Ever decreasing circles

I wrote some test code in Python to check that my firmware was working properly. This turned out to be incredibly easy using the socket and struct modules. I have not written a lot of Python before but it never ceases to amaze me how much more productive it is compared to C++. Handling the Ethernet UDP comms required only a few lines of code. Here are extracts from the constructor and destructor :-

from socket import *
from struct import *

class Hydra:
  def __init__(self, ip_addr):
      # Create sockets
      self.sendSock = socket(AF_INET,SOCK_DGRAM)
      self.sendAddr = (ip_addr,21000)

      self.recvSock = socket(AF_INET,SOCK_DGRAM)
      self.recvSock.bind(("",21001))
      self.recvSock.settimeout(0.1)

  def __del__(self):
      # Close sockets
      self.recvSock.close()
      self.sendSock.close()

Below is the code that sends a command and receives the reply. UDP packets are not guaranteed to arrive at their destination and if they do they are not guaranteed to arrive in the same order they were sent in, although on a small single segment LAN they generally do. Packets are protected by CRCs and checksums so if you do receive a packet you can be pretty sure it is not corrupt.

To make a reliable protocol timeouts, retries and packet sequence numbers are required. I also added a 4 byte magic number at the start of each packet just to make sure a stray packet from a rogue application or the Web could not cause HydraRaptor to lose the plot.

    def do_cmd(self,cmd):
      tries = 3
      while tries:
          self.sendSock.sendto('\x12\x34\x56\x78' + cmd,self.sendAddr)
          try:
              data,addr = self.recvSock.recvfrom(1024)
              return data
          except timeout:
              print "read failed"
              tries = tries - 1
      raise 'comms failed'

Here is the code to wait for the machine to be ready and then instruct it to go to to a 3D position.

   def get_status(self):
      return unpack('>hhhhBB', self.do_cmd('\x00\x00'))

  def goto_xyz(self,pos,delay):
      while 1:
          a, b, c, steps, seq, ready = self.get_status()
          if steps == 0 and ready:
              break
      seq = (seq + 1) & 255
      self.do_cmd(pack('>BBhhhh', self.goto_xyz_cmd, seq, pos[0], pos[1], pos[2], delay))

Notice how easy it is to marshal and unmarshal packets using the struct module. The big endian to little endian conversion is handled simply by putting a > sign at the start of the format string.

Below are some test patterns. The 25 circles in the centre are 1 - 25mm radius and are drawn with the minimum number of line segments to keep within 0.05mm accuracy. This is calculated with the formula

π / acos((r - 0.05) / (r + 0.05))

The outer circle is made with 360 line segments.



Not only can I now drive HydraRaptor from a script but I can also interact directly with it via the command line:

>>> hydra = Hydra("10.0.0.42")
>>> print hydra
At (9944,11943,400) steps = 0
>>> hydra.goto_xyz((0,0,0),1000)
>>> print hydra
At (3310,3976,133) steps = 3976
>>> print hydra
At (0,0,0) steps = 0
>>>

I noticed that although the Z axis is repeatable in the short term the distance to the paper varied by about 0.1mm over a two day period. I expect this is due to the MDF expanding and contracting slightly. I plan to make a tool height sensor mounted on the XY table to make it easy to switch tools. This should also compensate for the wood changes.

The next thing to do is use my existing milling setup to make some motor mounts to make an improved milling machine. To do this I will extend my Python script to allow me to define an outline as a list of lines and arcs and then mill around it.

I will then try to make the RepRap FDM extruder with the improved milling machine.

It speaks, but my PC has its fingers in its ears

I almost got UDP working this evening. I wrote a simple Python test script to send a "get status" command to HydraRaptor. Here it is :-

   from socket import *

 # Set the socket parameters
 hydra = "10.0.0.42"
 inPort = 21000
 inAddr = (hydra,inPort)
 outPort = 21001
 outAddr = ("localhost",outPort)
 buf = 1024

 # Create sockets
 sendSock = socket(AF_INET,SOCK_DGRAM)
 recvSock = socket(AF_INET,SOCK_DGRAM)
 recvSock.bind(outAddr)
 recvSock.settimeout(2.0)

 # Send a get status command
 cmd = "\x12\x34\x56\x78\x00"
 if(sendSock.sendto(cmd,inAddr)):
     print "Sending message ....."

 # Receive the reply
 try:
     data,addr = recvSock.recvfrom(buf)
     if data:
         print " Received message '", data, "'"
 except:
     print "Read failed"

 # Close sockets
 recvSock.close()
 sendSock.close()

HydraRaptor replies with its x,y,z coordinates and the number of steps remaining. The reply packet arrives at my PC which then sends an ICMP destination unreachable (port unreachable) message back. I have no idea why. netstat -ad shows :

  UDP    shuttle:21001          *:*                                    2412
[Pythonwin.exe]

Which looks to me to show that the port is being listened to. I have tried turning my firewall off but that made no difference. I am sure the problem is at the PC end because Ethereal shows me that the packets are well formed. I have had enough for this evening. Any Python socket experts out there? I have never done sockets in Python before.