Solar Charging Trials during COVID19

Solar Charging Trials - in COVID Times.

It is August 2020 in Melbourne, Australia. We haven't been allowed to travel any distance for many weeks now, so there's no sailing trials permitted for hobbyists. So no chance to test the new sail, or the new LoRa telemetry system now installed on Voyager 2.0.

This allows time to start looking ahead to Voyager 3.0, which will be a scaled up version of Voyager 2.0 at 2m LOA, and about 0.5m in beam.

The intention of Voyager 3.0 is that it has a large enough deck space to accommodate enough solar cells to allow for missions of indefinite duration. Currently the battery capacity of Voyager 2.0 is about 80 to 120 hours (4 or 5 days). The Wingsail battery life is about 200 hours (8 days).

The Solar Cells in use are the Sunpower C60 cells. These are a light weight and flexible (within limits) and can be sliced up. They can provide over 3A at around 0.5V in direct sun. I'm currently slicing them into thirds, which can yield around 1A at 0.5V per cell in direct sun, and of course when connected in series, the three cells can yield 1.5V at 0.5A.

The image below shows a typically test set up for working out the basic parameters of the charging setup.

Preliminary Test Set up for assessing Cell Quantity and Charging Circuits

In order to evaluate different Solar Charging arrangements in the real world, I commenced with the Wingsail by pressing into service the failed (overweight) WingSail #2. This was kitted out with 4 third size slices of Sunpower C60 Cells in conjunction with the LTC3015 Step up DC-DC converter charging a single 18650 Li-ion cell. The load is the current Wingsail Controller board, but without the Bluetooth module, which was replaced with an Ebyte LoRa module instead. The Wingsail Controller was programmed with software primarily intended to read 3 sets of Voltages and currents and transmit them to a monitoring station.

Mock up of the Voyager Wingsail and electronics for trialing Solar Charging

It will be necessary to place cells on both sides of the sail.

The current Wingsail (without Solar Charging) has two 18650 cells. It is expected that only one cell will be required once solar charging is used. This will partially offset the increased weight due to solar cells (but not by much).

The test rig charging circuits and the the test rig software  are still being adjusted, but the aim is to monitor the operation of the solar charged Wingsail over a period of a month or so.

During this time, we'll start on a mock up of the deck of  the future Voyager 3.0 and set up a solar charging test rig for her.

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.

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.

Resolutions from the First Long Voyage - March 2020

This post covers each of the issues identified after the March 2020 Voyage across Port Phillip, and the resolution.

Cross Track Error Resolution

The Cross Track Error (CTE) calculation has poor resolution when the waypoint is many miles away (e.g. 20 miles).
The reason for the poor resolution is that the CTE is calculated using Sine(CDA) x DTW. But the CDA is represented as an integer value. (CDA is Course Deviation Angle).

Illustration of  CTE Resolution

This has been corrected by ensuring that the calculation and relevant values are all floating point.

Course To Steer

The Course To Steer (CTS) for the simple case of sailing directly to a waypoint, is simply the Bearing To Waypoint (BTW). This is ok for short distances, but when the distance to waypoint is large, and maximum CTE is small by comparison, then the vessel can easily reach or exceed the boundary for the leg. This in turn may cause the vessel to take drastic corrective action and tack on to an inappropriate course to address the excursion beyond the course boundary.

The CTS needs to use CTE as part of the steering algorithm.

The design change has been to add a CTS Correction offset.

CTS Offset  = CTE/Max CTE x K

Where K is constant representing the CTE gain.
The constant K should be stored in the EEPROM as an adjustable parameter.
Possible values for K may be 10° or 20°.

This course correction is added to the BTW. The correction is proportional to the CTE, and hence should greatly improve course keeping.


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.

First Long Distance Voyage - March 2020

The Voyage

In mid March 2020 Voyager 2.0 completed its first voyage in salt water. Voyage of 40 hours across Port Phillip in Victoria. A distance of around 30 miles. Completing a voyage of this distance was a good result, despite not sailing very well. But at least it made it, despite the problems.

This was about one or two weeks prior to isolation rules being stepped up in Victoria, where this would not have been permitted.

This article covers observations about the voyage. Subsequent articles will discuss the design adjustments to overcome the problems observed.

Stepping through the pre-launch checklist

I've learnt from my mistakes that a pre-launch checklist is important. There's nothing more frustrating than launching the boat and then realising that you've forgotten something (like installing the SD Card).

