Change Over to LoRa for Telemetry

Change Over to LoRa Radio for Telemetry

The Telemetry Radio used onboard Voyager 2.0 for the past few years has been the standard 433Mhz APM Telemetry Radio.

It operates well over short distances of a few hundred metres, as a transparent full duplex serial link, with a default transmission rate of 57600 Baud.

Traditional APM Telemetry Radio

The design of the telemetry system was that the vessel would constantly broadcast telemetry messages, whether they were being received or not.

The specific messages being broadcast are enabled by bit mask.

The mission definition does support power control where the telemetry radio can be powered off or on during specific mission steps.

The problem with the standard low cost telemetry radio is that distance is very limited. More efficient antennas were used to extend the range to a few hundred metres, but the link was always marginal at that distance.

LoRa Radio

LoRa (Long Range) Radio offers long range communications up to several kilometers with line of sight. 

LoRa is proprietary technology developed by Semtech, who provide a range of transceiver chips, primarily the SX1278. Many PCB modules are available from multiple suppliers using the SX1278 device.

The module used here is the Ebyte 433MHz E32-433T20DT 100mW.

Ebyte 433MHz LoRa Radio

The Ebyte module incorporates additional microprocessing components in order to provide a serial interface, making it reasonably simple to incorporate into the existing Voyager Controller design.

Ebyte Modules mounted for use with USB for the Voyager Base Station and a Serial Connection for use onboard.

The Ebyte LoRa module uses an effective transmission rate of approximately 2400 Baud. It can be increased, but that is at the cost of reduced range. The default rates are being used here.

The design of the telemetry system has been changed to a "request" system, rather than a broadcast system. This means that the vessel remains mostly silent and only transmits telemetry messages in response to specific requests from the Voyager Base Station.

Hence the scheduling of telemetry messages is dictated by the Base Station.

This alleviates the need to consider power control for the Telemetry Radio on board the vessel, because it will only transmit on demand.

The main aim of changing to LoRa Radio is to gain increased range for telemetry and control of the vessel. This should allow faster reconfiguration and testing of the Wingsail and the vessel, without the need to keep returning to shore.

Note: This is part of the ongoing development of a low cost autonomous oceangoing sailing drones, utilising a self-trimming wingsail. This is the Voyager series of sailing drones.

Wingsail Development

Wingsail Development 

All sailing testing so far has been performed using the one sail. It was originally designed and made as a very conservative prototype, not trying to push the boundaries far.
Its been a tough and reliable sail, with its longest voyage being across Port Phillip over a couple of days.

Wingsail #1

Dimensions:

• Height 1000mm
• Chord Length 150mm
• Area 0.15 sq metres
• NACA 0015 (15% Chord)
• Weight 820g

Wingsail #1

Wingsail #2

This Wingsail was short lived. Not enough thought was put into its weight and the stability of the boat. I thought the stability margin was quite high and didn't need to be considered much.

That was very wrong !

Dimensions:

• Height 1100mm
• Chord Length 330mm max, tapering down to 165mm.
• Area 0.32 sq metres
• NACA 0018 (18% Chord)
• Weight 1298g

Wingsail #2 - too heavy

When Wingsail #2 was trialed in the water in a mild 10 knot wind, the boat simply laid over and wouldn't right itself. A big failure.

Stability Measurements

It was clearly important to stop assuming the stability margin would be ok, and actually take measurements.

A setup was established to measure the mast tip loading required to hold the boat flat at 90 degrees of heel.

With Wingsail#1, the mast tip loading was measured at 475g at a distance of 1210mm from the deck.

This sail has demonstrated good performance in strong winds.

With Wingsail#2, the mast tip loading was only 250g at the same height off the deck, and boat laid over in mild wind.

Wingsail #3

Wingsail #3 is the same size as Wingsail #2, but it has been designed to minimize weight and heeling moment while retaining as much strength as possible.

Dimensions:

• Height 1100mm
• Chord Length 330mm max, tapering down to 165mm.
• Area 0.32 sq metres
• NACA 0018 (18% Chord)
• Weight 980g

Changes:

• The printed components were all redesigned to reduce weight. Previously, the printed pieces were designed as mostly solid pieces, and printed with infill of about 10% to reduce weight, as well as incorporating large circular holes. The components were all redesign by shelling them to about 1.5mm.
  Continuous checks were made on the design of each component by performing the slicing operation and noting the length of filament that would be consumed, in order to estimate the weight of the finished item.
  The weight of the finished component was determined to be 7g per metre of filament, as reported by the slicer.
  The end result was a reduction of mass of the printed components by almost 50%.
• The film previously used was 250 micron A4 sized clear acetate film, used for binding documents. I'm now using 200 micron A3 sized film.
  This has plenty of stiffness for a sail of this size, and the larger A3 size allows for less overlapping seems, and hence a neater result with reduced weight.
• Lower the centre of mass.
  The electronics and the battery with its switch, were lifted on Wingsail#2 in an effort to ensure it was high out of the water. This was a mistake because of the significant cost in loss of stability.
  The battery and electronics are now as low as possible, while remaining forward of the mast.
• Lower the centre of mass.
  The tail section and the forward counterweight have been dropped by 200mm, so that they are as low on the deck as practical.
  This has a significant effect on stability.
• The 12mm aluminium mast has been replaced with carbon fibre.
  The carbon fibre mast now consists of  three sections:
• 500mm by 12mm OD and 10mm ID
• 500mm by 10mm OD and  8mm ID
• 1000mm by 8mm OD and 6mm ID
• The Carbon Fibre tubing fits nicely together as a press-fit, to form a tapered mast.
  The aluminium mast weighed 125g. The carbon fibre tapered mast weighed 76g.

Result

The new Wingsail #3 has a mass of 980g (roughly 320g less than #2).

It requires a tip loading of 500g to hold the boat flat a 90 degrees of heel. This is a great improvement, and is slightly higher than 475g of Wingsail #1.

Wingsail #3

Next Steps

The next step is to get the boat in the water for trials.
There is still room for improvement in reducing the heeling moment due to mass, by reducing the mass of the tail. This causes a second-order problem, because the counter-weight is unnecessarily large to balance the tail. Hence, any improvement in the weight of the tail should see almost a double improvement in overall weight.