Lab 2
Contents
1. breadboard and powered from the control module There are two sets of terminals for RS the large terminals connect to the source and load while the small header terminals connect to the energy meter circuit SOURCE LOAD Load SOURCE 4 Current scaling and offset 5V I 1 1 i i 1 1 i 1 asv 1 1 l i 1 1 l I 1 1 ale R8 5 UIA NEIMUR DO a7 C3 U1B 100nF 1 2 MCP602 i i Al Current sense Das ca ingest ek att Ss sas Gt is sig Pha cpa ae Sak Sa ah sh Sc a Se Fs J Figure 1 The energy meter sits between a source and a load It connects between the source and the load on the negative side Two analog inputs of the processor module AO and A1 are used to sense the load voltage and current respectively A voltage divider composed of resistors R1 and R2 scales the load voltage VL to the 0 2 5V input range of the processors ADC The voltage divider of R7 and R8 should set the zero input voltage on AO and A1 to 1 25V the center of the ADC range to allow bipolar positive and negative measurement The RS 10m current sense resistor connected between NL and NS converts the 10A load current into a 100mV voltage We connect LOAD to processor ground so that our voltage measurement is the voltage across the load which differs from the voltage across the source by the amount dropped across the current sense resistor We use operationa2. EE 155 255 Lab 2 Revision 1 September 28 2015 Lab2 Energy Meter In this lab youll build and program a meter that measures voltage current power and energy at DC and AC Assigned September 28 2015 Due Week of October 5 2015 Part 1 New Code Pull the latest code from the class Github repo using git pull If you are working on a private repos itory you will have to run git pull upstream master assuming you have mapped a remote to the class repository If you haven t set up a remote repository for the class Github you can through the com mand git remote add upstream https github com ndanyliw green electronics git The lab starter files will be under labs lab2 For this lab you should be able to do everything by editing lab2 c and lab2 h Code is already provided to read and write calibration data in EEPROM Part 2 Analog to Digital Conversion To measure power delivered to the load you need to measure the voltage across the load and current through the load nearly simultaneously Circuitry is provided to convert the load voltage and current into voltages which are safe for the microcontroller to measure The STM32F303 has 4x12 bit 5 Msps ADCs connected to 39 external channels There are many modes in which these ADCs can be operated and controlled For those who are curious please check out the chip datasheet and user manual The EE155 library operates in a dual conversion mode which allows capturing data s
3. EEPROM and the calibrate_ functions write it back to EEPROM when its updated Page 5 of 6 EE 155 255 Lab 2 Revision 1 September 28 2015 Part 5 Part 6 Testing Use a heating element as the load This device is almost purely resistive and can dissipate lots of power enough to burn you so dont touch it after you start delivering significant power to it For a DC supply use one of the 100V 10A power supplies Demonstrate that your energy meter correctly measures the power and energy delivered to the load for three different operating points Apply a known amount of power for a known amount of time and verify that the total energy delivered is as predicted How accurate is your meter How much error does it show under heavy load 1kW and under no load Use the infrared thermometer to measure the temperature of the load after delivering 1kJ Signoffs 1 Show DC power and energy measurements for at least three different operating points Show correct four quadrant measurements 1 Exchange the source and the load to invert the current 2 Exchange the source terminals to invert the voltage Page 6 of 6
4. e found in the ge_adc c source To enable a channel use the adc_enable_channel int chan function which will enable the specified channel on both ADC1 and ADC2 After enabling the channels you will need to specify callback functions for them which can be done through adc_callback const int chan void callback uint_t32 The callback function takes in a 32 bit integer which is the results of ADC2 and ADC1 concatenated ADC2 ADC1 Starting Conversions After configuring the ADC library to actually start conversions use adc_start which will launch the timer and begin conversions To stop the converter use adc_stop Processing Results The results of each conversion will be passed to the callback functions If your processing is fast enough you can parse the results in that function Otherwise you should save the data to a global variable and process it later ADC example Listing 1 ADC example code we Calculate power using the ADCs include ge_adc h volts per division define V_PER_DIV 0025 amps per division 10 define A_PER_DIV 001 Part 2 continued on next page Page 2 of 6 EE 155 255 Lab 2 Revision 1 September 28 2015 Part 2 continued ADC callbacks void calculate_power uint32_t data 15 float power_result initialize ADCs adc_init 20 set sampling frequency at 50kHz adc_set_fs 50000 add callback to channel 1 adc_callback 1 amp calculate_pow
