# Force Fight and Its Elimination in a Prototype of a Redundant Direct-Drive Avionic Actuator
## 7 Experimental Validation
### Faulty Operation Mode and Force Fight Phenomenon  
The force fight phenomenon can occur for any configuration of operation. According to the application, active-active configuration is selected to study and to analyze this phenomenon. Indeed, this phenomenon is more significant when four BLAC motors are operating simultaneously.  
Using a multicore dSPACE platform where four control loops are implemented, all the BLAC motors of the designed actuator are active. In addition, a deviation of offset (+) is created manually on motor lane 4 in order to observe this phenomenon in real time.  
Figure 14: Position response in faulty mode.  
Figure 13: Test bench of the designed actuator. (**1**): dSPACE platform 1006. (**2**): Direct-Drive Actuator (prototype). (**3**): Load torque machine. (**4**): Power electronic stages. (**5**): Current sensors. (**6**): Position sensors. (**7**): Torque sensor. (**8**): Temperature sensor.  
Based on the position command given by Figure 14, a real-time co-simulation is carried out between Simulink and AMESim in faulty operation mode for 80 s. Among the faults listed above, a variable fault offset on position feedback information is created at t = 3 s on motor lane 4. Figure 14 shows that the desired position of the designed actuator is achieved with good performance, namely response time, minimum overshoot, and steady-state error of zero. Even though a fault appeared on one or two motor lanes, the desired position was achieved with the same performance listed in the specifications of the application.  
In addition, it can be noticed that when the offset is changing between positive and negative values (%), the current command (Iq_command) of the faulty motor lane (Motor Lane 4) given by Figure 15 is changing too. Indeed, as shown in Figure 9, the multiloop control generates the velocity command from the position error, while the velocity controller generates the Iq command from the measured velocity and the velocity command. Its value depends indirectly on the angular position error, which is computed from the measured position associated with the created offset and the angular position command. If the position offset is positive, the position error is negative; Iq current will be also negative. Similarly, if the offset is negative, the position error is positive; Iq current will be positive. However, the change of Iq has a direct effect on the torque. This is due to the direct relationship between both [9]. For the other motor lanes, the offset does not affect Iq current responses of the other motor lanes. This is due to the independent control loops of all the motor lanes and their optimized PID parameters.  
As the torques are images of Iq currents, Figure 16 shows that the torque response of the faulty motor lane is not following the same response as the other motor lanes. Indeed, this motor lane challenges the other lanes to move the shaft to a different position even though the other motor lanes  
Figure 15: Iq current responses of the four motor lanes in faulty operation mode.  
are following the same angular position command. The sign of the motor torque of the faulted motor lane (cyan on Figure 16) depends on the sign of the offset (**+**) as shown in Figure 14. However, the other motor lanes handle this problem by creating more torques in order to maintain the position shaft to the position command whatever the motor torque of the faulted motor. This behavior induces more heat losses by a factor of three for the healthy lanes and a factor of six for the faulted lane [2].  
In order to eliminate the force fight phenomenon between the lanes of the designed actuator and to keep good performance, three methods were suggested previously. Among them, the master/slave approach is selected here because of its easy implementation compared to the others.