Lab 3 Final report: Embedded Systems Digital Potentiometer Subsystem TEAM: RAR

Transcription

1 Lab 3 Final report: Embedded Systems Digital Potentiometer Subsystem TEAM: RAR EE 300W, Section 6 Professor Tim Wheeler Rui Xia, Yuanpeng Liao and Ashwin Ramnarayanan

2 Table of Contents Introduction...2 Rationale Implementation...3 Explanation of Code and Circuitry...4 Verification Testing...11 Validation Testing...19 Results from Integrated System...19 Discussion Conclusion...24 Appendices

3 Introduction As the complexity of today s computer system grows, we can no long use a single system to meet the requirement. At this time, we need to utilize embedded system to break the main function into several subsystems. The purpose of this lab is to integrate the four subsystems together and analyze the function of a black box. The four subsystems contain digital-to-analog converter subsystem (D2A), digital potentiometer subsystem (Digipot), analog to digital converter subsystem (A2D) and keypad subsystem. D2A subsystem and Digipot subsystem will generate and input a waveform into the black box. The output of the black box will be analyzed and sampled by the A2D subsystem. Users can control the entire embedded system using the keypad subsystem. By comparing the input and output of the black box, we can analyze the behavior of the black box. Our team is assigned to build the digital potentiometer subsystem. Digitpot subsystem is used to scale the output of D2A subsystem and input the scaled waveform into the black box. The output amplitude of the digital potentiometer subsystem is selected by the keypad subsystem. It is crucial to generate a desired and consistent input to the black box because we need to compare the input and output of the black box. Rationale Theory of Operation In digital potentiometer subsystem, we need to scale the DAC subsystem output (0-3.3V) to have a voltage range of 0 to +1, 2, 3, 4, or 5 V. Digital potentiometer is the most important part in our subsystem because we need it to change the gain of the amplifier and thus scale the input waveform. The digital potentiometer works as an analog potentiometer. The wiper position Dn of the DigiPot selects the two resistors values RA and RB. RA = RAB * (256 Dn) / 256 and RB = (RAB * Dn) / 256. RA + RB = RAB = 50kΩ for MCP41050 DigiPot. We choose to place RA between the input and inverting terminal, and RB on the feedback path. Once we change the wiper position 2

4 based on the C++ code, the gain of the inverting amplifier will also change to our desired value. In our C++ code, the subsystem outputs different scaled amplitudes using 5 case statements. Theory of Interfaces Mebed is connected to PC to load C++ code to our circuit. We use SPI to communicate the mbed to the DigiPot. The connections used were MOSI (Master-Out Slave-In) and SCLK (clock). PuTTY can send and receive messages from the mebed using USB port. CAN bus connects our subsystem and the keypad subsystem so that the commands received from keypad subsystem will send to our subsystem. Implementation Circuitry: Our subsystem should scale and map a 0 to 3.3V input from DAC to a 0 to 1V, 2V, 3V, 4V and 5V by selection. The selection of output range will be made via mbed according to the can message from the keypad subsystem. An inverting amplifier circuit model with a voltage divider is chosen. 3

5 Single-supply, programmable, inverting gain amplifier using a digital potentiometer Vin is the subsystem input and Vref is connected to ground. The gain equation is now Vout = - Vin(Rwb/Rwa) Variable gain Rwb / Rwa allows us to output a range no matter it is larger or smaller than the input voltage. A second inverting amplifier with gain of 1 is added at Vout to produce a positive output. In experiment, we found that the resistors used in the second amplifier should be much larger than the resistors in the first one to make an output waveform without any clipping. We chose 2M ohm which is much larger than 50 k ohm (Rab). Also, a 10k ohm resistor is added at Rb to stabilize the performance when Rwb is low. 4

6 Circuit design Our equation is now: Vout = Vin((Rwb+10k)/Rwa) Where Vin = 3.3V, Vout = 1V, 2V, 3V, 4V, 5V, Rwb + Rwa = 50k ohm. By solving the equation, we get the resistance values of Rwb for different outputs. Output (V) Rwb resistance (ohm) k k k k k The hex wiper position is calculated by the equation 5

7 Dn = 256 * Rwb / Rab where Rab = 50 k. Adjustments are made to the calculated values based on experiments. The final wiper positions are: Output (V) Hex wiper position 1 0x14 2 0x21 3 0x31 4 0x33 5 0x46 Code: An LPC 1768 mbed microcontroller will be coded with C language to realize the following functions: 1. It gets the CAN-bus message from keypad subsystem via MCP 2551 CAN transceiver to determine desired digital potentiometer output 2. It writes corresponding hex wiper position values to MCP digital potentiometer. In the code, the can message will be read and stored by this line CANMessage msg; We used an if statement to test if there is any message received by the mbed and show in LEDs: if(can2.read(msg)) { 6

