AGI One Axis Accelerometer for CubeSat
In this short note are reported some general indications and requirements concerning the possibility to use an AGI accelerometer in a CubeSat mission.
Sommario
2 Accelerometer linearity and its max range. 3
3 Proposed interfaces between the accelerometer and the CubeSat. 4
1 Accelerometer Data Sheet
In the table_1 is reported the accelerometer data sheet.
Table_1 Acc_AGI main characteritics
|
Caracteristics |
Value |
|
Sensitivity |
|
|
Acquisition Frequency (Hz) |
0.1, 0.2, 0.5, 1.5, 10, 20 |
|
Output |
Analogic or digital |
|
Data rate (10Hz acc and T) |
|
|
Internal thermometer Pt10000 |
Sensitivity better than |
|
Interface of communication |
Rete RS232 full-duplex/rate; RS485 with adapter |
|
Standard of communication |
NMEA |
|
Dimension |
80X55X25 (mm) |
|
Weighs |
0,320 (Kg) |
|
Voltage supply via USB or external |
5 (V) |
|
Dynamic |
120 dB |
|
Power Dissipation |
150 (mW) |
|
Linearity |
> 80 dB |
There is also the possibility to implement an active thermal control, with the necessary power dissipation strictly connected to the temperatures variations to compensate, to their frequencies and to the preliminary knowledge of the average value of the in orbit temperature.
1 Accelerometer linearity and its max range.
Figure_1. Accelerometer calibration curve.
In the figure_1 it is presented the calibration curve of the implemented accelerometric element. The characterization has been obtained disposing the Acc_AGI with its axis in a horizontal plane and imposing a tilt to this plane (it is necessary to remember that the angle variation in rad is equal to acceleration in g that acts on the Acc_AGI). The zero output of the Acc_AGI corresponds at a measured angle at which its sensitive axis is perpendicular to the local gravity ( in this calibration).
From this calibration it is possible to see that the Acc_AGI is quite linear for signals ranging from . It can answer also at more big signals but the linearity is not guaranteed.
1 Proposed interfaces between the accelerometer and the CubeSat
1.1 Electrical interface
Connector position
The accelerometric module is provided with eight pin male connector (P6), it is the primary electrical interface connector. Here are reported the connector pin-out and functionality as well as electrical signal specification.
The following picture shows the P6 connector position:
The connector number and the pin number 1 is show on the printed circuit board too; a one-side, wired, 0.4m length cable with female connector is provided with the accelerometer.
Connector pin-out and functionality
The following picture shows the P6 connector pin-out:
The following table shows pins functionality :
|
Pin number: |
Electrical type: |
Max: |
Min: |
Functionality: |
Cable wire color: |
|
1 |
Input |
|
|
GND |
Black |
|
2 |
Input |
+4.8 V |
+5.2 V |
+5 V |
Red |
|
3 |
N.a. |
|
|
N.C. |
N.a. |
|
4 |
N.a. |
|
|
N.C. |
N.a. |
|
5 |
Input |
|
|
Rs232-GND |
Blue |
|
6 |
N.a. |
|
|
N.C. |
N.a. |
|
7 |
Input |
+5.2 V |
+4.8 V |
Rs232-RX |
Green |
|
8 |
Output |
+5.2 V |
+4.8 V |
Rs232-TX |
Yellow |
Serial interface specification
Data telemetry and command are managed via three pin serial asynchronous
interface. The asynchronous link is a fully compliant RS232 standard with the following parameters:
|
Parameter: |
Value: |
|
|
|
|
Boudrate: |
115200 bit/s |
|
Voltage levels: |
+/- 5V |
|
Parity: |
None |
|
Data bit: |
8 bit |
|
Stop bit: |
Yes (1) |
Power consumption
The system total average power consumption is 150mW +/- 10% in standard operation mode.
Boot sequence take 1.5 seconds to complete, in this short time the system power consumption is 300mW +/- 10%.
1.1 Software interface
The accelerometer provide telemetry as well as telecommand via NMEA 0183 standard serial protocol.
More information about NMEA protocol can by find at following link:
http://www.nmea.org/content/nmea_standards/nmea_0183_v_410.asp
Accelerometer telemetry format:
After power up sequence, accelerometer starts to send telemetry via serial interface at desired frequency that can be chosen via telecommand. Factory made frequency is fixed at 20Hz (twenty samples per second).
