Here's my python code:

from H3DInterface import *
from H3D import *
from H3DUtils import *
import math

hdev = None
di = getActiveDeviceInfo()

if( di and len( di.device.getValue() ) > 0 ):
hdev=di.device.getValue()[0]


Delta_A1=-4
Delta_A2=-20
Delta_B1=-5
Delta_B2=-6
class force_z(TypedField(SFString,(SFBool,SFBool,SFBool,SFBool,SFVec3f))):

def update(self, event):
global Delta_A1
global Delta_A2
global Delta_B1
global Delta_B2

up, down, right, left, pos = self.getRoutesIn()

posz=pos.getValue().z

if posz < 0.2 :

if event == up :
Delta_A1=Delta_A1+5
return "(z^2)*"+str(Delta_A1)+"+z*"+str(Delta_A2)+"-10"
if event == down :
Delta_A1=Delta_A1-5
return "(z^2)*"+str(Delta_A1)+"+z*"+str(Delta_A2)+"-10"
if event == right :
Delta_A2=Delta_A2+5
return "(z^2)*"+str(Delta_A1)+"+z*"+str(Delta_A2)+"-10"
if event == left :
Delta_A2=Delta_A2-5
return "(z^2)*"+str(Delta_A1)+"+z*"+str(Delta_A2)+"-10"

else:
if event == up :
Delta_B1=Delta_B1+5
return "(z^2)*"+str(Delta_B1)+"+z*"+str(Delta_B2)+"-10"
if event == down :
Delta_B1=Delta_B1-5
return "(z^2)*"+str(Delta_B1)+"+z*"+str(Delta_B2)+"-10"
if event == right :
Delta_B2=Delta_B2+5
return "(z^2)*"+str(Delta_B1)+"+z*"+str(Delta_B2)+"-10"
if event == left :
Delta_B2=Delta_B2-5
return "(z^2)*"+str(Delta_B1)+"+z*"+str(Delta_B2)+"-10"

force_tastiscroll=force_z()

and I get this error:
"Warning: update()-function for Python defined field of type Force.force_z does not return a value. "
an then this other one, in infinite loop:
"Syntax error: Premature end of string
Could not parse function expression: "" in ParsedFunction node(zfun). "

Now, I know that the problem is here:

up, down, right, left, pos = self.getRoutesIn()
posz=pos.getValue().z
if posz < 0.2 :

because when I wasn't routing pos (the SFVec3f) the remaining was working just fine. The vector contains the position's coordinates of the end effector of the haptic device .

Can anybody help me???

thanks

The first warning is because one of your paths (looks like default) does not return anything. You should probably have a statement at the end of your update function.

The second error is probably caused because of either a null string or some garbage string since nothing is returned from the function. The ParsedFunction perhaps expects a non-null string. I do not know, check the strings you return and also check if you are using any characters that the function string treats as special (if there are any, if it is it says so in the documentation of the node).