improvements in magfiels/ips_mercury

- read voltage
- fix a bug with ._init name conflict

secop_psi/magfield.py

@@ -82,19 +82,21 @@ class Magfield(HasLimits, Drivable):
         'wait time to ensure field is stable', FloatRange(0, unit='s'), readonly=False, default=31)
 
     _state = None
-    _init = True
+    __init = True
     _super_sw_check = False
     _last_target = None
     switch_time = None
 
     def doPoll(self):
-        if self._init:
-            self._init = False
+        if self.__init:
+            self.__init = False
             self.switch_time = time.time()
             if self.read_switch_heater() and self.mode == Mode.PERSISTENT:
                 self.read_value()  # check for persistent field mismatch
                 # switch off heater from previous live or manual intervention
-                self.write_target(self.persistent_value)
+                self.write_target(self.persistent_field)
+            else:
+                self._last_target = self.persistent_field
         else:
             self.read_value()
         self._state.cycle()