More on ISRO's TVC actuators :
Details on Linear Electro-Mechanical Actuator for TVC
Electro-mechanical actuators (EMAs) are finding increased use in the field of Thrust Vector Control (TVC)of launch vehicles. In earlier days, electro-hydraulic actuators were used especially in the lower stages of the launch vehicles where there is a requirement for large actuation forces. However, electro-mechanical actuators are being scrutinized for this purpose in an effort to provide lighter, cleaner and more reliant control actuators.
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The power of electro mechanical actuators was limited by the ability of drive electronics to handle the large electrical power, availability of high discharge light weight batteries and non-availability of high torque motors. The recent developments in Lithium ion batteries with high energy density have made usage of high current / power possible in launch vehicles.
SYSTEM CONFIGURATION
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Engine will have one actuator for one plane (pitch or yaw) gimballing and will have two actuators for two plane gimballing (pitch and yaw). Two engines with two actuators mounted on each engine will provide three plane gimbal controls (pitch, yaw and roll).
The following are the specifications that have been used as design input.
- Max angular deflection, θ : 6 deg
- 10% system bandwidth, f : 4 Hz
- Engine Inertia, Ie : 4000 kg-m 2
- Control arm length, L : 1100 mm
ACTUATOR CONFIGURATION
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The actuator configuration consists of a housed 3 phase quadruplex brushless DC (BLDC) torque motor driving a roller screw through spur gear based compound gear train. The rotational motion of electrical motor/gear is transformed in the linear motion by constraining the rotational motion of the roller screw nut. The linear motion of roller screw nut is extended to the engine by piston which is rigidly connected to roller screw nut.
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A LVDT (Linear Variable Differential Transformer) sensor, with probe sliding inside the roller screw and being rigidly connected to the piston which follows the motion is used as the linear position feedback sensor to close the position control loop.
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Lubrication is provided to the gears and other moving elements by grease plating the components before assembly. Krytox grease based on perfluoropolyether oils is used since they can survive continuous high temperatures up to 260oC and provide exceptionally long life time. The connectors used for electrical connectivity of the actuator to the drive electronics belongs to high quality MIL-38999 III series.
BATTERY CONFIGURATION
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Batteries are the power source for electro mechanical actuators. Lithium ion batteries have been selected for use in this configuration mainly due to their ability to handle large currents and higher energy density.
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Each coil of the BLDC motor can demand a maximum of 25 A, but for a short duration only. After taking into account the power demand of an actuator undergoing a typical flight duty cycle, the configurations that can be used were found to be either two 80 AH batteries or four 40AH batteries. Four 40 AH batteries were selected and connected in such a way that each battery is powering one coil of the motor. Hence, in case of a battery failure, each motor will still have three healthy coils which can cater to the supply of adequate control force and ensure a successful mission.
DESIGN :
Source :
https://web.archive.org/web/2019052...0af0/0cd520371bb5be95d60cda797b46a460eab3.pdf
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