Change AFM._check_target_deflection argument name to `deflection`.

Also check for negative deflections.  It can't hurt ;).

diff --git a/pyafm/afm.py b/pyafm/afm.py
index 406b15f9eb089b775f542ac34e342eedf7644ce0..12543e4834d89a54682c029ebbd752b9da054f2e 100644 (file)
--- a/pyafm/afm.py
+++ b/pyafm/afm.py
@@ -87,7 +87,7 @@ class AFM (object):
 
         zero = _convert_volts_to_bits(axis.config['channel'], 0)
         target_def = _convert_volts_to_bits(defc.config, setpoint)
-        self._check_target_deflection(target_def)
+        self._check_target_deflection(deflection=target_def)
 
         _LOG.debug('zero the %s piezo output' % self.axis_name)
         self.piezo.jump(axis_name=self.axis_name, position=zero)
@@ -150,7 +150,7 @@ class AFM (object):
             'positioned %g m into the surface at stepper %d, piezo %d (%g m)'
             % (depth, self.stepper.position, target, target_m))
 
-    def _check_target_deflection(self, target_deflection):
+    def _check_target_deflection(self, deflection):
         defc = self._deflection_channel()
         max_def = defc.get_maxdata()
         if target_deflection > max_def:
@@ -158,6 +158,11 @@ class AFM (object):
                         'than the maximum deflection value of {:d} bits'
                         ).format(setpoint, target_deflection, max_def))
             raise ValueError(setpoint)
+        elif target_deflection < 0:
+            _LOG.error(('requested setpoint ({:g} V = {:d} bits) is less '
+                        'than the minimum deflection value of 0 bits'
+                        ).format(setpoint, target_deflection))
+            raise ValueError(setpoint)
 
     def _stepper_approach_again(self, target_deflection, min_slope_ratio, far):
         _LOG.info('back off %d half steps and approach until deflection > %g'