Automatically acquiring and maintaining the line of sight of a video camera or focal plane sensor is often required in various aerospace, defense, and security system applications. One way to mechanize such a system is to reflect the field of view through two independently-controlled mirrors that each rotate in axes orthogonal to one another. The object of the control system is to acquire the target, and by controlling rotation of each mirror, move the line of sight coincident with the target angle. This places the virtual image of the target in the center of the focal plane. Once the image of the target is acquired on the focal plane, an error in azimuth and elevation can be determined by a variety of image processing techniques, such as contrasting, differencing, and area parameter calculations.
For this simulation, such a mechanism is assumed, with a pipeline image processor providing direct angular azimuth and elevation measurements. The following design decisions are also assumed:
Motor type: Permanent magnet DC synchronous motor with Hall sensors for commutation sensing and control.
|
Motor parameter |
Value |
Units |
|
Operating voltage |
28 |
Volts |
|
Magnetizing inductance |
0.0009 |
Henries |
|
Stator inductance (per phase) |
0.001 |
Henries |
|
Stator resistance (per phase) |
0.5 |
Ohms |
|
Torque constant |
0.1035 |
Nm/A |
|
Number of poles |
2 |
|
|
Rotor moment of inertia |
8.5 E-06 |
kg-m2 |
|
Rotor shaft viscous damping factor |
5.695 E-06 |
kg m2/s |
For the simulation, a PWM Brushless Servo Amplifier block is used with a base frequency of the PWM at 9000 Hz, along with a Hall Sensor block for commutation.
Precision current sense resistors produce voltage that is fed into a processor. An encoder provides motor shaft position and velocity. Encoder angle measurement and phase current measurements are used to obtain direct and quadrature current estimates through Clarke and Park transforms. Current and speed loops are used to set stiff inner loop performance.
Mechanical Load: Precision λ/4 flat oval mirrors mounted on a gear reducer shaft with rotation center coincident with reflecting surface represent the main load moment of inertia. A torsional spring with preload tension is used to help minimize backlash hysteresis. An optical encoder is provided with 16000 lines to measure mirror angle. PI compensation is used for controlling line of sight. Load parameters are:
Gear reduction 20:1
Backlash 0.0005 radians
Load moment of inertia 0.001 kg – m2
Load viscous damping 0.01 kg – m2/s
Load spring constant 0.01 N-m/rad
Load spring preload 0.1 N-m
Pipeline Image Processor: Provides 60 Hz frame rate acquisition of target from focal plane array. Pixel resolution is sufficiently higher than expected control requirement of less than ± 3 degrees between target angle and line of sight in both axes. Hierarchical classification and size discrimination of blobs with subsequent calculation of the target centroid determine target position.