Connect Lego Hub (Spike/Inventor) to PC/Android with low latency

[czuryk]

Hi All,

I need to develop my own Android app to control the hub as a remote control with advanced features. Using only an Xbox controller isn't sufficient for my needs, as I require additional real-time telemetry data to demonstate it on smartphone screen.

I found a potential solution here: https://pybricks.com/projects/tutorials/wireless/hub-to-device/pc-communication/.
But I found a bit specific (connection to Hub before program runs). But Ok, i have test it with my test C# program with BLE support (I have useed this service: c5f50001-8280-46da-89f4-6d8051e4aeef), and while it works fine, but I noticed significant latency. The program just send's command (value from 0 to 255) while I changed the slider. And ofcourse I have limit the amount of data by using a 15ms interval. (It is perfectly work's with Lego native firmware SPIKE/Inventor with native BLE implementation).

To evaluate the issue, I created a small test program. It transmits a single value and runs one motor to measure the response time. However, I observed that commands are getting queued, causing the motor to execute stacked commands, changing speed and direction with noticeable delay.

Please review my POC for hub. I've stripped out everything that could increase the latency:

from pybricks.hubs import InventorHub
from pybricks.pupdevices import Motor
from pybricks.parameters import Port, Color
from pybricks.tools import wait
import ustruct

# Standard MicroPython modules
from usys import stdin, stdout
from uselect import poll
import urandom

hub = InventorHub()
motor = Motor(Port.A)

stdout.buffer.write(b"rdy")
while True:
    cmd = stdin.buffer.read(1)
    b, = ustruct.unpack("B", cmd)
    b = (b - 127) * 5
    #print(b)
    #motor.dc(b)
    motor.run(b)

My question is: is there an alternative solution for sending and receiving data without delays (similar to the Xbox controller approach) that would be compatible with connecting to an Android smartphone via BLE for use with a my custom app?

I’m hoping there’s a native solution similar to this example: https://github.com/pybricks/micropython-bleradio/blob/master/examples/custom_irq.py.
However, I haven’t been able to find one so far. The example uses specific channels like BLERadio(observe_channels=[4, 18], broadcast_channel=5, ble=ble), but I can’t integrate this with my Android app.

Thank you.

[czuryk]

I think my starting post was not clear, so I will try to simplify my needs:
I believe what I’m looking for is a GATT implementation provided by the micropython bluetooth library. I think I can develop this implementation by myself using this library, but perhaps there’s already such implementation included in the pybrick firmware that supports this feature already and this will be pretty usefull to save some space on Hub just utilize such implementation.

@laurensvalk , could you guide me in the right direction here?

Here’s what I need to achieve:

1. Start advertising.
2. Establish a connection with a device (e.g., an Android smartphone).
3. Send and receive data to/from another device.

Sound's pretty easy, but I still cannot find this for pybrick.

Thanks.

[laurensvalk]

Is it still slow with motor.dc instead of motor.run?

Motor.run smoothly decelerates, so changing speed rapidly is going to appear slow when it is just physically taking time to stop or reverse.

[czuryk]

Yes, as you can see, I've tried both options, and they are slow. I can shot a demo video for you with this issue.

[czuryk]

I noticed that when I change the interval for sending new positions to 100ms, the performance improves, but only for a single value. However, when I increase the packet size to 6 (for example), the latency decreases significantly, even though some packets are dropped.

Here’s the snippet:

    cmd = stdin.buffer.read(6)
    a, b, c, d, e, f = ustruct.unpack("BBBBBB", cmd)
    b = (b - 127)
    motor.dc(b)

[czuryk]

Hi @laurensvalk , apologies for the annoying questions, but I’m completely stuck here. I decided to write my own custom BLE class, similar to what I previously did for the legacy hub firmware.

I tried using the Bluetooth class in the PyBricks project, but I encountered the following error: ImportError: no module named 'bluetooth'.

From what I understand, it seems there’s no way to directly access BLE with full BLE features. Is that correct?

[czuryk]

As I realized, there is no other way to connect to the Hub under Pybricks control except the option mention above. I found a data transmission delay that allowed me to control the hub correctly.
So I wrote my own Android app as an alternative way to control the Hub using a smartphone. I have shared the current realisation of my own Android BLE app with some basic functions:
https://github.com/czuryk/Lego/tree/main/PyBricks/BLE

Of course, this won't replace the more preferred option of controlling it with an Xbox controller, but in order to not only control the model but also receive, processing and represent hub telemetry and sensor data, using a smartphone GUI this approach is much more convenient (at least for me).

Here's a screenshot of the app:

This is a demo video of how app connecting and controlling a test model with some feedback features like hub voltage, gyro and distance sensor data:

https://youtube.com/shorts/6E0ajDd5fnM

@laurensvalk, How do you think this example of app could be useful to add to the Hub-to-Hub Communications tutorials section on pybricks.com?