Tried to restore controlfile from a backup set by RMAN, but it failed with ORA-19697.
RMAN> restore controlfile from '/home/oracle/031r656h_1_1';
ORA-19697: standby control file not found in backup set
ORA-19697 means that the backup set you want to use does not contain any controlfile. You have to choose the right one to restore it.
Please note that, "standby control file" in error message may be somewhat misleading for some cases.
To know what content is in a backup piece, there's a procedure to find out. Most likely, the one with smallest size contains controlfile.
If you cannot find the right backup set to restore it, you still have a chance to restore controlfile from AUTOBACKUP or a copy, not necessarily a RMAN backup set.
Be careful, the restored controlfile may not meet your expectations, especially the point in time of the backup is too old to be useful.