt Trigger slope Waveform of selecte
22. lugged directly into the breadboard Figure 1 For the basic operation of the SimpleDAQ chip on the breadboard only a few additional external components are necessary i e oscillator with capacitors USB connector and 3 3 V power supply Figure 1 Using 5 V to 3 3 V voltage regulators the power can be obtain directly from the USB bus as shown in Figure 1 Sensors 2014 14 9760 Figure 1 Example of using the SimpleDAQ chip on the breadboard The resolution of the internal ADC is 12 bits and the input range 1s from 0 V to 3 3 V The code width of the SimpleDAQ chip is as follows pode ei 1 ey tesglikon in bits oN 2 3 SimpleDAQ Module On the basis of the SimpleDAQ chip a SimpleDAQ module has been further developed This module contains all the components oscillator USB connector efc that are necessary for the basic operation of the SimpleDAQ chip This module is powered from USB therefore any external power is unrequired The module contains a SimpleDAQ chip labelled U3 in Figure 2 within the SMD package an oscillator Y1 mini USB connector J1 voltage regulator U1 for power supply from USB reset button R S1 general purpose button B S2 two 12 pin extension connectors J3 and J4 for the customary application board debugger connector J2 and other less necessary components The module can be plugged directly into the breadboard or it can be used in combination with a custom developed application board
23. message form host Process request message Send response message s to host Background tasks ma M a ee ee ae ee La Sensors 2014 14 9762 SimpleDAQ is developed as a USB human interface device HID The USB HID class is a part of the USB specification for computer peripherals it specifies a device class for human interface devices such as keyboards mice game controllers and other devices 25 Most of operating systems natively support the USB HID class therefore the end users do not need to install additional drivers The maximum transfer rate for USB HID devices 1s limited to 64 kB s SimpleDAQ firmware is developed in the form of a simple server application i e the firmware periodically checks for incoming requests messages from USB Figure 3 An application on the PC client needs to initiate a communication session with the SimpleDAQ and sends a request s When the SimpleDAQ receives an incoming request it performs the requested task s and transmits the results back to the client application Figure 3 Requests and responses are in the form of byte messages packets An example of a request response message is presented in Table 3 Table 3 An example of a request response message acquire waveform from analogue input Message Type Byte Number Byte Meaning 0 Command CMD READ ANALOG WFM 1 Sampling channel pin number 2 Reference 0 internal 1 external 3 Trigger 0 disable 1 enable 4 Sampling
24. nd pressing the MODIFY button In the Pin Settings window that appears Figure 9 the user can select one of the pre set options for the selected pin Pin labels are taken from the microcontroller s datasheet 24 as Sensors 2014 14 9767 briefly explained in Table 4 By selecting an appropriate option in Pin Assignment dropdown menu additional options may appear that are specific to the selected option For example if an option RAO digital input or output is selected the Pin direction and Initial value appear under the Additional settings section When using the first option the user can select the pin direction input or output whilst the second option enables the setting of the pin s initial value 0 or 1 when the pin is configured as digital output Figure 8 SimpleDAQ configuration utility 3 SimpleDAQ Configuration Utility MODIFY ANI PRINT ANZ i i ANS VDDCORE RAS VSSL OSCI DSC RCO RCL RC2 VUSB D D akl leca Figure 9 a Pin settings window b Pin assignment menu 43 Pin settings Pin assignment Pin number h Fie RAO w Additional settings Pin direction 4 Input Initial value F 7 yf a b The configuration utility creates the configuration data stream and sends it to the SimpleDAQ chip via USB The SimpleDAQ firmware configures the pins according to the received configuration stream and also stores the retrieved configuration within the flash memo
25. ose are used for establishing and closing USB connection with the SimpleDAQ device At each While loop iteration one sampled value is retrieved using Read Analog 1Ch 1Samp function The software clock is defined using Wait Until Next ms Multiple timer function inside the While loop Figure 11 In the presented algorithm an analogue signal that is connected to pin 2 of the SimpleDAQ device is acquired every 50 ms Figure 11 Acquisition of single sample using a software clock Pin number VISA resource name k SimpleDAQ Wait Until Next ms Multiple 28 cma Acquisition of a finite number of samples using the hardware clock on a SimpleDAQ device is shown in Figure 12 The use of a hardware clock allows faster and more accurate signal acquisition In this acquisition mode the sampled data are initially stored within the buffer on the SimpleDAQ device Sensors 2014 14 9770 At the end of the acquisition process the acquired data is transmitted to the PC application Acquisition using a hardware clock is achieved using the Read Analog Wfm function in the LabVIEW algorithm Figure 12 This function requires the following input parameters 1 analogue input channel chip pin number 2 sampling frequency in Hz and 3 the desired number of samples Figure 12 Acquisition of waveform using a hardware clock Pin number Sampling frequency Number of samples 100 Waveform VISA resource name k SimpleDAQ
