**3.1 Principle of operation: Positional synchronization and ratio control**

All operation was based on setting an analogue synchronization between the speed and direction control modules first. This has been achieved by feeding a common speed reference voltage to the drives and tuning the drive speeds in order to get them into an approximate synchronism. A ratio agreement was significant for the slave drive. This analogue pre-synchronization was matched the two speeds within an error range of approximately 0.8%, confirming the earlier results of Ref. [31]. Presently, the digital synchronization has to balance for the analogue speed errors in order to get an absolute, angular and positional synchronization with no drift and no cumulative displacement of the motor shafts. This required a digital feedback of the angular shaft position of the drives. In our case, incremental shaft encoders (e.g., encoder simulation from a resolver system) were used for this feedback purpose.

The synchronizer (CPU and encoder modules) was continuously checked the two shaft positions and immediately responded by an analogue correction signal when an angular error started to appear. This analogue correction signal, added to the slave's reference with the correct polarity, was maintained the shaft positions of master and slave inline. The synchronized speed of developed system was monitored by using LCD. It was noted that the synchronizer responds within only microseconds to each individual encoder pulse and the slave was practically have no chance to drift away. We have been adopted the way to generate feed forward signal (FFS) given in Ref. [31]. Currently, this FFS was generated internally from the frequency of the master encoder and no external voltage was applied to the analogue input. It was observed that the encoder frequency always represents the real actual speed of the master. Moreover, a FFS was needed to run the drives, and no external correction voltage was added to receive the total slave speed reference. It was significant that the digital FFS used here was higher than at maximum speed, for encoder frequencies [7, 8, 31].
