RFID Sensor [2007.05.11]

• Introduction
• Configuring the RFIDSensor
• Choosing the right SensingMode
  • Radius
  • Obstacle
  • Attenuation
• Detecting RFID Tags
• Reading RFID Tags Memory
• Writing RFID Tags Memory
• Erasing RFID Tags Memory
• Conclusion

Introduction

RFIDSensor simulates a RFID reader and is used to detect RFID tags that are in sensor range. It allows also to read and write their memory. RFID tags can be added in the virtual world both from UnrealEd or can be dynamically released by the RFIDReleaser effecter.

Refer to http://en.wikipedia.org/wiki/RFID for further information on RFID technology.

Configuring the RFIDSensor

Like any other USARSim class, the sensor can be configured from the USARBot.ini file. In the [USARBot.RFIDSensor]
section you can set these parameters:

• SensingMode [default: Attenuation]: it can be Radius, Obstacle, Attenuation. It specifies what signal propagation model to use.

Radius and Obstacle mode parameters:

• MaxRange [default: 3] (m): sensor range. It's only used by Radius and Obstacle sensing modes.
• MaxSingleShotRange [default: 6] (m): same as MaxRange but for single shot tags.

Attenuation mode parameters:

• dBmTXPower [default: 36] (dBm): sensor TX power in dBm.
• dBmRXSensibility [default: -88] (dBm): sensor RX sensibility in dBm.
• dBObstacleAttenuation [default: 3.5] (dB): attenuation due to an obstacle.

Other parameters:

• bTraceRFIDs:[default: false] : if set to true then USARSim will draw 3D lines from sensor to rfid tag (slow, use it only for debugging).
• bAlwaysReadRFIDmem [default: false]: if you want to read tag memory every time you detect it. That means that if you sense 10 times per second, you will read the memory content of all tags that are in range 10 times per second.
• HiddenSensor [default: true]: show/hide the sensor in the simulator. We recommend setting it to true if it’s not necessary to show the sensor. When you want to confirm if the sensor is placed in the correct position and has the correct direction, you can temporarily set it to false.
• bWithTimeStamp [default: true]: indicates whether include the timestamp in the the sensor data message.
• Weight [default: 0.4] (kg): The weight of the sensor in kg.
  

Choosing the right SensingMode

Radius

Radius mode only takes into account the sensor range (MaxRange). The sensor can read any tag in this range ignoring obstacles.

SensingMode: Radius

Traced lines (if bTraceRFIDs is true): all the lines are green.

Obstacle

Obstacle mode is the same as Radius, but it also takes into account obstacles. Tags that are in sensor range but lie behind an obstacle are invisible to the sensor.

SensingMode: Obstacle

Traced lines (if bTraceRFIDs is true): green lines for reachable tags, red lines for hidden tags.

Attenuation

Attenuation mode uses a signal attenuation model. It consider both distance and obstacles. The signal strength is function of the transmitter power (dBmTXPower) and distance:

(from: MaxStram Application Note - Indoor Path Loss)

note: the factor 2 means that we are considering forward and backward propagation (considering the passive tag as the source of the reflected signal).

Signal strength over distance with dBmTXPower = 36

If the received signal is lower than the sensor sensibility (dBmRXSensibility) then the tag is out of range. If there is an obstacle we attenuate the signal by a fixed amount (dBObstacleAttenuation). In this simplified model we consider only one obstacle. This is an acceptable approximation because of the very short range of the passive RFID tags.

SensingMode: Attenuation

Traced lines (if bTraceRFIDs is true): line color represent the signal strength (from green to red). If the signal is too low the line is black (tag is unreachable).

Detecting RFID Tags

When you are near one or more RFIDTags you will receive the following string:

SEN {Type RFID} {Name RFID} {ID 1} {Mem 0}

if you detect one RFID, or:

SEN {Type RFID} {Name RFID} {ID 1} {Mem 0} {ID 2} {Mem 0} ...

if you detect more then one at the same time.

• ID: is the unique RFIDTag identifier.
• Mem: Is the RFIDTag memory content, "0" if empty. This field can be deactivated by setting to false bAlwaysReadRFIDmem.

Reading RFID Tags Memory

To read the memory of the RFIDTag you can use the following command:

SET {Type RFID} {Name RFID} {Opcode Read} {Params RFIDTagID}

Where RFIDTagID is the ID of the RFIDTag you want to read.

For example, failure can happen when the RFID tag is out of the sensor range.

Writing RFID Tags Memory

Using the same RFIDSensor you can write the RFIDTag memory using this command:

SET {Type RFID} {Name RFID} {Opcode Write} {Params RFIDTagID MemoryContent}

Where RFIDTagID is the ID that identifies the RFIDTag you want to write, MemoryContent is the string you want to write in the RFIDTag.

Erasing RFID Tags Memory

To erase the RFIDTag memory you can both write a "0" string or, more easily:

SET {Type RFID} {Name RFID} {Opcode Write} {Params RFIDTagID}

Where RFIDTagID is the ID that identifies the RFIDTag you want to erase.

Conclusion

The RFIDSensor is a fast sensor so it will not load the CPU.

The Attenuation mode uses a very approximated model. It could be improved considering more obstacles, antenna gains and tag/sensor relative rotation (anisotropic radiation field). But the disadvantage would be a higher CPU load and a whole pile of new, obscure, parameters.

Another way of improving the RFIDSensor is to adopt a new syntax for its messages :-)