26. ry At each SimpleDAQ reset the firmware configures its pins regarding the settings stored in the flash memory Sensors 2014 14 Pin Number Pin Assignment Options 2 Yn BW 11 12 13 17 18 21 22 23 24 25 26 27 28 Table 4 Pin assignment options in current version of firmware RB4 CCP4 SCK1 RBS CCP5 SDI 3 3 LabVIEW Driver RAO ANO RA1 ANI RA2 AN2 RA3 AN3 RAS AN4 RCO RCI RC2 AN11 RC6 RC7 SDO1 RBO AN12 RB1 AN10 RB2 AN8 RB3 AN9 RB6 CCP6 RB7 CCP7 Assignment Meaning Digital I O Analogue input Digital I O Analogue input Digital I O Analogue input Digital I O Analogue input Digital I O Analogue input Digital I O Digital I O Digital I O Analogue input Digital I O 9768 Digital I O Serial peripheral interface SPI data out Digital I O Analogue input Digital I O Analogue input Digital I O Analogue input Digital I O Analogue input Digital I O PWM SPI clock Digital I O PWM SPI data in Digital I O PWM Digital I O PWM A complete LabVIEW driver for the SimpleDAQ chip module was developed This driver enables easy access to the SimpleDAQ I O peripheral using LabVIEW application on a PC After the driver s installation all the SimpleDAQ related virtual instruments VI s appear on the SimpleDAQ palette Figure 10 The short meanings of the more important SimpleDAQ VI s are presented in Table 5 4 1 SimpleDAQ Figure 10
27. s chemistry or mechanical engineering where most technicians are not proficient in embedded programming but usually have a good knowledge of the LabVIEW programming environment Unlike many related solutions where a microcontroller based DAQ has been developed for a specific application this solution is configurable and can be utilised for different customised applications In comparison to the commercially available DAQ devices Table 1 this solution has some advantages and disadvantages Commercial products include analogue front end with configurable analogue inputs range In contrast the presented solution is without analogue front end and consequently the input range for I O pins is limited to the microcontroller voltage level between 0 V Sensors 2014 14 9773 and 3 3 V If the users have to use different input voltage ranges they have to develop their own application board with appropriate voltage level shifters However the possibility of using a custom developed analogue front end that is specific to the used sensors could actually be an advantage in some applications DAQ applications vary according to the required I O peripheral For example some applications require more analogue inputs whilst others more digital I O s From this point of view SimpleDAQ is more flexible because the I O peripheral can be changed whilst the I O peripheral of commercial available DAQs 1s fixed for example DAQ contains 8 AI 2 AO 8 DI 8 DO In regar
28. sor An application program on the PC was developed using Visual Basic Datta et al 7 presented a DAQ Sensors 2014 14 9757 device for the current voltage frequency and power factor measurements of electric motors The DAQ 1s connected to the PC using RS 485 serial interface and enables sampling rates of up to 960 S s An application program on the PC was developed using Visual Basic In 8 Gupta et al describe a low cost DAQ for electrocardiogram ECG monitoring The sampling rate of the developed DAQ is 1 kS s and is connected to a PC using the RS 232 serial bus whilst an application program was developed using MATLAB In 9 the author presents a microcontroller based DAQ for differential thermal analysis The DAQ is based on the PICI8F4550 microcontroller and two K type thermocouples Cold junction compensation and digitisation of a signal from a type K thermocouple was achieved using a Maxim MAX6675 chip connected to the microcontroller via an SPI bus The DAQ enables sampling rates of up to 4 S s The user interface was realised using LabVIEW A similar project is presented in 10 In this case the device is based on a PIC18F4550 microcontroller and an ADS595AQ monolithic thermocouple amplifier that enables temperature measurements of K type thermocouples The GUI was developed in Visual Basic In 11 the authors present a USB air quality monitoring system that can measure and analyse the concentrations of major air pollutant gases In
29. the peak to peak amplitude of the applied sine wave In order to measure the ENOB a sine wave test signal with peak to peak amplitude 3 V and DC offset 1 65 V was applied to the ADC input of the SimpleDAQ module from the Tektronix AFG 3021B arbitrary function generator The module was supplied from the PC via the USB port The sine wave was then acquired using a SimpleDAQ LabVIEW driver and the SINADgpg was calculated using a LabVIEW built in analysis function Coherent sampling and uniform distribution of the samples in this phase were achieved by setting the input frequency according to 26 J r 3 where fi Input signal frequency fs Sampling frequency M Number of samples in the test sequence J Integer number of input signal periods within the set sequence which is relatively prime to the M The highest sampling frequency was 100 kHz and the number of samples was limited by the microcontroller s memory to 1024 The test results for different frequencies between 10 Hz and 1 MHz are given in Figure 6 Note that the input frequencies above the Nyquist frequency f 2 are aliased and that slightly better results could be achieved with low pass and band pass filtering of the input signal Figure 6 ENOB results for SimpleDAQ module 12 11 10 ENOB Lo 101 102 10 107 10 10 Frequency Hz 3 1 3 Serial Peripheral Interface SimpleDAQ also contains the serial peripheral interface SPI bus through which it is possible
