The word “telemetry” has become a little unfashionable recently (under a deluge of “wireless this” and “IoT that”) but historically it was the first and primary word used to describe low power (ISM band) radio modules. Even today we talk about “radio telemetry modules” as a generic term.
| “Tele-metry”: derived from the Greek roots tele, 'remote', and metron, 'to measure'. |
Now “telemetry” is obviously far from being the only, or even the primary task to which wireless device are put (they are also used for remote control of all manner of devices, for asset tracking, or for remote monitoring of user inputs, such as alarms buttons) but remote sensing is (and is likely to remain) a major low power wireless application.
The characteristics of short range radio devices are well enough understood (low power, small size, limited communication bandwidth (=data rate) and moderate range) that we can concentrate here on the specifics of using them in conjunction with suitable sensor devices to form simple “telemetry” systems
Sensors come in a bewildering variety, designed to monitor almost any process or parameter that can be imagined (from, for example, the soil moisture content in a plant-pot to the neutron flux in a fission reactor core). For that reason, we need to consider each sensor in terms of what it outputs to the monitoring system or operator, rather than what it is actually measuring.
Real world sensors typically fall into a small number of categories:
1. “Switch” type (the output is an on or off, a digital yes/no state)
These are things like level (float) switches, pre-set alarms, limit switches, door monitors and the like
2. “Meter” devices (output is an analogue value, typically a voltage or a loop current)
Temperature or humidity sensors, fluid level meters, light level monitors, magnetometers, wind speed
3. “Data stream” based devices (where the output is a formatted serial data packet)
GPS modules, data loggers, engine management systems
4. “Complex waveform” real time signal based devices (where the output is a baseband signal)
Seismic microphones, machine vibration monitors.
The challenges associated with implementing systems based around the different sensor types are really quite disparate:
The first group present what are fundamentally digital bit outputs. The low power wireless industry already offers many “control orientated” products [for example, the CTA88 range from Radiometrix] well suited to such simple signals.
The second group are more interesting. Here the analogue value will need to be conditioned, digitised and then transmitted over the radio link. There are “analogue input” products in the industry (especially for the common 4-20mA current loop standard interface) but a (semi) custom approach, using something like an Arduino to handle the analogue to data packet conversion at the sensor end, is often the better, more flexible, approach.
Group three is populated by devices that have already acquired the sensed parameter and coded it into a digital message. These sensors only require a “radio modem” type device (with the main concern being the provision of adequate radio-link data rate to accommodate the sensor device output)
Finally, the fourth group represent quite different problems. The obvious solution (digitise in real time, and send a data stream) may be usable, but there are (still) cases where the trade off between (ADC) resolution and necessary signal bandwidth makes the transmission of the actual baseband “audio” from the sensor over the radio link the only practical approach (seismic microphones typically fall into this class).
| The crucial thing to remember is that, no matter what your sensor, somewhere in the low power radio industry there will be a wireless link to match it, or the potential for one to be designed for it. You just have to ask. |
By Myk Dormer for Radiometrix Ltd, August 2023.