RC Submarine 4.0 – radio (7/10)

Submarine needs a low-frequency radio controller that is able to penetrate water. At least 27 MHz, 40 MHz and 75 MHz bands should work.

Those are hard to find. Nearly all available radio controllers use 2.4 GHz. I’ve resorted to buying cheap toy submarines that use proper radio frequencies, and cannibalize them for the receiver board and use their transmitter. It is a sub-optimal solution, but it has worked well enough in my three previous submarines.

Once gain, I continued the same path, using the same no-name Chinese toy manufacturer as before.

A toy submarine

I bought a mini u-boat toy submarine of a chinese brand Mliu. It cost 33 euros in Amazon store TAIPPAN, link:
https://www.amazon.de/-/en/Control-Military-Electric-Rechargeable-Swimming/dp/B08T66M8P7/

The main criteria in buying this particular toy, besides the fact it uses 27 MHz radio frequency, was the nice looking transmitter this has. Very toyish, but it still appeals to my eye.

Radio transmitter.

The transmitter has 6 buttons for control: left, right, forward, backward, dive, surface.

After I tested that the toy submarine works, I disassembled it. I cut all wires leading to motors and took out the single board it contains.

Here is the board that came out.

Here is a closeup of the board from both sides, after I removed the big green cylinder that was a LiPo battery.

External power supply

In previous submarines I’ve supplied the radio board with a LiPo battery. Now I wanted to remove that and use power from the Lego battery box. It would make recharging the submarine easier, as there would be only one battery on-board.

But does the radio board work with voltage other than 3.7V it it designed for?

I tested it using an adjustable power supply, and found the minimum voltage that made the board function to be 2.3V. That means I can use the Raspberry Pi 3.3V output to power up the radio board. Great!

I also verified that the control range was not affected. It was around 7 meters regardless of supply voltage.

Connection to Raspberry

First I checked if I need pull up/down resistors to protect the Raspberry. Raspberry GPIO pins can take 0-3.3V. I measured voltage from the radio board motor wires that I’m using to read button presses. The voltage was either 0V or 3.3V. Great, I can connect them directly to Raspberry.

Then I connected the wires to Raspberry GPIO pins. 6 wires for buttons, one ground wire and one power wire.

Radio board wires. The blue wire on the left is the antenna. I added a DuPond connector to it for testing different antennas.

Code

I first created Button instances for all the GPIO inputs and one output for powering the radio.

#prepare radio control
radioPower = DigitalOutputDevice(25)
buttonForward = Button(24, hold_time=1.0)
buttonBackward = Button(23, hold_time=1.0)
buttonLeft = Button(9)
buttonRight = Button(10)
buttonDive = Button(11, bounce_time=None, hold_time=1.0, hold_repeat=True)
buttonSurface = Button(8, bounce_time=None, hold_time=1.0, hold_repeat=True)

tachoPower.value = 1

Then for left and right buttons I added handlers that write values to the motor output. Here is a snippet of the relevant parts.

motorLeft = DigitalOutputDevice(17)

def buttonLeft_pressed():
    motorLeft.value = 1
def buttonLeft_released():
    motorLeft.value = 0

buttonLeft.when_pressed      = buttonLeft_pressed
buttonLeft.when_released     = buttonLeft_released

For forward and backward buttons I added a little crawl period for the first second, in which PWM duty cycle is only 60%. It helps maneuvering in tight places.

motorForward = PWMOutputDevice(22)
motorForward.frequency = 1000

def buttonForward_pressed():
    motorForward.value = 0.6
def buttonForward_held():
    motorForward.value = 1
def buttonForward_released():
    motorForward.value = 0

buttonForward.when_pressed   = buttonForward_pressed
buttonForward.when_held      = buttonForward_held
buttonForward.when_released  = buttonForward_released

Surface and dive buttons don’t control motors directly, instead they change target depth or target distance for the PID controller.

def changeTargetDepth(value):
    global controlMode
    global targetDepth
    global targetDistance
    if controlMode == MODE_PID:
        targetDepth += value
        if targetDepth < 0:
            targetDepth = 0
        if targetDepth > MAX_DEPTH:
            targetDepth = MAX_DEPTH
    elif controlMode == MODE_PID_LASER:
        targetDistance -= value
        if targetDistance < 0.1:
            targetDistance = 0.1
        if targetDistance > MAX_DISTANCE:
            targetDistance = MAX_DISTANCE

def buttonSurface_pressed():
    changeTargetDepth(-0.05)
def buttonSurface_held():
    changeTargetDepth(-0.5)

buttonSurface.when_pressed   = buttonSurface_pressed
buttonSurface.when_held      = buttonSurface_held

Change control mode

I need to change between modes. The PID can be controlled using a pressure sensor or a laser distance sensor, so I need to change between those two. Also, I need to be able to bypass PID control and drive syringe directly, e.g. to empty the syringe after dive.

But the radio transmitter has only 6 buttons which all have a function already. The only way I can think of is to use button sequences. Three button presses in a certain order in 1.5 seconds to change the mode.

