Generic Telemetry Module
To assist with the Small Sounding Rocket Program
(SSRP) ASRI have requested the following 'loose' specifications to promote an initial discussion among the HackerSpace/Lunar Numbat communities:
- ADCs with a sample rate of 100 Hz (baseline), with an option for much faster sample rates if necessary. (e.g. Mach shock wave recording.)
- At least 12 bits of ADC resolution.
- ADC samples at 100 Hz are stored on local memory, but 1 in 10 of those samples (at 10 Hz, in real time) are transmitted via RF telemetry.
For example, 16 channels might be a desirable number, divided up as follows:
- 7 x channels from devices or instruments in the user's payload
- 3 x accelerometers
- 3 x gyroscopes
- 1 x temperature sensor
- 1 x barometric pressure
- 1 x spare, or for user instruments.
Note that the GPS receiver is not an analog sensor device, and therefore it has nothing to do with the ADCs. GPSs typically output ASCII data (standardised NMEA strings) over a digital serial interface. 5-10 Hz is the fastest update rate I've seen for commercially available GPS modules.
Before deciding upon the number of ADC channels, however, I think it is valuable to consider what kind of sensors we actually want to use and whether these sensors have analog output and require external ADCs. Many of these kinds of sensors available today have digital outputs.
For example, if we look at the ADXL345 3-axis accelerometer
(just as one example), this accelerometer has built-in ADCs and has a digital I2C?
We can implement both the 3-axis accelerometer and the 3-axis gyroscope using sensors with digital outputs (using e.g. the Analog Devices ADIS16400 IMU module). We can also implement the barometric pressure sensor and the temperature sensor digitally, using a SCP1000 and DS18B20?
(A Zuni will experience acceleration of about 70 g during engine burn. Is it possible to get MEMS accelerometer chips that can measure accelerations that large!?)
The Freescale MMA3302 accelerometer
is a two-axis MEMS accelerometer that operates up to +/- 100 g in the X-axis and +/- 50 g in the Y-axis,
Therefore, we only need 8 12-bit ADC channels available, and we can make them all available for interfacing with sensors or circuits in the user payload.
We also need digital interface pins available, of course, for the SPI, I2C?
and 1Wire interfaces for these digitally-interfaced sensors. The GPS is interfaced onto a serial UART.
Flash memory for local storage is also needed - I think something removable like an SD card would be the best way to go here.
- 2 x standard definition cameras at 25(?) FPS
- Integrated "Wrap-around" type would be preferable for TLM/VID/GPS
- 100 Mhz to xmit telemetry, video & GPS data.
- 127 mm diameter x 150 mm long
- Conform with standard bolted interface. Ref ASRI payload user guide.
- Able to withstand an acceleration of 70 g.
- Storage chip protection via resin potting or similar.
- Option for RF uplink connection