30. tial requirements 1 Microcontroller must have integrated USB 2 0 stack The data exchange between a PC and the microcontroller must be realised directly using universal serial bus USB without any additional components such as USB to serial converters In this way the device would be simple and as low cost as possible 2 Microcontroller must have integrated ADC with at least 12 bit resolution This resolution should be good enough for a majority of data acquisition applications 3 Microcontroller must be procurable as a dual in line package DIP or DIL and also in a surface mount device SMD package The DIP package is very useful for experimentation because it can be plugged directly into an experimental board breadboard 4 Microcontroller must be low cost The main idea was to develop a low cost DAQ device therefore the limit was set at 5 per unit Sensors 2014 14 9759 In regard to the stated requirements and detailed market review a Microchip PICI8F27J53 microcontroller was selected as our main platform This microcontroller is a member of the Microchip PICI8F47J53 24 family of microcontrollers that incorporates a fully featured USB communications module with a built in transceiver that is compliant with the USB Specification Revision 2 0 The microcontroller also includes two independent Enhanced USARTs and two Master Synchronous Serial Port MSSP modules capable of both Serial Peripheral Interface SPI and I2C Master and S
31. to send data to other peripheral devices that support SPI communication A SimpleDAQ chip has the role of master that can send the data to the various SPI devices slaves The master determines using a chip select pin as to which slave the data should be transmitted The pin number of the desired chip select pin is retrieved from the request message form of the client application This pin must be configured using the SimpleDAQ Configuration Utility as a digital output pin and must have an initial value of 1 Sensors 2014 14 9766 When the microcontroller retrieves the request for sending forwarding the data to SPI bus Figure 4 it firstly extracts the chip select pin number from the input message and then initiates the SPI communication by turning the chip select pin to a low state The microcontroller then forwards the received message from the USB to the SPI byte by byte When it has finished sending the data it turns the chip select pin back to high state and disables the SPI communication 3 1 4 PWM Outputs The SimpleDAQ chip also has up to four PWM outputs The firmware enables generation of PWM signals with a frequency of between 2 9 kHz and 100 kHz and a duty cycle of between 0 and 100 3 1 5 Bootloader SimpleDAQ firmware includes a bootloader that enables upgrading of the firmware without the need for an external programmer or debugger The bootloader code starts executing on device reset If the bootloader condition is met
32. within the module The calibration process is automated and is initiated after each device reset The calibration routine performs an offset calibration and stores the results internally After that all subsequent readings are automatically compensated Figure 5 SimpleDAQ firmware flowchart Waveform acquisition process Acquire waveform without trigger DAQ parameters sampling channel frequency etc te m N of samples to acquire i 0 of acquired samples Start ADC conversion Store ADC result into buffer Wait for interrupt ae e e e e e e e a e e e e e e e e e E E e E O E E O E O E E O E O E O O E O E O O O O O O O e e e e e o e e e a X X Dynamic Performance of an ADC In addition to the properties of the used ADC given in the microcontroller data sheet 24 the effective number of bits ENOB were measured in accordance with IEEE standards 26 The ENOB is a way of quantifying the dynamic performance of an analogue to digital conversion 27 For an input sine wave of specified frequency and amplitude the ENOB is the number of bits of an ideal ADC for which the root mean square rms quantization error is equal to the rms noise and distortion of the ADC under test The ENOB can be calculated from the signal to noise and distortion ratio as expressed in dB SINADas SINAD 1 76 oriog E A 2 PP ENOB 6 02 Sensors 2014 14 9765 where FSR is the specified full scale range of the ADC and A is
33. work we will also develop example applications for other operating systems and programming environments Java Visual Basic MATLAB Delphi etc Author Contributions The presented work was carried out in collaboration between both authors Darko Hercog developed the software and wrote the manuscript while Bojan Gergi designed the hardware and supervised the work Both authors have read and approved the final manuscript References 1 Juca S C S Carvalho P C M Brito F T A Low Cost Concept for Data Acquisition Systems Applied to Decentralized Renewable Energy Plants Sensors 2011 71 743 756 2 Sumphao T Thanachayanont C Seetawan T Design and Implementation of a Low Cost DAQ System for Thermoelectric Property Measurements Proced Eng 2012 32 614 620 3 Hyertaker B T Maad R Schuster E Almas O A Johansen G A A data acquisition and control system for high speed gamma ray tomography Meas Sci Technol 2008 19 doi 10 1088 0957 0233 19 9 094012 Sensors 2014 14 9774 10 11 12 13 14 15 16 17 18 19 20 21 22 Hwang Y S Kim H J Park H Kang H Kim W Kim S Development of Embedded DAQ System for Flux Counting J Nucl Sci Technol 2008 45 Supplement 5 582 585 Murovec B Kocijancic S A USB based data acquisition system designed for educational purposes Int J Eng Educ 2004 20 24 30 Singh N M Sarma K C Singh N G
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