Onwards and Upwards, Your near space guide

Transcription

1 The NearSys BalloonSat Sound of Near Space Flight Computer is a BalloonSat flight computer with one major purpose, to experiment with the effects of near space vacuum on the sound transmission. It creates a combination of six tones for a video camera and records the pressure each time the tones are produced. After recovery, the experimenter can play back the sounds and analyze the transmission of sound through the near vacuum of near space and then relate the volume and tone to the air pressure. In addition to sound production, the BalloonSat Sound of Near Space is also capable of operating a camera. The heart of the flight computer is a PICAXE-14M2, a microcontroller that makes the BalloonSat Sound of Near Space Flight Computer programmable. The PICAXE-14M2 s data memory is limited to 256 bytes, but that s more than enough for a mission. Power for the flight computer comes from a four AAA battery pack and the weight of the flight computer, including battery, is only 110 grams. That leaves a lot of weight available for the BalloonSat airframe and two cameras. Onwards and Upwards, Your near space guide Overview of the BalloonSat Sound of Near Space An experimenter can program this flight computer to make a multitude of tones, although only six at a time are typically created and recorded during a near space mission. Each time the flight computer generates a tone, it also illuminates one of six LEDs to signify which tone is being produced. The PICAXE-14M2 also measures and stores the current air pressure and triggers a camera to take a picture. A digital video camera pointed at the sound unit records the audio tones and LED illuminated. Software, like Audacity (a free sound editing program) can analyze the tone in terms of frequency and volume and discover how each tone is affected by transmission through near vacuum. The numeric analysis of the tone and the recorded air pressure are then correlated through the use of a spreadsheet. Since the PICAXE-14M2 can produce 128 tones and only six are typically tested on each flight, there s lots of opportunities to refly the flight computer on new near space missions.

2 Figure 1. A completed BalloonSat Sound of Near Space flight computer. Main Unit on the left and Sound Unit on the lower right. Parts List Main Unit: Flight Computer C1 C2 C3 J1 J2 J3 J4 J5 J6 R1 R2 R3 R4 R5 R6 R7 R8 R9 U1 U2 U3 U4 U5 47 uf uf 220 uf 2-pin straight header 2 by 8 receptacle 4k7 10k 22k 500 ohm trimmer 10 ohm 91k 1k 1k 91k LM2940T5 PICAXE-14M2 LM386 MPS-3138 TLC272

3 Sound Unit D1 LED D2 LED D3 LED D4 LED D5 LED D6 LED R1 680 R2 680 R3 680 R4 680 R5 680 R6 680 The remaining items are required to complete the NearSys BalloonSat Photometer, but they do not have a reference on the PCB. Wire (#24 AWG) Two ¼ tall nylon spacers 4 AAA Battery Pack #2-56 hardware Speaker Theory of Operation Figure 2. NearSys BalloonSat Sound of Near Space Schematic.

4 The SOUND command allows the PICAXE to emit one of 128 tones. Each time the PICAXE emits a tone, the signal is amplified by a LM386 audio amplifier for a one inch speaker. While the PICAXE is emitting a tone, it also illuminates one of six LEDs to indicate which tone is being produced and when. That way a digital video camera can record a visual signal of the tone while recording it. That way, should the tone grow too faint to hear in near space, evidence of the tone s presence is still available. The MPS-3138 is a Wheatstone-based pressure sensor that produces a voltage that is directly proportional to the air pressure. However, that voltage is too small for the PICAXE to digitize. Therefore, the pressure sensor voltage is sent to a TLC272 op-amp for amplification. Then the output of the op-amp is digitized by the PICAXE and stored into memory. After a near space mission, a program downloads all the pressure readings are downloaded and imported into a spreadsheet along with the volumes of the tones emitted during the flight. The air pressures are compared to the volume and tone of the audio signals to learn how air pressure affects the transmission of sound in near space. The relay is used to short out camera shutter switches so that the flight computer can record images during its mission. There are four Commit Ports located around the edges of the Main Unit PCB. The PICAXE is programmed to not begin collecting data until after a Commit Pin is removed from the Commit Port. This allows ground crews to turn on the BalloonSat Sound of Near Space during balloon filling, but prevent it from recording data until the balloon is ready to launch. Assembling the NearSys BalloonSat Photometer Flight Computer Main Board Figure 3. Parts layout for the BalloonSat Sound of Near Space Flight Computer.