#change depth control mode
#  triggered by button sequences
#    MODE_DIRECT    = Surface-Dive-Surface
#    MODE_PID       = Surface-Dive-Right
#    MODE_PID_LASER = Surface-Dive-Left
changeMode = False
if buttonSurface.value == 1:
    if modeButtonStep == 0:
        prevModeButtonTime = perf_counter_ns()
        modeButtonStep = 1
    if modeButtonStep == 2:
        modeButtonStep = 3
        controlMode = MODE_DIRECT
if buttonDive.value == 1:
    if modeButtonStep == 1:
        modeButtonStep = 2
if buttonRight.value == 1:
    if modeButtonStep == 2:
        modeButtonStep = 3
        controlMode = MODE_PID
if buttonLeft.value == 1:
    if modeButtonStep == 2:
        modeButtonStep = 3
        controlMode = MODE_PID_LASER
if modeButtonStep != 0 and (perf_counter_ns() - prevModeButtonTime) / 1e9 >= 1.5:
    modeButtonStep = 0

Problem with auto-surface

While testing the board, I kept having problems with the surface button. The surface motor wire would turn on apparently by itself and the surface button stopped working. What was going on?

It took me awhile to figure it out. The manual calls it “automatic lifting function”. When no buttons are pressed within 10 seconds, the submarine will automatically rise to the surface. This is a new feature in this brand of toy submarine, which is why I hadn’t noticed it in previous submarine projects.

I spent a lot of time trying to bypass it. First I reverse engineered the operation principle. The little toy submarine has a piston ballast, like I’m making here. 🙂 The ballast has limit switches to indicate when to stop driving the motor. The auto-surface feature is waiting for the ballast to reach the limit, and then it will stop driving the motor. I connected the switch wire to Raspberry and put code in to trigger it every 5 seconds, to make the board think it is at surface. It worked most of the time, but sometimes the surface button still stopped responding. What made it worse, if the auto-surface thing was turned on only for a moment, you would need to press dive button once to reset it. Otherwise the surface button was dead.

Eventually, I came up with a brutal idea. I just reset the whole board. Since I’m powering it through Raspberry GPIO pin, I can control when to power it.

I put a code to reset the board very quickly every 9 seconds. Or more precisely, after 9 seconds of not pressing any buttons, since I can detect all button presses in code. The power is cut off for one loop interval, 25 ms, which is long enough to reset it.

#radio controller fix
#  the radio board has an auto-surface safety feature
#  after 10 secs of silence it will make the surface button to not work
#  to fix it, we will quickly power off and on the radio every 9 secs
if (buttonDive.value == 1 or buttonForward.value == 1 or
   buttonBackward.value == 1 or buttonLeft.value == 1 or buttonRight.value == 1):
    prevRadioResetTime = perf_counter_ns()
secsSinceRadioReset = (perf_counter_ns() - prevRadioResetTime) / 1e9
if secsSinceRadioReset > 9:
    radioPower.value = 0
    prevRadioResetTime = perf_counter_ns()
else:
    radioPower.value = 1

That took care of it – the radio worked always after that.

Problems with range

When I got to test the radio inside the hull, with Raspberry and all other parts running, I noticed the range was only 3 meters. That was odd. I had tested it before disassembly, and the range should be 7 meters.

Maybe extending the antenna will help? In my earlier submarines I added a longer antenna and increased the range from 7 to 10 meters. I tried it, but this time it had no effect.

Maybe the Raspberry’s noisy supply voltage causes frequency interference? After all, the voltage from Lego rechargeable battery box, that is initially clean, coming from two LiPos, goes first to a 5V switching regulator, and then to Raspberry’s internal 3.3V regulator, before reaching the radio board. So it probably has a lot of noise.

I tried to supply the radio from a clean power supply, and that seemed to fix the problem at first. Range was increased to 7 meters. But when I added a separate LiPo for the radio board, the range was dropped back to 3 meters. I was getting mixed test results.

Okay, maybe the radio picks up electromagnetic interference from wires and other parts inside the hull? After all, it is located on top of the Raspberry, very close to all other electronic parts.

The first location was on top of the Raspberry.

I tried a few different locations. If the receiver was 20 cm away from other parts, range was 7 meters, but dropped to 3 meters when close to other parts. Okay, I’ll locate the radio board as far as possible inside the hull, about 10 cm away.

I moved the location to the back, on top of the main propeller motor.

That increased the range to 4 meters. Still quite bad.

Once again I tried different antennas. 20 cm long straight antenna didn’t improve the range but a big looping antenna along the hull cylinder increased range to 6 meters at first. But when I tested it again in swimming pool, it had no effect, range was only 3-4 meters, which is the same range I got with a short 3 cm antenna.

At this point, I was having a lot of mixed information and test results. This radio interference feels like black magic. Probably all the things I tried have some effect, I just can’t measure it in any reliable way.

I gave up trying to fix it. The radio control range is only 3-4 meters, just deal with it. It was enough to do the tests and videos I needed to do, including tests in a swimming pool and through a very small river. But it was annoying, sometimes having to press buttons multiple times to get through.

Testing the radio control in a swimming pool.