bt

I’ve started some development for a new throttle controller, this one is based on an Android tablet for the user interface. My approach is a bit different than what is out there for tablets and smartphones though. I don’t like ‘sliders’ on a touch screen for controlling vehicles, so the idea here is to interface a small analog joystick (the parallax unit) and a potentiometer to do the actual control. The tablet will be used to pull up user interface screens for the locomotives and also have screens for controlling other things.

Basically, I’ve connected a generic bluetooth interface (about $10) to my Xbee Widget controller. The Widget can be configured to have up to 8 analog inputs, I’m using 3 of them here. I have the bluetooth interface on the spare serial port. On the other serial port is the Xbee which actually sends the commands to the locomotive.

I’m using PyGame to build Android apps with because I love Python!

Right now, the only thing on the screen that actually works is the speedometer. This is a screen shot of a Positive Train Control screen I dug up on the internet. I’m not sure what most of the other things on there are so this is just for playing around for now. But hey, it looks neat!

So anyhow, since I looked high and low and found ONE reference on how to scan the BT devices with Python, I decided to post up some python source to help out anyone looking to do this. As with most Python things, it’s really quite easy once you get the particulars ironed out.

To use this, just pass in the name of your bt device after you instantiate it. This is the same name you see when you pair your device with your tablet. In my case, it’s ‘HC-06’.



bluetooth = Bluetooth()
bluetooth.prepare("HC-06")
bluetooth.write("--testmessage--")


Here is the method. Note that the UUID in the device create is the same for all BT devices (as far as I know).

Jnius is a nice package that lets you call Java from Python so this could be used for other things besides Bluetooth I guess, but that’s another project. I really detest Java so anything I can do to write Android apps in python is great.




#
# Bluetooth interface class for PyGame- Python Android Games and Graphics development
# you will need the jnius python library to use this
#

import pygame
import sqlite3
import math

try:
   import android
   from jnius import autoclass
except:
   android = None

class Bluetooth:
    def __init__(self):
        self.BluetoothAdapter = autoclass('android.bluetooth.BluetoothAdapter') 
        self.BluetoothDevice = autoclass('android.bluetooth.BluetoothDevice')
        self.BluetoothSocket = autoclass('android.bluetooth.BluetoothSocket')
        self.UUID = autoclass('java.util.UUID')
        self.deviceValid = False

    def prepare(self, name):
        paired_devices = self.BluetoothAdapter.getDefaultAdapter().getBondedDevices().toArray()
        for device in paired_devices:
            if device.getName() == name:
               self.socket = device.createRfcommSocketToServiceRecord(self.UUID.fromString("00001101-0000-1000-8000-00805F9B34FB"))
               self.recv_stream = self.socket.getInputStream()
               self.send_stream = self.socket.getOutputStream()
               self.socket.connect()
               self.deviceValid = True

    def write(self, sendString):
        if self.deviceValid:
           self.send_stream.write([ord(b) if ord(b) <= 127 else ord(b)-256 for b in sendString])

    def read(self):
        datastring = ""
        if self.deviceValid:
           c = self.recv_stream.available() 
           if c > 0:
              for i in range(c):
                  datastring = datastring + chr(self.recv_stream.read())
        return datastring
        
    def close(self):
        if self.deviceValid:
           self.socket.close()



P1040103

P1040101

P1040102

Here are a couple of shots of my control system going into my Aristocraft RS3. The power is all in the back end, I have a 5000mah 14.8v lipo pack driving this beast with a Pololu 18v7 motor controller powering the trucks. A very potent drive train. Anyhow, these pics show the brains- the Atmel 1634 board, the Xbee Series 1 and the MDFly mp3 sound card. Not seen is the RFID reader on the fuel tank- I’ll post that up later. Phew, some work and lots of engineering spits and fails but it’s now pretty clean and works well. I did downsize the controls a bit, I’m only driving the motor, the two coupler servos and the sound card. I left the lights on a manual switch and there is a current sensor in there but I’m not looking at it right now. As mentioned, the RFID is also connected and works so I do have the basics of a computer controlled system. The main control boards are also reasonably accessable by taking off just the short hood of the locomotive so tweaking the firmware, sounds and the pololu motor controller won’t require the entire locomotive to be taken apart (which is a BITCH to say the least!)

This is a quick project I’ve been working on to test the feasibility of using wifi to directly control something.

It uses a slider on an android app to control the position of a servo in real time using my widget and the Sparkfun WiFly Module.

pygame

There are three parts, the hardware, which is the widget and the Wifly module, the microcode that goes into the widget and the app code on the tablet.

(This is a rather lengthy post so I’ve moved this to a page of it’s own)

You can find it here- Simple Wifi Servo control with Android

As far as conclusions from this, I don’t particularly care for 802.11 for control. In my case, the primary objection is well, it’s not really meant for that sort of thing and I don’t like all the complicated message transactions this entails. Your mileage may vary on that. The other thing is my cheap tablet doesn’t have a very good wifi range so at 30 ft or so I start to loose control. Granted, much of this could probably be resolved with an improved algorithm on the app side, or perhaps a better device (I’ve resisted getting a smart phone). But for me, I prefer the Xbee series mentioned elsewhere on this blog so I will be sticking with that project before I come back and put any more effort in this one.