THE ACTUATOR
One of the most important goals of the actuator design was to achieve an
highly reliable device: hundreds of them have to be mounted on the antenna,
in a location difficult to reach, so failures could result in a no easy and
long time spending replacement, keeping the mirror in a wrong shape in the
meantime. Moreover, device failures could reduce the antenna observing
efficiency. For reliability reasons we decided to implement the most simple
device, for instance without using limit switches (the step motor is
current controlled so the mechanism is intrinsecally safe). No sensor
position is needed (encoder) because the motor position is stored in the
controller memory and a simple mechanism to check if motor loses steps
assure the reliability of the position.
The actuator network has been organized so that it is composed of
subgroups of 5 actuators, a failure of one of them will block five panels,
at most, corresponding to a negligible amount of area not actuated: in this
way the maintenance can be postponed after the end of the observation.
The solution adopted avoids the RFI generation by putting the electronics
near the motor and using multiple shielded cables: this avoids long cables
where a chopping signal could irradiate interferences.
The mechanics part include a reduction gear connected to a pre-loaded
ball screw. A linear slide, with a slithering bearing, is mounted in
order to avoid radial loads on the ball screw. An anchor plate screwed
to the slide accomodates four bolts for connecting the actuator to the
four corners panels.
(pict)
The worm gear axis is connected to a step motor controlled by a commercial
microncontroller/driver. The controller communicates via a RS485 serial link.
The electronic board provides the power supplies, rectifying a 48Vac,
the current motor regulation with respect to the temperature
(the friction changes due to the viscosity of the lubricant), and a
simple mechanics able to check if the motor loses steps.
(pict)
The specification reported in the table originates from tests performed,
such that loading test, temperature tests, environmental, EMC, communication.
(pict)
Actuator dimensions are forced by the available space on the antenna
structure. Accuracy is partly due to the reduction gear backlash and partly
to the non linearity of the mechanical system. The survival load has to be
intended as relative to the survival wind of the antenna and not as a
destructive load. Operating load is relative to a wind value equal to 80km/h.
Temperature range is conservative, in order to take into account that the
controller is a 0÷70 °C commercial device: tha actuators were
tested in a larger range.
