NXT Python Facade Class

NXT Python Facade Class

Nearly all functions of the NXT brick can be controlled using NXT Python. However, because utilising all functions is complex, the facade class will only focus on those functions we want to use. The main functions of the NXT are:

  • Input
    • Set motor speed
    • Control the speaker
    • Display messages
  • Output
    • Read motor encoder values
    • Read sensors (microphone, ultrasonic, touch, light)
    • Read system information
  • Other
    • Search for a NXT
    • Connect to a NXT
    • Read files from the NXT
    • Read the NXT firmware

There is little meaning to putting all of these functions into the facade class. When another function is necessary, the facade can be extended to include it. In keeping with this, the facade described here will supply the functions necessary for the robot we will control.

  • Input
    • Set motor speed: setMotors()
  • Output
    • Read motor encoders: getMotors()
    • Read sensors (microphone, ultrasonic, touch, light): getSensors()

A class made along these lines is shown below.

 #!/usr/bin/env python
 # @file NXTBrick.py
 # -*- coding:shift_jis -*-
 
 import nxt.locator
 from nxt.sensor import *
 from nxt.motor import *
 
 class NXTBrick:
     def __init__(self, bsock=None):
         """
         Constructor
         Connect to a NXT brick, control motors and sensors, and reset odometry.
         """
         if bsock:
             self.sock = bsock
         else:
             self.sock = nxt.locator.find_one_brick().connect()
 
         self.motors = [Motor(self.sock, PORT_A),
                        Motor(self.sock, PORT_B),
                        Motor(self.sock, PORT_C)]
             
         self.sensors = [TouchSensor(self.sock, PORT_1),
                         SoundSensor(self.sock, PORT_2),
                         LightSensor(self.sock, PORT_3),
                         UltrasonicSensor(self.sock, PORT_4)]
         self.resetPosition()
 
     def close(self):
         """
         Close the connection to the NXT.
         """
         self.sock.close()
 
     def resetPosition(self, relative = 0):
         """
         Reset the NXT motor encoders.
         """
         for m in self.motors:
             m.reset_position(relative)
 
     def setMotors(self, vels):
         """
         Receive an array, set the motor power levels.
         If the length of vels is not equal to the number of motors,
         the smaller of the two values is used.
         """
         for i, v in enumerate(vels[:min(len(vels),len(self.motors))]):
             self.motors[i].power = max(min(v,127),-127)
             self.motors[i].mode = MODE_MOTOR_ON | MODE_REGULATED
             self.motors[i].regulation_mode = REGULATION_MOTOR_SYNC
             self.motors[i].run_state = RUN_STATE_RUNNING
             self.motors[i].tacho_limit = 0
             self.motors[i].set_output_state()
 
     def getMotors(self):
         """
         Read the motor encoder angles.
         
         """
         state = []
         for m in self.motors:
             state.append(m.get_output_state())
         return state
 
     def getSensors(self):
         """
         Read the sensor values, return them in an array.
         """
         state = []
         for s in self.sensors:
             state.append(s.get_sample())
         return state
 
 
 """
 Test program
 Set a suitable motor value, read their encoders.
 Read sensor values and display them.
 """
 if __name__ == "__main__":
     import time
     nxt = NXTBrick()
     print "connected"
     
     # Motor test
     for i in range(100):
         nxt.setMotors([80,-80,80])
         print "Motor: "
         mstat = nxt.getMotors()
         for i, m in enumerate(mstat):
             print "(" , i, "): ", m
         time.sleep(0.1)
     nxt.setMotors([0,0,0])
 
     # Sensor test
     for i in range(100):
         sensors = ["Touch", "Sound", "Light", "USonic"]
         sval = nxt.getSensors()
         for s in sensors:
             print s + ": " + sval
             print ""
             time.speel(0.1)

The final section from if name == "main": is a test program. When this module is executed independently, the test will run. The module should be tested and corrected until it passes the test succesfully.

The above NXT facade class is very simple, only setting motor values, reading the encoders and reading the sensors. Trying to do everything from the start leads to a class whose purpose is difficult to understand. The class can be extended at any time, so begin with something simple that works as it should.

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