5 Solder the following parts R1 4k7 (yellow, violet, red, gold) R2 10k (brown, black, orange, gold) R3 22k (red, red, orange, gold) R4 500 ohm trimmer R5 10 ohm (brown, brown, black, gold) R6 91k (gray, brown, orange, gold) R7 1k (brown, black, red, gold) R8 1k (brown, black, red, gold) R9 91k (gray, brown, orange, gold) C uf (marked 473) Note: Solder the headers into the PCB by their short pins. Leave the long pins above the PCB so the shorting block and camera cable can be plugged into them. J1 J2 J3 J4 J5 J6 2-pin straight header 2 by 8 receptacle Note: Bend the leads of the LM2950 voltage regulator to a right angle where they become slightly more narrow. Then bolt the heat sink of the voltage regulator to the PCB using the ½ long 2-56 bolt and nylock. Then solder the leads to the PCB. U1 LM2940T5 Note: The MPS-3138 is a surface mount component. It has a notch (groove) at one end that might be easier to see from the underside of the device. That side is solder to the PCB where the notch is drawn in the top silk pattern. U4 MPS-3138 Note: Do not insert the integrated circuits into the IC socket until after completing and testing the flight computer. U2 U3 U5 14-pin IC socket 8-pin IC socket 8-pin IC socket Note: Watch the polarity of C1 and C3. Capacitor C1 is marked with a small + symbol to indicate the positive lead of the capacitor. Capacitor C3 is marked with a band containing one of more negative dashes. C1 47 uf

6 C3 220 uf Note: The relay contains a diode to protect the electronics from damage created by the collapsing electromagnetic field inside the relay. Therefore, the relays are polarized and will not work if soldered in backwards. Solder the relays so that their faint white letter is inside the PCB and not against the outside edge of the PCB. The white stripe on the relays in Figure 3 is an attempt to show the lettering printed on the side of relays. Relay Insert the black and red wires of the battery pack into each strain relief hole and solder all the wires to the Main Board. Note: The strain relief holes are the two large holes near the edge of the PCB. The battery pack wires pass through the holes from the bottom of the PCB and then bend over so the bare ends can be soldered into the smaller holes near the strain relief holes. Using strain relief prevents normal use from breaking the wires off the PCB. Figure 4. An example of a wire and the strain relief hole. Notice that the wire remains insulated as it passes through the strain relief hole. The only place the wire is bare of insulation is where it is soldered to the inside soldering pad. Assembling the NearSys BalloonSat Sound of Near Space Sound Unit Figure 5. The parts layout for the Sound Unit.

7 Solder the resistors R1 680 ohms R2 680 ohms R3 680 ohms R4 680 ohms R5 680 ohms R6 680 ohms Note: The LED is a polarized device. The short lead is normally the cathode of the device. This is also the side of the LED body with the flattened edge. The other lead is the anode and this is the lead that is inserted into the pad closet to the A printed on the PCB. Figure 6. The anode and cathode of an LED. Note: Which color LED goes into which location on the PCB is not important and it does not affect the Sound Unit. D1 D2 D3 D4 D5 D6 LED LED LED LED LED LED Solder two cut resistor leads to the solder pads of the speaker Slide the cut resistor leads into the PCB at the two solder pads Note: Do not solder the resistor leads to the PCB yet. Bolt the speaker to the PCB using the ¼ long nylon spacers Solder the resistor leads to the PCB and trim Cut the wire into eight equal lengths Strip ¼ of insulation from both ends of each wire Solder one end of each wire into PCB using the strain relief holes like you did for the battery pack Solder the other end of the striped wires to the Connector PCB Solder the 8-pin right-angle header into the Connector PCB

8 Figure 7. Completed Sound Unit with Connector. Using the BalloonSat Sound of Near Space Flight Computer The BalloonSat Sound of Near Space Sound Unit must be mounted outside the BalloonSat airframe. Preferably on a boom so that a digital video camera inside the airframe can record the LEDs and speaker of the Sound Unit. The cable from the Sound Unit is routed to the Main Unit inside the airframe and plugged into J6. J6 and the Connector are marked with GND to indicate how the ground pins in both align and are plugged together. Digitizing Voltages The BalloonSat Sound of Near Space flight computer needs a program to digitize the voltage produced by the pressure sensor and then save it into memory. In addition, the program needs to generate tones, operate one camera and monitor the Commit Pin, and send status signals with its Indicator LED. There are two BASIC commands to digitize voltages, which is referred to as analog to digital conversion (ADC). The first converts a 0 to 5 volt signal to an 8-bit value between 0 and 255. READADC 0,B0 The second example converts a 0 to 5 volt signal to a 10-bit value between 0 and READADC10 0,B0