The data in ASCII format is 38 bytes long string, showed in the following picture:
The following table describes the data field in detail:
|
Data |
Bytes |
Offset |
Description |
Min value |
Max value |
|
$ |
1 |
1 |
Fixed start code |
N.a. |
N.a. |
|
AC |
2 |
2 |
Fixed talker code |
N.a. |
N.a. |
|
DAT |
3 |
4 |
Fixed telemetry type |
N.a. |
N.a. |
|
, |
1 |
7 |
Fixed comma data delimiter |
N.a. |
N.a. |
|
Scientific data #1 |
12 |
8 |
Acceleration (m/s^2) |
-9.999999999 |
+9.999999999 |
|
Scientific data #2 |
10 |
21 |
Temperature (°C) |
-99.999999 |
+99.999999 |
|
System status |
2 |
32 |
System status register |
See table below |
See table below |
|
* |
1 |
34 |
Fixed End-Of-Data code |
N.a. |
N.a. |
|
CRC |
2 |
35 |
Cyclic redundancy check |
See description |
See decription |
|
13 10 |
2 |
37 |
Fixed Line feed |
N.a. |
N.a. |
Start code, talker, message type:
This string is fixed, and it is always “$ACDAT” for data telemetry. This Ascii code identify the data telemetry coming from the accelerometer, the “$” code have to be used as data starting “lock” code.
Scientific data #1:
This telemetry field contain the signed measured acceleration in m/s^2.
Scientific data #2:
This telemetry field contain the signed measured temperature in C°.
System status register:
This 16 bit (2 Bytes starting from index 32) register store the accelerometer status as well as housekeeping messages, the following picture show the bit detailed fields of the register:
The following table describes the system status register bits functionality in detail:
|
Bit number: |
Name: |
Description: |
Note: |
|
0 |
CMD Error |
Command Error flag (0 = No errors) |
|
|
1 |
TMP Ctrl ON |
Temperature control mode (0 = No TMP Ctrl) |
Not used |
|
2 |
PRM Error |
Command parameter error (0 = No errors) |
|
|
3 |
O.o.D. |
Out of dynamics warning (0 = No O.o.D.) |
|
|
4 |
Acq. Freq. |
Acquisition frequency (see Note) |
110 = 0.5 Hz 000 = 1 Hz |
|
5 |
|||
|
6 |
|||
|
7 |
Spare |
Spare |
Not used |
|
8 |
CMD Ack |
Command Acknowledge (1 = Ack) |
|
|
9 |
Heat Flag |
Temperature heat flag |
Not used |
|
10 |
TMP TCS |
Thermal Control System ON/OFF |
Not used |
|
11 |
Ext. LED |
External Led ON/OFF |
Not used |
|
12 |
Cal. Mode |
Calibration mode ON (0 = Cal.Mode OFF) |
|
|
13 |
Wave Mode |
Low pass filter mode (0 = Wave Mode OFF) |
|
|
14 |
A.A. Filter |
AntiAliasing Filter (1 = A.A Filter ON) |
|
|
15 |
Spare |
Spare |
Not used |
CRC:
This field contain the cyclic redundancy check code in ASCII HEX format, it is calculated as exclusive OR (XOR) of data starting after “$” code and ending before “*” code.
Line Feed and Carriage Return (13 10):
The last two bytes are the telemetry terminators.
Valid data telemetry examples:
Telemetry examples for 20Hz acquisition frequency with AntiAliasing filter ON:
$ACDAT,+0.000418246,+29.875020,À°*01
$ACDAT,+0.000504345,+29.879159,À°*0C
Telemetry examples for 10Hz acquisition frequency with AntiAliasing filter ON and CalibrationMode ON:
$ACDAT,+0.001435824,+04.119101,Ð *0F
$ACDAT,+0.000719788,+04.119503,Ð *0C
Accelerometer telecommand format:
The accelerometer is able to accept a set of telecommand in order to change functional parameters. Commands are always 15 ASCII long string with no CRC or terminators.
