The RFID detection system consisted of unique active RFID tags worn by any worker entering the robot workspace, and an RFID reader mounted in the front of each mobile robot. When an employee wearing a tag moves within a specified range of any reader, the reader will send information to the central path planner including:
• Detected tag identity
• Estimate of tag proximity and location relative to robot base
• Timestamp
From this data, the central server had a rough knowledge of each employee’s location on the floor. If multiple robots detect the same beacon, it may also be possible to further refine the position estimate by trilateration, as demonstrated in other research [55].
Depending on the type of antenna selected, the reader could only pick up tags specifically in front of the drive, which would further increase the certainty of tag location [65].
Because of the potential for seconds-long communication delays between drive platforms and the central server, each robot must react independently to a very close RFID reading to avoid sudden collisions. Since the worst-case stopping distance is about 5 feet, any robot should begin to decelerate if it comes within 9.8 feet of a human. Drives could resume operation once the RFID tag leaves a larger safety zone. Assuming the worst case scenario of a human jogging directly at a loaded robot moving at full speed, the robot would have to react to the human and begin decelerating while it was still 21 feet away to be stopped by the time the human reaches it. Expecting a human to continue to jog at 6mph toward an oncoming loaded drive may be unrealistic, but RFID detections at that range are possible [52] [67] [55].
The RFID readers will use signal strength to approximate distance to a detected tag. To estimate distance, we will use a modification of the Friis transmission equation [51]:
𝑃!
𝑃! ∝ 𝐺!𝐺! 𝜆 𝐷
!
Equation 1: Friis transmission equation
where !!!
! is the ratio of received power to transmitted power, 𝐺! and 𝐺!are the gains of the receiver and transmitter antenna, respectively, λ is the wavelength of the signal, D is the distance between the transmitter and receiver, and n is an experimentally determined variable, usually between 3 and 5. This simplification allows us to adjust for the multipath and broken line of sight interference expected in a real system.
To prototype the RFID system, we were considering using one of the following development kits:
1. OpenBeacon, an open-source hardware beacon and receiver operating at 2.4GHz.
With this system, we have access to the microcontroller source code, and plenty
56 of control over the low-level hardware. The documentation is not as detailed as the other two solutions, and a lower operating frequency would be less susceptible to interference from the environment. These readers to not provide RSSI
information, meaning that they would only detect, not estimate the distance to RFID tags. The receivers report data over serial or Ethernet. The entire
development kit costs about $100 [68].
2. Synapse RF engine, a modular system that comes in 868 MHz and 2.4 GHz variants. These modules are mounted on a microcontroller development board, measure signal strength, and can use PCB or external antennas. There is no premade “ID card” beacon, but there are low-power modules that will operate off batteries. Data can be output over USB or RS-232. This development kit is also around $100 [69] [52].
3. Wavetrend 433 MHz TGP active tags and RX202 reader. These are the ID card type tags we propose to use, and the reader can report signal strength with a “high resolution” that is not specified in the product information. Reader data is output on USB, RS-232, or RS-485. Including the tag, reader, antenna, cables and power supply, this kit costs around $650 [70] [71].
With the understanding that we would be able to get less interference and signal
attenuation at lower frequencies, we decided to use the Wavetrend 433 MHz TGP active tags and the RX202 reader.
57
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