9 Which command you use is up to you. Just remember that the 10-bit conversion requires one word (two bytes) of memory to store (as opposed to only one byte of memory to store the 8-bit value). However, the greater resolution afforded by the 10-bit conversion is four times greater than the 8-bit version. You can trick the PICAXE into storing a 10-bit value into an 8-bit memory if you remember to add 255 to the final value after downloading the data. Generating Tones Tones are generated using the SOUND command. The command looks like this, SOUND I/O pin, (tone, duration). The I/O pin is B.1. The tone is a value between 0 and 127 and the duration is in units of 10 microseconds (so a value of 100 is approximately one second long). While a tone is being generated, its corresponding LED should be illuminated to indicate the tone is being played. That makes it much easier to analyze the resulting video file when the tone is difficult to hear. This means a tone command should look like this, HIGH C.4 SOUND B.1,(20,100) LOW C.4 PAUSE 1000 Operating the Camera A relay (connected to PICAXE B.2) triggers the camera shutter (that is if the camera has wires soldered to the contacts in its shutter switch). To operate a camera shutter, issue the HIGH command to close the switch and LOW command to open the switch. Put a PAUSE command in between to give the camera enough time to focus. HIGH B.2 PAUSE 1000 LOW B.2 Monitoring the Mission Commit Pin The Mission Commit Pin prevents the BalloonSat Photometer from recording data prior to launch. This way, the launch team can power up the flight computer while the balloon is being filled with gas, but not record until launch time. The simplest way to make the flight computer wait to record until after the Commit Pin is removed is to use the snippet of code below. Commit: IF PINC.3 = 0 THEN Commit While the Commit Pin is in place, C.3 is pulled low. And until the pin is removed, PINC.3 will never go high.

10 Using the LEDs as Indicators The LEDs are an easy way to have the PICAXE signal that it s ready to record data or that it s recording data. To turn on the LEDs, HIGH pins C.1, C.2, C.4, B.3, B.4 and/or B.5. and then turn them off with the LOW command. The following code snippet will rapidly flash an LED five times. FOR counter = 1 to 5 HIGH C.1 PAUSE 200 LOW C.1 PAUSE 200 NEXT For redundancy, it might be a good idea to generate tones at the same time an LED is being flashed to signify the start of the experiment. That way there s two ways for the flight crew to know the flight computer is ready to start generating data. Saving Data Data is saved in I2C EEPROM at a memory location given in an address. Writing data to memory therefore requires two pieces of data, the value to write and the location to write that value into. Prior to using the EEPROM however, it must be set up for one word records and fast communication with the PICAXE. The command to do that is listed below. I2CSLAVE % ,I2CFAST,I2CWORD A subroutine to write data into memory looks like this, StoreData: WRITEI2C counter,(reading) counter = counter + 1 PAUSE 10 RETURN In this code snippet, the variable named counter keeps track of the memory address and must be updated after every byte is saved to memory. The variable named reading is the one byte of data stored in memory and it s updated upon each digital conversion of voltage. Downloading Data Data is downloaded to a PC using the Terminal program of the PICAXE Editor (option F8). The Terminal program s baud rate must be set to Each data record must be separated by a comma in order for a spreadsheet program to import it as a commadelimited file (CVS in Excel). A subroutine to download data looks like this,