The following table shows the list of the acceptable commands:
|
Command name |
Command description |
String format |
Parameters |
Note |
|
ACQ-FC |
Change acquisition frequency |
$SMCMD,FC,XXX.X |
XXX.X: 001.0 100.0 |
Permanent mode. |
|
CAL-Mode |
Enter/Exit calibration mode |
$SMCMD,SM,SWCAL |
Toggle mode. |
Not permanent. |
|
AA-Filter |
Enable/Disable AA-Filter. |
$SMCMD,SM,SWAAF |
Toggle mode. |
Not permanent. |
|
W-Mode |
Enable/Disable LP-Filter. |
$SMCMD,SM,SWAVE |
Toggle mode. |
Permanent mode. |
|
SW-RST |
Software Reset |
$SMCMD,SM,SWRST |
|
|
“Permanent mode” has to be intended as a mode that is stored into instrument memory and not affected by instrument reboot.
The following picture shows the detailed description of ACQ-FC telecommand:
Telecommands description:
ACQ-FC:
Command format: $SMCMD,FC,XXX.X Effected status param.: Acq. Freq., CMD Ack, PRM Error
This telecommand change the accelerometer acquisition frequency, factory made acquisition frequency is 20 Hz (twenty samples per seconds). The execution store the acquisition frequency into instrument non volatile memory and it has a permanent effect after reboot or power up sequence.
Command Parameters: XXX.X
Acquisiton Frequency values (Hz): 000.5, 001.0, 005.0, 010.0, 020.0, 050.0, 100.0
Note:
This command effect the status register Acq. Freq.
If correctly executed enable CMD AcK status bit for 2 seconds, if parameters is not correct it enables PRM Error status bit for 2 seconds.
CAL-Mode:
Command format: $SMCMD,SM,SWCAL Effected status param.: Cal. Mode, CMD Ack
This telecommand change the accelerometer acquisition mode; into calibration mode the accelerometer sends scientific data in a voltage format, this allow the user to calibrate the accelerometer.
Command Parameters: No parameters, this is an a “Toggle command” (see note).
Note:
This command effects the status register Cal. Mode bit, this command is a “toggle command”, it enables the calibration mode if it is not enabled and viceversa, you can check the status of the instrument by reading the status register bytes.
If correctly executed enable CMD AcK status bit for 2 seconds.
AA-Filter:
Command format: $SMCMD,SM,SWAAF Effected status param.: A.A. Filter, CMD Ack
This telecommand enable/disable the accelerometer antialiasing filter, this filter is factory made enabled to avoid high frequency aliasing into measured band.
Command Parameters: No parameters, this is an a “Toggle command” (see note).
Note:
This command effects the status register A.A. Filter bit, this command is a “toggle command”, it enables the Antialias Filter if it is not enabled and viceversa, you can check the status of the instrument by reading the status register bytes.
If correctly executed enable CMD AcK status bit for 2 seconds.
W-Mode:
Command format: $SMCMD,SM,SWAVE Effected status param.: Wave Mode, CMD Ack
This telecommand enable/disable the accelerometer 1Hz cut off frequency low pass filter. This filter can be usefull in calibration mode to remove high frequency signal.
Command Parameters: No parameters, this is an a “Toggle command” (see note).
Note:
This command effects the status register Wave Mode bit, this command is a “toggle command”, it enables the Low Pass Filter if it is not enabled and viceversa, you can check the status of the instrument by reading the status register bytes.
If correctly executed enable CMD AcK status bit for 2 seconds.
SW-RST:
Command format: $SMCMD,SM,SWRST Effected status param.: CMD Ack
This telecommand reboot internal instrument software, this command take effect after 2 seconds.
Command Parameters: No parameters.
Note:
If correctly executed enable CMD AcK status bit for 2 seconds.
1.1 Mechanical interface
In the following drawings are reported the mechanical interfaces concerning the ACC_AGI one-axis accelerometer with the cube-sat.
Accelerometer Max external dimension.
Accelerometer Mechanical Interface
The main elements are:
1) the supporting plates on the top and on the bottom of the accelerometer;
2) the sensor;
3) the electronic pre-amplifier for the accelerometer;
4) the analog electronics;
5) the digital electronics.
In the following figures are shown some prospective view of the accelerometer installed in one-element of CubeSat.
|
|
In this picture the supporting plate on the top is not shown.
The volume reserved to the accelerometer in the CubeSat is the one in green colour.
The accelerometer fits in the volume (green) reserved to it.