5. er enable channel 1 adc_enable_channel 1 start ADCs 30 adc_start wait for interrupts while 1 35 void calculate_power uint32_t data uintl6_t chanl data amp 0x00ff uintl6_t chan2 data amp Oxff00 gt gt 16 power_result float chanl V_PER_DIV float chan2 A_PER_DIV 4o Part 3 Hardware Your energy meter should be able to measure 200V and 10A for a total instantaneous power of 2kW It should work on both DC and AC Our power and energy meter connects between a power source and a load The meter senses the load voltage VL by measuring the voltage between the positive and negative load terminals The meter senses the load current by measuring the current that flows from LOAD to SOURCE A schematic of our power and energy meter is shown in 1 Most of the component values are left for you to calculate as a lab exercise The meter uses our microprocessor module for analog measurement and display the current sense resistor on the component module to measure the load current and an operational amplifier to amplify the voltage across the current sense resistor The current sense resistor RS is on the component board which has terminals for using various passive Part 3 continued on next page Page 3 of 6 EE 155 255 Lab 2 Revision 1 September 28 2015 Part 3 continued high power parts All of the circuit except RS R1 and R2 should be built on a
6. excess noise on the readings Part 5 Calibration The ADC measurements are not perfect and the resistor values used in the voltage divider and current sense circuit are not exact The result is that OV and OA will not typically produce an ADC value of 2047 and other voltages will not produce exactly what you expected when you calculated the resistor values To correct for this you need to measure at least two points to find the conversion from ADC counts to voltage or current Since any change to the circuitry could change the offset or scale the calibration procedure needs to be simple and automated To do this add a special mode to your program to measure OV OA a known voltage and a known current Make the known values small enough to be safely handled when testing but large enough to avoid excessive rounding error a single ADC count is not a good choice Set CAL_VOLTS and CAL_CURR in lab2 h to the values you choose The defaults are 10V and 3A The main loop provided to you uses pushbutton 1 to select among four screens live measurements calibrating offset and calibrating voltage and current scale factors Pressing the pushbutton 2 in a calibration screen causes one of the calibrate_ functions to be called which is where you should calculate and store the new calibration data Calibration data is stored in EEPROM so you dont have to recalibrate every time you turn on the meter meter_init reads this calibration data from
7. imultaneously on converter channels This allows measuring things like current and voltage at the same time so you can calculate instantaneous power The reference for the ADCs is set to 3V so analog signals will have to range between 0 3V to be converted properly The converters are set to operate in a single ended mode so a result of 0 corresponds to OV and Oxfff is 3V On the fast channels the conversions take 0 19 us This can be sped up to 0 16 ps if reducing the resolution to 10 bits The EE155 libraries provide a very simplified interface to the ADCs with a small subset of their possible func tionality To fully utilize the converters on the board you will have to expand on the libraries functionality using STM s abstraction libraries check out stm32f30x_adc h c Part 2 continued on next page Page 1 of 6 EE 155 255 Lab 2 Revision 1 September 28 2015 Part 2 continued For a good overview on the ADCs check out the STM32F3_ADC PDF in the Github repository ADC Initialization Before you can use the ADCs you will have to initialize the library The command is adc_init This command sets up the ADC registers and prepares them for conversion To set the sampling frequency use adc_set_fs float fs This will set up TIM2 to trigger the ADC at the appropriate intervals Setting up Channels The ADC library is configured to use specified channels for carrying out the conversions What external pins these map to can b
8. l amplifier U1A connected as a differential amplifier to amplify the 100mV voltage across the sense resistor to fit the 2 5V input range of the ADC We use a differential amplifier here rather than a single ended inverting or non inverting amplifier to cancel any noise that exists between LOAD and the actual processor ground The gain of this amplifier is set by the choice of resistor values For simplicity set R4A R6 and R3 R5 Capacitor C2 forms a low pass filter with R5 to reduce high frequency noise that may be above half the ADC sampling rate Since R1 has a high voltage connected to it the connections on R1 and R2 must be solid and reliable This voltage divider should be soldered together R1 should be at least 100k Page 4 of 6 EE 155 255 Lab 2 Revision 1 September 28 2015 Part 3 Part 4 Filtering A common and simple approach to smoothing signals is to use a one pole IIR filter Yn ayn Bx where the DC gain is thay x We want a to be slightly less than one to provide a low pass response To keep the implementation fast and simple well choose a 1 sit and 1 where shift is some integer gt 0 This can be written as Yn 1 Yn Yn 1 7 Shift 2 The DC gain is 2shft so to produce unity gain the output needs to be divided by 2 These divisions can be done with bit shifts if working with integers so they are fast You will need to write filtering functions for your power meter to avoid
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