improvements in magfiels/ips_mercury

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

secop_psi/convergence.py

@@ -89,6 +89,10 @@ class HasConvergence:
         """to be called from write_target"""
         self.convergence_state.start(self.state_approach)
 
+    def interrupt_state(self):
+        """to be called from stop"""
+        self.convergence_state.start(self.state_instable)
+
     def state_approach(self, state):
         """approaching, checking progress (busy)"""
         state.spent_inside = 0
@@ -157,3 +161,10 @@ class HasConvergence:
         else:
             state.spent_inside = max(0, state.spent_inside - state.delta())
         return Retry()
+
+    def state_interrupt(self, state):
+        self.status = IDLE, 'stopped'  # stop called
+        return self.state_instable
+
+    def stop(self):
+        self.convergence_state.start(self.state_interrupt)