Designing low power telemetry systems for instrumentation
Advances in wireless technology have enabled instrumentation manufacturers to expand their product range, creating a new generation of products for remote monitoring. By Brett James.Whet
Designing low power systems is always a difficult compromise between the required or desired battery life and the frequency of readings or the reactiveness of the device. With some applications demanding small physical battery volumes the choice of battery also becomes critical.
The basic power saving model is to get your device into a very low current state in between times when it has to be awake to take readings or transmit data. From waking up it is imperative that each operation is conducted as fast as possible and using the minimum amount of current, which may mean powering different parts of the circuitry during the cycle.
For example, in some telemetry modules that wake periodically to take a reading from a strain gauge, the sequence of events is very critical. We would first wake the MCU and power up the analogue circuitry. Whilst the minimum number of readings are averaged from the A/D, we would start to power up the radio module. As soon as the readings have been taken, the analog circuitry is powered down and the data transmitted. Once transmitted, the radio is powered down and the entire device re-enters a low current sleep mode until the next wake-up.
The entire wake, take readings and transmit sequence can take as little as 20mS, so even with transmission rates as high as 25Hz we can still save some power between transmissions. Once the transmission rate drops to 1 per second or below, we can start to see very long battery life from reasonably small batteries.
The Mantracourt T24 telemetry range was designed around a 3V supply, so that a pair of D, AA or AAA batteries can be used to power the modules. Many customers feel happier that batteries can be replaced in an emergency by sourcing from the local garage.
It is also very important that the devices can operate down to low supply voltages. The lower they can operate, the more capacity of the battery can be utilised. For example a pair of D cell batteries may be specified by the manufacturer as having a capacity of 10Ah but this is usually down to a voltage such as 0.9V. The circuitry usually cannot operate that low, so given that it can only operate down to 1.1V, this may mean that we can only use 8Ah capacity of the battery.
Battery self discharge also determines battery choices, because sometimes the batteries will discharge themselves long before the attached electronics. Improvements in alkaline battery technology means that shelf life is very good, so these batteries can now be used instead of non-rechargeable lithium batteries when very long life is required e.g. 5 years.
For rechargeable batteries, lithium polymer has proved to be a very reliable and efficient battery chemistry. The 3.7V output is ideal for telemetry modules and the associated charging and regulating modules means that batteries can be recharged in situ.
An area that must be sacrificed for longer battery life is the use of LEDs to inform users of operating modes and errors. A better approach is the use of pulsed LEDs and sequenced flashes. By utilising the techniques mentioned above it is possible to provide transmissions every 30 seconds from a load cell sensor powered from a small lithium button cell for months.
Due to the low power requirements of radio modules, energy-harvesting solutions such as solar cells can also be used to allow the products to be used without having to worry about changing batteries. This is an area Mantracourt is currently researching for development.
Antenna and frequency choice
The antenna choice will mostly be determined by the application. As discussed above, wireless instrumentation tends to operate in harsh environments, so a chip antenna mounted on the PCB provides a low cost and robust solution. A similar alternative is a PCB antenna with omni-directional characteristics. Of course, an external antenna can be used and does give additional range, but may not be practical in some circumstances.
Mantracourt chose 2.4GHz for its latest radio modules after previous experience of 915MHz and 868MHz equipment. Different geographical regions requiring different frequencies meant it was previously necessary to not only manufacture and stock two variants of all products but also to make decisions for customers as to which type to purchase, as not all customers retained the product themselves. Coupled with the limitation of duty cycles for 868MHz band, the 2.4GHz licence exempt band was investigated as a solution which, in consultation with customers, proved to be a good solution. Distribution partners had less stock to carry with only one, not two versions, and an easier decision for customers who may have not known exactly where the devices would operate in the world.
Some concerns over adopting the 2.4Ghz frequency band included reduced range and the crowding of this band with other radio equipment, such as Bluetooth and Wi-Fi, not to mention microwave ovens. However, the radio techniques used has resulted in no issues operating with any of the other equipment. The reduction in range compared to 868MHz and 915MHz was apparent but the line of sight range was more than acceptable and indoor range could be boosted with repeaters.
A range of 100M in an open field site is stated for Mantracourt’s chip antenna modules and this has been tested to 200M in a true open field site. About 20M indoors through a wall can also be achieved. Using the larger antennas, as in the boxed modules, results in 400M in a true open field site. Indoors range is about 40M through a single wall and less if passing between two walls.
Transmitting the data
From the original design concept of the Mantracourt radio module it was decided that a general purpose module would be more useful than one that specialised in a particular area, so a proprietary protocol was designed in favour of the existing radio protocols. This gives the ability to transmit up to 1000 packets per second and run for up to 5 years on a pair of AA batteries (at usable delivery rates).
Avoiding the complexity of requiring and configuring coordinators was also a requirement. For speed, flexibility, low power and complexity, Mantracourt decided against mesh networking. This has proved to be a good decision as a very high percentage of applications are solvable using standard modules along with smart repeaters in the few cases that require them. Compared to a lot of radio systems this approach has proved to be simple to configure and working systems can be added to at later dates with minimal fuss and with usually no change to existing modules.
Whether considering developing, or simply buying-in wireless instrumentation, hopefully this article has provided an insight into the issues surrounding wireless instrumentation and also provides a level of expectation as to what is possible at this time. This technology is helping engineers worldwide monitor loads, torques and strains in difficult to access locations and on moving systems.
Author profile: Brett James is a Design Engineer at Mantracourt Electronics Ltd.