8 printf("message received: %d\n", msg.data[0]); led2 =!led2; } Then, a case statement is used to determine the desired wiper position to the digipot. For each case, a wiper position is selected and a message is printed to show the case And the following lines will write the selected wiper position: cs = 0; spi.write(0x13); z = setvoltage(msg); spi.write(z); cs = 1; 7

9 cs=0 will select the chip first. The position 0x13 is the selected command bit. The selected wiper position is written next. Finally, cs = 1 would deselect the chip. Verification Test: Our subsystem, digipot, is responsible to map the output of Digital-to-Analog Converter system (0 3.3V, 1Hz or 10 Hz solenoid or square wave) to a waveform of different amplitude (0 0-1V, 2V, 3V,4V or 5V). And the output amplitude is selected by keypad via a local CAN bus. And the output of our system will be directly sent to the black box. Our module and interface connection: 8

10 Testing Method: 9

11 We used a function generator outputting a sinusoidal wave spanning 0 to +3.3 V to mimic the function of DAC subsystem. The Can bus message is set in code for output range selection. The output waveform is observed with an oscilloscope. The plots of the oscilloscope are recorded for different cases: 10

12 1V: 2V: 11

13 3V: 4V: 12

14 5V: Validation test: 13

15 Most of our result waveform almost exactly matched the desired amplitude. And all the percentage errors are no more than 10% Desired voltage(v) Actual voltage(v) Percentage Error (%) Our subsystem successfully fulfills the requirements. Our subsystem receives command from keypad, converts the output of DAC and output a waveform to the black box. The accuracy of subsystem s output can directly affect the observation of the black box behavior. Digipot is an indispensable part in the whole system. Result from integration: We didn t finish integration, But we tried to use our part and oscilloscope to identify the black box. We tried various kind of input to the black box. By observe the reading from oscilloscope (ADC subsystem), we determined the behavior of the black box. Firstly, the output of black box does not depend on the input frequency. The output is the same for both 1Hz and 10Hz. We also observe that the output amplitude is no more than 1V in any case. Trying different input value, we observed that the gain is about 0.1. The black box behavior can be express as: 14

16 Vout = 1V- 0.1 Vin Our group thinks that the black box is an inverting amplifier with a 1 volt offset. Vout = - Vin (1k/10k) *(1 + 1k/10k) Discussion The design that we chose met the requirements by using one MBED microcontroller, one digital potentiometer, two op amps and one CAN transceiver. The utility of this design is in the usage of the digital potentiometer as a resistor on the op amp design whose resistance can be changed as needed. This is better than the alternatives of using a lot more components and breadboard space. The team used the provided Agilent function generator to generate our input signal. We also used the two power supplies provided to us to get our +15V,-15V and +5V supply. The team initially faced a non working circuit simply because the ground was not shared. Eventually, after following the proper wiring, the circuit began functioning to a certain degree. In the beginning, the circuit produced nothing but noise. This was corrected when we debugged our circuit to find that the voltage input was not getting through to the 15

17 MBED. After rewiring, we obtained a periodic function and resembled a wave. This was due to incorrect resistor values as we found out later on. After, the values were changed, the code had to be changed in the case structures for the HEX values. Finally, after the final debugging we were able to obtain our sine wave. This entire process took up the entirety of our time. Thus, the team was unable to work with the other teams for a significant amount of time. The teams were unable to link the CAN buses together to get a common voltage signal through them and control the black box using the keypad. This is the result of time management and communication issues both within and outside the team. This would be the main area of improvement for the group moving forward. Finally, the teams tried identifying what was in the black box. We were successfully able to get the 1 Volt offset answer correct but were unable to calculate the slope. The final waveform function was: Y = -0.1X + 1 V Future areas of improvement would involve proper communication amongst the team members in order to manage time efficiently. A lot of work was required to be done outside the given lab period of time which was not used properly. Value Statement: In this lab, we are required to communicate with other groups to accomplish an integrated project. This is the first time we encounter this setting. However, we can certainly imagine that this kind of scenario is far from rarely happen in an engineer s career. We were a little confused at the beginning. This resulted in the failure that we could not finish the integration. This experience greatly enhances our communication skill and prepares us for the inevitable. Technically, this is also the first time for most of us to use LCP 1768 mbed and an online compiler. Being able to work with various languages and compilers is a great contributor to a successful career. This lab shows us a continuously learning process example and gives us some experience about it. 16

18 Conclusion Team RAR successfully completed the DigiPot subsystem that received the input signal and could control the resistor values in 5 different cases. The teams were unsuccessful in integrating the subsystems together whose entire purpose was to divide the work instead of loading all of it on one program. The utility of this project goes into security and control systems. The CAN buses enables us to integrate multiple subsystems into one giant process. This allows for continuous improvement to a given process and simultaneously makes debugging much easier for the engineers. Appendixes Appendixes A: Gantt chart Appendixes B: Bill of materials 17

19 Appendixes C: mbed code 18

20 19

21 Appendixes D: Block diagram 20