11 Download_Data: FOR counter = 0 to 150 READ counter,pressure SERTXD (#pressure,13,10) NEXT Erasing Old Data in Memory The PICAXE will overwrite old data in memory each time the BalloonSat Photometer starts. If there s more data in memory that was overwritten, it can be difficult to tell where the new data ends and the old data begins. The following program will overwrite old data in memory. Clear_Memory: FOR counter = 0 to Max_Records WRITEI2C counter,(0) PAUSE 10 SERTXD (#counter,",") NEXT SERTXD ("Memory cleared") Preparing the Pressure Sensor The pressure sensor consists of a Wheatstone Bridge in which one of the elements changes its resistance based on the air pressure. The resistance of each of the four resistors in the Wheatstone Bridge are not identical and must be compensated for. This is done by adjusting the trimmer potentiometer to maximize the reading of the pressure sensor. Afterwards, we ll calibrate the pressure sensor. Calibrating the Pressure Sensor Program your flight computer to read and display the output voltage of the pressure sensor. Open the Calibration spreadsheet on a PC. Find the current air pressure (this can be found on the Internet) and record under the Current Air Pressure Cell (I1). Write the units of air pressure in the cell next to the Current Air Pressure cell. Push the syringe plunger all the way forward to expel all air from the syringe. Connect one end of the vinyl tubing to the pressure sensor port. Connect the other end of the vinyl tubing to the syringe s slip tip. Adjust the trimmer (with a small Phillips screwdriver) so that the READADC value increases. At some point, the value will stop increasing, which should be around 213. Back off the trimmer until the READADC value is 200, or very close to 200 (setting back the pressure sensor will allow it to measure slightly higher pressures without topping out). Record the ADC value in the spreadsheet in cell B2. Pull the syringe plunger back to 1 milliliter. Record the READADC value in the spreadsheet in cell B3.

12 Pull the syringe to the rest of the volumes listed in column A of the spreadsheet and record the corresponding ADC values in column B. Note: The vinyl tube has a volume close to 1 milliliter (ml), so the actual volume of the syringe is actually 1 ml greater than displayed on the syringe. You can be even more accurate by suctioning water into the syringe and tubing (after taking pressure sensor readings) and observing the listed syringe volume when water first appears in the bottom of the syringe. Write the displayed syringe volume in cell I2. Go to the Air Pressure Chart worksheet of the Calibration spreadsheet and write down the equation of the Trendline listed at the bottom of the page. Note: The process above will also work if the READADC10 command is used. Post Mission Data Processing After BalloonSat recovery and data download, you will have a text file of data. Import the text file into a spreadsheet and use the equation for the Trendline to convert the ADC values into units of air pressure. An example is shown on page three of the spreadsheet. Air Pressure Experiment Don t lose the syringe, syringe barb, or tube; you ll need them n the future to verify or recalibrate the pressure sensor in the future. In the meanwhile, try performing this experiment. Push the syringe plunger all the way forward to expel all air from the syringe. Insert the tip of the syringe into a cup of water and suction up a small amount of warm water (about 1 ml). Point syringe upward and push the syringe plunger forward a bit to force out any air inside the syringe. Slide the red #8 thread protector over the syringe s slip tip (to make an air tight seal). Tip the syringe downward and pull back on the syringe plunger until the volume reads 35 ml. Look closely at the water inside the syringe you should be able to see bubbles are forming inside the water. While still holding the syringe plunger at 35 ml, pull off the thread protector from the slip tip and let air flow back into the syringe. Observe closely and you will see water vapor condense on the interior of the syringe. The Background Story The molecules (or atoms) in liquids are in dynamic equilibrium between the liquid and gas phase. The pressure liquids exert to become gas is called the vapor pressure of the liquid. Vapor pressure is higher when the liquid is hotter and lower when the liquid is colder. When the vapor pressure of a liquid reaches the air pressure exerted on the liquid, the liquid is not longer stable and its molecules begin to convert into a gas in mass. We call this process boiling. This means the two ways to make a liquid boil is to either raise the temperature of the liquid until its vapor pressure equals the air pressure exerted on the

13 liquid, or to reduce the air pressure until it equals the vapor pressure of the liquid. By pulling back on the sealed syringe, you are creating lower air pressures until the water inside the syringe begins to boil. Hotter water inside the syringe will make this effect more even apparent. When you hear someone claim that your blood would boil in space, he or she is referencing this fact. In near space, the air pressure equals the vapor pressure of water at 98.6 o F (body temperature) at an altitude of 63,000 feet, or what is called the Armstrong Line. Analyzing Sound Data How you analyze the sound file from your experiment depends on the software you use. However, Audacity is free software that can import audio files and play them back in a controlled fashion. Analyze tones by high lighting a particular tone and then click on the Analyze option in the top menu. In the drop down menu, click on Plot Spectrum. The quality of the tone and its volume (in decibels or db) will be shown. 22 December 2016