The motors of each actuator have to be driven individually, as they have to position the fiber on individual targets. For this purpose, a basic electronic board has been developed. This board will be placed inside the envelope of each robot, minimizing the area consumed by each actuator in the focal plate.
Small scale electronics
On one hand, the components of the electronics have been chosen following ultra-low power consumption requirements. This characteristic allows to minimize the thermal noise in the focal plane, reducing overall errors in the telescope images. On the other hand, all components have to respect the volume constrains required by the actuator dimensions. The available volume for the electronics next to the two Ø 4mm motors is 1x6mm for the PCB and 1x4mm for the components as shown in the figure below:
Open Loop Control
Closed loop control for high reliability
The figure below shows the overall electronics block diagram. The main component of the system is a 32-bit microcontroller, in charge of receiving commands from the top communication nodes, processing them and managing information flowing into the actuator. This component will receive data through a fast I2C bus up to 100MHz, and will control with high accuracy the position of each motor through a mosfet-based bridge system. The position of each motor is acessible to the microcontroller thanks to a magnetic encoder system embedded in each motor. This information will allow the microprocessor to drive the motors in a closed loop control scheme, performing time synchronised trajectories demanded by the Master system. On the other hand, this information will allow the system to detect unexpected behavior in case of failure (motor defect, arrive to a physical zero, collision between actuators, etc.).
To optimize the consumption of the electronics, the microcontroller can run in two modes, Active and Sleep. The first one is used to process data, to monitor constants and to position the robot, whereas the second one is used in waiting command periods, reducing consumption to a minimum when not required. This feature is managed by an interrupt line that allows the Master software manipulate directly the consumption state of each actuator.
Finally, the motor control algorithms involved in this electronics calculate the torque that the actuator has to use to move the fiber. This feature allows to detect mechanic failure of the actuator or blocking of the movement by a hardstop or collision with a neighbour. In order to detect this kind of error, the hall sensor encoder system will be used to check the existence of real movements, and an error is detected if the calculated torque is outside a window around the expected torque for a given trajectory.