GY-85 — A quick datasheet study
ADXL345 How it Works #arduSerie 34
In this post we will unlock the accelerometer — ADXL345— off the board.
In this other post we already work with compasses…
What about the board GY-85, what’s so special about it?
GY-85 Features: Model : GY-85 — Multiple sensors in a chip or on a board.9DOF : Sensor Module — Nine degrees of Freedom — 9 axis modules Ultralow power : as low as 23 μA in measurement modeFixed 10-bit resolution Full resolution, up to 13-bit resolution at ±16 g Free-fall detectionSupply voltage range : 2.0 V to 3.6 V I2C digital interfaces : this board connect CS to 3.3V to tell the sensor we will be using it as an I2C device, and not an SPI device3 Chips on board : They are (With their respective 7-bit addresses):ADXL345 - 0x53 — Three axis acceleration
ITG3205 - 0x69 — Three axis gyroscope
HMC5883L - 0x1E — Three axis magnetic fieldps: for accel chip the alternate IIC address of 0x53 can be chosen by grounding the SDO/ALT ADDRESS pin (Pin 12). That's exactly what this board does!
Now, back to accelerometer…Datasheet says:
ADXL345 — THEORY OF OPERATION
The ADXL345 is a complete 3-axis acceleration measurement system. The ADXL345 has a measurement range of ±16 g minimum.
The accelerometer can measure the static acceleration of gravity in tilt-sensing applications as well as dynamic acceleration resulting from motion, shock, or vibration.
The sensor is a polysilicon surface-micromachined structure built on top of a silicon wafer. Deflection of the structure is measured using a differential capacitor that consists of independent fixed plates and plates attached to the moving mass.
ADXL345 — MECHANICAL SENSOR
The ADXL345 uses a single structure for sensing the X, Y, and Z axes. As a result, the three axes’ sense directions are highly orthogonal and have little cross-axis sensitivity.
ADXL345 — APPLICATIONS INFORMATION — POWER SUPPLY DECOUPLING
For most applications, a single 0.1 µF capacitor, CDC, placed close to the ADXL345 supply pins adequately decouples the accelerometer from noise on the power supply.
ADXL345 — USE WITH OPERATING VOLTAGES OTHER THAN 3 V
The ADXL345 is tested and specified at VS = 3 V; however, it can be powered with VS as low as 1.8 V or as high as 3.6 V. Note that some performance parameters change as the supply voltage is varied.
ADXL345 — OUTPUT IS RATIOMETRIC
Ratiometric (Adjective)?
(electronics) Describing any system in which an output is directly proportional to an input.
The ADXL345 output is ratiometric, therefore, the output sensitivity (or scale factor) varies proportionally to the supply voltage. At Vs = 2.5 V, the output sensitivity is typically 256 LSB/g (see code below).
The zero g bias output is also ratiometric, thus the zero g output is nominally equal to VS/2 at all supply voltages .
ADXL345 — POWER CONTROL & AXES OF ACCELERATION
Table 16. Register Map - ADXL345 Datasheet
Address
Hex Dec Name Type Reset_Value Description
0x2D 45 POWER_CTL R/W 00000000 Power-saving features control.
(…)
0x32 50 DATAX0 R 00000000 X-Axis Data 0.
0x33 51 DATAX1 R 00000000 X-Axis Data 1.
0x34 52 DATAY0 R 00000000 Y-Axis Data 0.
0x35 53 DATAY1 R 00000000 Y-Axis Data 1.
0x36 54 DATAZ0 R 00000000 Z-Axis Data 0.
0x37 55 DATAZ1 R 00000000 Z-Axis Data 1.The device will come up in sleep mode upon power-up, so this is the first register we should worry about: POWER_CTL!
ADXL345 — HOW IT WORKS
Standby or Measurement : At power-up, the device is in standby mode, awaiting a command to enter measurement mode, and all sensor functions are off. After the device is instructed to enter measurement mode, all sensor functions are available.
Standby Mode: For even lower power operation, standby mode can be used. In standby mode, current consumption is reduced to 0.1 µA (typical).
In this mode, no measurements are made. Standby mode is entered by clearing the measure bit (Bit 3) in the POWER_CTL register (Address 0x2D).
Placing the device into standby mode preserves the contents of FIFO.