Voyager heads off into the evening at 8pm Saturday March 14, 2020.

The launch took place at dusk to ensure the boat would be well clear of the coast in daylight, to reduce the chance of passing by pleasure craft.

The actual course and the waypoints

The typical winds for the majority of the voyage were 10 to 20 knots south easterly.

Plot of positions from the Satellite Tracking.

The satellite position transmitter was programmed to send a position every 15 minutes. These positions are shown in the image above. The spacing gives a sense of the speed variations.

Shared View: Voyager 2.0 March 2020 - SPOT Tracking (findmespot.com)

The boat didn't quite make it to its intended destination. Mid-morning on the second day, travelling slowly with light winds, and about three miles from the finish, the boat was picked up by some passing fishermen in a half-cabin power boat. They phoned me and we arranged or a hand over at their boat ramp.

I thanked them for assisting with picking up the boat, but it was a pity that it wasn't allowed to complete the journey by itself. They were not to know that the boat did not need rescuing.

 

Voyager 2.0 just prior to being picked up

The hand-over.

A lot of data was recorded on to the SD card during this voyage. The recorded data is useful for analysing the performance of the boat and working out areas to be improved. Some of the interesting date plots are shown below.

The voyage was about 40 hours in duration or about 2400 minutes.

Cross Track Error

Unfortunately the boat went off course early in voyage, but did eventually recover.  The plot below show the Cross Track Error (CTE) in metres.

The plot highlights an error in the CTE calculation. The CTE calculation is based on the SIN of the angle between the rhumb line bearing and the current Bearing to Waypoint (BTW).  The angle is stored as an integer which means that the calculated CTE can be seen "stepping", rather than changing continuously.

Cross Track Error (metres)

Steering Servo Signal

The steering servo is standard RC servo, with a neutral of 1500us, with a full range from about 1100us to 1900us.

There are clear periods of the voyage when steering was easy and when steering was difficult. The periods with lots of large steering movements correspond to the periods when the boat was not on course.

Steering Servo Signal - microseconds

Rudder Servo Movements 

The Rudder Servo made approximately 100,000 movements over the 40 hours of the voyage. This is corresponds to about 2500 movements per hour. 

The number of movements per hour remains fairly constant over the whole voyage as shown in the plot below. But the amplitude of the movements does vary greatly depending on conditions, as shown by the image above.

Accumulated Rudder Servo Movements

Rudder and Wingsail Trim Tab Servo Movements per Hour

The next plot shows the quantity of movements in each hour for both the rudder servo and the wingsail trim tab servo, rather than accumulated movements.

If the vessel is sailing well and holding course on one tack, then there should no movement in the wingsail trimtab. This was the case for a few hours. This can be seen in the blue line plot below.

The wingsail trim tab movements at other times are related to waves rolling of the vessel when reaching, or due to rolling while running downwind.

Rudder and Wingsail Trim Tab Servo Movements per Hour

Power Consumption - Battery Voltage and Current

The battery is 2S LiPo battery made up of twenty 18650 cells. 

Fully charged the battery voltage is 8.4V and it may discharge down to about 6.0V

The rate of discharge appears to be fairly constant, and actually seems consistent with tests performed on land.

Speed Over Ground

SOG is measured in metres per second. One metre per second is approximately 2 knots.

The first few hours shows the boat averaging around 0.5m/s (1 knot) in reaching conditions with wind of about 15 knots.

An increase in wind speed to about 20 knots lifted the average speed toward 1m/s, almost 2 knots. Then as the wind faded in the later part of the voyage the average speed dropped to well under 0.25m/s, less than  0.5 knots.

Temperature within the Electronic Housing

The plot of temperature clearly shows the diurnal cycle, with overnight lows of around 17°C and daytime highs of over 35°C inside the equipment housing.

The overnight lows are very close the published water temperature for Port Phillip at this time.

The daily highs are well above the outside air temperature. The equipment housing is clear plastic and the interior clearly heats up in the direct sunlight.

COG, BTW and CDA

Course Over Ground, Bearing to Waypoint and Course Deviation Angle.

The yellow plot of COG highlights the poor course keeping during the voyage. It clearly shows long periods of time spent off course. It does eventually recover and get back on course.

The blue plot of BTW clearly shows the passage past three waypoints, with the third waypoint involving a more noticeable course change from about 230°T to about 285°T.

As each waypoint is approached and closely passed, the BTW goes to extreme values until it goes behind the beam of the boat, and mission steps to the next waypoint.

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.