Il sensore TPA81 e' costituito da una matrice di sensori sensibili alla gamma, infrarossa, in un range che va dai 2um ai 22um. La banda e' simile a quella rilevata dai sensori PIR che si trovano negli allarmi / antifurto o nei dispositivi che accendono le luci. La caratteristica di questi sensori, e' che possono rilevare soltanto il movimento degli oggetti, e questo li rende limitati nel settore della robotica. Ad esempio, non possono essere utilizzati per misurare la temperatura di una sorgente calda statica.
Un tipo differente di sensori sono i 'thermopile array'. Questi vengono utilizzati nei misuratori di temperatura all'infrarosso, che non richiedono il contatto con la sorgente da misurare. Hanno un ampio campo di rilevamento di circa 100? e spesso sono utilizzati con una lente che restringa il campo di misura a circa 12?, molto piu' utile in un'applicazione di misura.
Recentemente sono disponibili sul mercato dei sensori dotati di lenti al silicone, questo e' il tipo utilizzato nel TPA81.
Il TPA81 puo' misurare la temperatura di 8 punti adiacenti, contemporaneamente. Puo' anche controllare un servo per muovere, sull'asse orizzontale, il sensore e costruire una immagine dei valori termici rilevati. il TPA81 riesce ad individuare la fiamma di una candela a 2 metri di distanza ed e' insensibile alla luce ambientale.
|Corrente Operativa Tipica||mA|
|Accuratezza Range 4-10?C||+/-3?C|
|Accuratezza Range 10-100?C||+/-2?C +/-2%|
|Campo di rilevamento (FOV)||41? x 6? (8 pixel of 5? x 6?)|
|Uscite||1 ambiente + 8 temp degli 8 pixel|
|Controllo Servo||32 passi per 180? di rotazione|
|Dimensioni||31 x 18 mm|
The TPA81 is a thermopile array detecting infra-red in the 2um-22um range. This is the wavelength of radiant heat. The Pyro-electric sensors that are used commonly in burglar alarms and to switch on outside lights, detect infra-red in the same waveband. These Pyro-electric sensors can only detect a change in heat levels though - hence they are movement detectors. Although useful in robotics, their applications are limited as they are unable to detect and measure the temperature of a static heat source. Another type of sensor is the thermopile array. These are used in non-contact infra-red thermometers. They have a very wide detection angle or field of view (FOV) of around 100° and need either shrouding or a lens or commonly both to get a more useful FOV of around 12°. Some have a built in lens. More recently sensors with an array of thermopiles, built in electronics and a silicon lens have become available. This is the type used in the TPA81. It has a array of eight thermopiles arranged in a row. The TPA81 can measure the temperature of 8 adjacent points simultaneously. The TPA81 can also control a servo to pan the module and build up a thermal image. The TPA81 can detect a candle flame at a range 2 metres (6ft) and is unaffected by ambient light!
The response of the TPA81 is typically 2µm to 22µm and is shown below:
Field of View (FOV)
The typical field of view of the TPA81 is 41° by 6° making each of the eight pixels 5.12° by 6°. The array of eight pixels is orientated along the length of the PCB - that's from top to bottom in the diagram below. Pixel number one is nearest the tab on the sensor - or at the bottom in the diagram below.
Here's some numbers from one of our test modules:
For a candle, the numbers for each of the eight pixels at a range of 1 meter in a cool room at 12°C are:
11 10 11 12 12 29 15 13 (All °C)
You can see the candle showing up as the 29°C reading. At a range of 2 meters this reduces to 20°C - still around 8°C above ambient and easily
detectable. At 0.6 meter (2ft) its around 64°C. At 0.3 meter (1ft) its 100°C+.
In a warmer room at 18°C, the candle measures 27°C at 2 meters. This is because the candle only occupies a small part of the sensors field of view and the candles point heat source is
added to the back ground ambient - not swamped by it. A human at 2 meters will show up as around 29°C with a background 20°C ambient.
All communication with the TPA81 is via the I2C bus. If you are unfamiliar with the I2C bus, there is a tutorial which will help. The TPA81 uses our standard I2C 5 pin connection layout. The "Do Not Connect" pin should be left unconnected. It is actually the CPU MCLR line and is used once only in our workshop to program the PIC16F88 on-board after assembly, and has an internal pull-up resistor. The SCL and SDA lines should each have a pull-up resistor to +5v somewhere on the I2C bus. You only need one pair of resistors, not a pair for every module. They are normally located with the bus master rather than the slaves. The TPA81 is always a slave - never a bus master. If you need them, I recommend 1.8k resistors. Some modules such as the OOPic already have pull-up resistors and you do not need to add any more. A servo port will connect directly to a standard RC servo and is powered from the modules 5v supply. We use an HS311. Commands can be sent to the TPA81 to position the servo, the servo pulses are generated by the TPA81 module.
The TPA81 appears as a set of 10 registers.
Ambient Temperature °C
Used for Calibration - do not write
|2||Pixel 1 Temperature °C||Used for Calibration - do not write|
|3||Pixel 2||Used for Calibration - do not write|
Only registers 0, 1, 2 and 3 can be written to. Register 0 is the command register and is used to set the servo position and also when changing the TPA81's I2C address. It cannot be read. Reading from register 0 returns the TPA81 software revision. Registers 1, 2 and 3, are used for calibration of the sensor. Do not write to these registers or the sensors calibration data may erased. (There is protection against this, in that a specific 3 byte command sequence similar to the I2C address change sequence, has to provided to enable calibration mode). Calibration requires the use of two temperature controlled black body heat sources, unless you have these, you will not be able to calibrate the module. All modules are calibrated in our workshop as part of the testing process.
There are 9 temperature readings available, all in degrees centigrade (°C). Register 1 is the ambient temperature as measured within the sensor. Registers 2-9 are the 8 pixel temperatures. Temperature acquisition is continuously performed and the readings will be correct approx 40mS after the sensor points to a new position.
Commands 0 to 31 set the servo position. There are 32 steps (0-31) which typically represent 180° rotation on a Hitec HS311 servo. The calculation is SERVO_POS*60+540uS. So the range of the servo pulse is 0.54mS to 2.4mS in 60uS steps. Writing any other value to the command register will stop the servo pulses.
|0||0x00||Set servo position to minimum|
|nn||nn||Set servo position|
|31||0x1F||Set servo position to maximum|
|160||0xA0||1st in sequence to change I2C address|
|165||0xA5||3rd in sequence to change I2C address|
|170||0xAA||2nd in sequence to change I2C address|
Changing the I2C Bus Address
To change the I2C address of the TPA81 you must have only one module on the bus. Write the 3 sequence commands in the correct order followed by the address. Example; to change the address of a TPA81 currently at 0xD0 (the default shipped address) to 0xD2, write the following to address 0xD0; (0xA0, 0xAA, 0xA5, 0xD2 ). These commands must be sent in the correct sequence to change the I2C address, additionally, No other command may be issued in the middle of the sequence. The sequence must be sent to the command register at location 0, which means 4 separate write transactions on the I2C bus. Additionally, there MUST be a delay of at least 50mS between the writing of each byte of the address change sequence. When done, you should label the sensor with its address, if you lose track of the module addresses, the only way to find out what it is to search all the addresses one at a time and see which one responds. The TPA81 can be set to any of eight I2C addresses - 0xD0, 0xD2, 0xD4, 0xD6, 0xD8, 0xDA, 0xDC, 0xDE. The factory default shipped address is 0xD0.
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