FIFO: accumulates samples, holding the latest 32 samples from measurements of the x-, y-, and z-axes.
Retrieving Data from FIFO: The FIFO data is read through the DATAX, DATAY, and DATAZ registers (Address 0x32 to Address 0x37). When the FIFO is in
FIFO, stream, or trigger mode, reads to the DATAX, DATAY, and DATAZ registers read data stored in the FIFO. Each time data is read from the FIFO, the oldest x-, y-, and z-axes data are placed into the DATAX, DATAY and DATAZ registers.
Measure Bit : A setting of 0 in the measure bit places the part into standby mode, and a setting of 1 places the part into measurement mode. The ADXL345 powers up in standby mode with minimum power
consumption.
ADXL345— [PSEUDO]CODING
Here is a six-steps PSEUDOCODE for ADXL345:
Notice that we have 6 questions to answer (5, actually…):
1 — Which device (address) are you interest in ?
2 — Which Register do you want to talk to ?
3 — What do you want the register to transmit?
4 — What do you want to ask about the specific register ?
5 — Now wait for the data looping in a while …
6 — Now how do you want the configuration of the three axes numbers?Here is my breadboard:
Now upload this code to your arduino and have fun!
CURIOSITIES ABOUT ACELLERATION:
ROLLER COASTER The swooping, sickening sensations you experience on a roller coaster come courtesy of brief g-forces of up to 5 g. Rides have to be designed so people don’t black out.Our tolerance of g-forces depends not only on the magnitude and duration of the acceleration or deceleration but also on the orientation of our body. We are most vulnerable to a force acting towards the feet, because this sends blood away from the brain. Five to 10 seconds at 4 to 5 g vertically typically leads to tunnel vision and then loss of consciousness.FIGHTER JETSFighter jets can pull up to 9 g vertically, and the more a pilot can take without blacking out, the better their chances in a dogfight. Some pilots wear “g-suits” which help push the blood away from their legs and towards the brain. People with the highest g tolerance are known as “g-monsters”. “We have had people who have been perfectly conscious at 6 g,” says physiologist Alec Stevenson of UK-based defence firm Qinetiq. Others pass out at 3 g, he says. Pilots can boost their natural g tolerance by training inside centrifuges, like the one Qinetiq has in Farnborough in Hampshire. They learn to tense their leg and abdominal muscles to push blood to the upper body, and to breathe in a special way, straining hard as if defecating when constipated, to raise blood pressure. The greatest vertical force anyone has withstood is 31.25 g, although for that the subject, NASA doctor R. Flanagan Gray was in a special water tank that pressurised his body to help him take the g’s. US air.F1 PILOTSWhat is it like to be a F1 pilot?
The acceleration figure is usually 1.45 g (14.2 m/s2) up to 200 km/h (124 mph), which means the driver is pushed by the seat with a force whose acceleration is 1.45 times that of Earth’s gravity. The carbon brakes in combination with tyre technology and the car’s aerodynamics produce truly remarkable braking forces. The deceleration force under braking is usually 4 g (39 m/s2), and can be as high as 5–6 g when braking from extreme speeds, for instance at the Gilles Villeneuve circuit or at Indianapolis. In 2007, Martin Brundle, a former Grand Prix driver, tested the Williams Toyota FW29 Formula 1 car, and stated that under heavy braking he felt like his lungs were hitting the inside of his ribcage, forcing him to exhale involuntarily.The Turn 8 at the Istanbul Park circuit, a 190° relatively tight 4-apex corner, in which the cars maintain speeds between 265 and 285 km/h (165 and 177 mph) (in 2006) and experience between 4.5 g and 5.5 g for 7 seconds — the longest sustained hard cornering in Formula 1.
ROCKET LAUNCHERS
What G-forces do different launchers cause?
This graphic above indicates that current satellite launchers are limiting peak acceleration to about 4g. The STS (Shuttle) did the same. In the Apollo days is that they threw 6g and 8g figures around for peak launch accelerations, or possibly peak re-entry forces.Download All File for This Project
Credits & References:
Breakout — 3-axis Analog Accelerometer ADXL345
Maxed out: How many gs can you pull?
Launch Accelerations: Values, history
Tap, Tap, Drop. ADXL345 Accelerometer + Arduino from http://bildr.org
— — — — — — — — — — published at Jun/2017 — — — — — — — — — —
