ZigBee Telemetry Subsystem

This page describes the telemetry subsystem that transmits the analog signal from the load cell in the dyno car, via radio, to a portable display near the operator. We decided to use radio to transmit the signal rather than having a display on the car or having the car record the data. After a brief, and in retrospect, silly, flirtation with pulse width modulation over analog FM on the VHF band, we came to our senses and decided to use off-the-shelf digital RF modules.

Our Requirements

• small to fit inside car
• reasonably low power to allow battery operation
• high frequency to keep antenna small
• good RF noise immunity and reliability
• repeatable accuracy to maintain calibration
• sufficient resolution to provide good dynamic range for the measurements
• expandable to support other data channels in the future
• support for analog output and/or computer readable output
• peer-to-peer or mesh topology to allow multiple tx and rx in the future

IEEE 802.15.4 at 2.4 GHz

After some research, we selected the IEEE 802.15.4 protocol operating in the 2.4 GHz ISM band. The 802.15.4 protocol is commonly known as ZigBee. It is very popular in the embedded systems world with many chips and modules to choose from. ZigBee is a Personal Area Network (PAN) and is intended for short range communication, like BlueTooth, but is not limited to point-to-point communication. The ISM bands are set aside by the FCC for Industrial, Scientific, and Medical (ISM) use and there is no license required for end users to operate approved devices in an ISM band.

The 2.4 GHz ISM band is shared by many devices including cordless phones, baby monitors, wireless video links, WiFi, BlueTooth, and microwave ovens. The band is crowded and noisy to say the least, but offsetting that is the fact that 802.15.4 uses Direct Sequence Spread Spectrum (DSSS) transmission, which offers excellent noise immunity. In addition to strong Forward Error Correction (FEC) codes that are part of the DSSS layer, the higher layer protocol can retransmit packets that are damaged or not received at all. 

XBee 802.15.4 OEM RF Modules

We selected the XBee 802.15.4 OEM RF Modules, manufactured by Digi International, for their availability, flexibility, size, and cost. The firmware on the XBee supports sampling analog and digital inputs, packetizing the data, transmitting it over the radio link, and outputting it to analog and digital outputs on the receiving node. The data is also available from any node by computer interface over the devices serial port. All of this functionality can be configured by sending simple AT–style text commands over USB without the need to write any code or attach an external processor.

Our first attempt at using the XBee modules was less than successful because we had unwittingly ordered Series 2 modules assuming that they were replacements for the Series 1 modules. After several hours trying to get the modules to work, we discovered that the Series 2 modules lack the analog output capability that we were looking for so we had to abandon them and order Series 1 modules. This was an unfortunate waste of time and money that I blame on confusing product names and a general lack of documentation adequately explaining the differences between XBee module typesVREF Input Pin