Servo Pan-Tilt
Dual-axis orientation and pointing for desktop and remote-controlled portable applications.
 
 
 Servo Pan-Tilt
 
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The Servo Pan-Tilt director is designed to perform two-axis orientation or pointing of small payloads, such as miniature cameras, lasers and feedback sensors, in the desktop and workbench environment, as well as from shelves, window sills and small mobile robot platforms. Control input and monitoring is by a front panel interface and by serial data link.

Manual position and velocity adjustments may be controlled and monitored via the push-buttons and illuminated LCD display on the front panel, or internally loaded paths and patterns may simply be triggered (and monitored).

A standard ASCII terminal interface, such as HyperTerminal, may be used for panel-oriented operations at a distance, similar to the above. The serial interface will also accept direct position command data in real-time, for playback of path files or variable control by external computer.

Various real-time display screens are accessible, showing current positions, velocities and assorted counters and states, covering each axis. The primary variables for user input and monitoring are the angular positions, in decimal degrees, for each of the pan and tilt axes, measured with respect to a user-adjustable zero for each axis. More precise measurement is also typically provided for reference, in many display screens, in angular units that are internal to the controller.*
An R/C servo is by nature a self-contained, closed-loop, error-feedback controlled position system, which lends itself to fairly direct input of a position command signal. The position (Loc) for each axis of the servo pan-tilt can be incremented (at varying step sizes) using the front panel interface.

In addition, a velocity input control mode has been added for each axis, to allow operation similar to that found on larger surveillance platforms. The user may set a fixed target speed at which the axes will rotate when panel buttons are pushed when in this mode. (A trapezoidal velocity ramp profile is implemented; acceleration is also user adjustable.)
A built-in sinusoidal motion generator allows for easy adjustment and control of reciprocating sinusoidal position on each axis, with independently adjustable frequency and amplitude, for each axis. The start, stop and phase of the axes are independent, which generally allows for complex motions.

The axes can also be linked to the same sinusoid path vector (common frequency and amplitude), with their phase relationship fixed (but adjustable), to generate more regular two-dimensional straight or ellipsoidal path motion. For instance a phase difference of 90 or 270 degrees allows a mounted laser to trace a circle on a target in either a clockwise or counter-clockwise direction.
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Various system and function parameters can be examined and adjusted.
External manual control may be performed using the RS232 serial interface (9600 bps) and a remote computer running a standard terminal emulator program. The front panel buttons, LCD-display data and functions are essentially duplicated (and extended in some cases) in the remote terminal session.
External control by a remote computer or microcontroller is possible using an alphanumeric command string and the serial interface noted above. A simultaneous command for both axes consists of a 10-character (ascii) string, where each 4-digit axis location is in units of the internal controller (~0.033°/step). The locations in the three commands shown are separated by about 1° in each axis.
Architectural Specifications
 
• Range
      Pan: 190°
     Tilt: 105°
 
• Precision
      Control: 0.1° manual (0.033° w/computer)
     Deadband: 0.8° (0.1° w/optional servos)
 
• Bandwidth (-3dB, unloaded)
      Pan: 4 Hz
     Tilt: 5 Hz
 
• Sample Rate
      Internal Cycle: 195 Hz
         Servo Pulse: 50 Hz
       External Ctrl: 25 Hz (2-axis pair)
 
• Built-in Sinusoid Generator (each axis)
     Frequency Range: 0.01 Hz – 9.00 Hz
     Amplitude Range: 0.1° – 95.0°
 
• Panel: 16x2 backlit LCD, 4 buttons
 
• External Interface
       Serial Port: RS232 / 9600 bps / ASCII
       Manual Ctrl: Terminal Interface
     Computer Ctrl: 10-char string
 
• Power supply: 1.25 A @ 6 Vdc (AC adapter)
Additional Notes
* The two drive actuators are standard analog R/C servo motors, each of which is controlled according to a well-established pulse-width modulation control signal specification. A typical position servo range of at least about 180 degrees can be the spanned by varying the control input pulse-width over a range of say about 2000 microseconds, or say from 500 to 2500 microseconds, with a central position obtained with input in the vicinity of 1500 microseconds. (A fairly standard fixed pulse interval of 20 ms is used in this system.)

The internal base integer unit of each control variable in this pan-tilt system corresponds precisely to a 1/3 microsecond pulse width, such that dividing a displayed internal position value by 3 will yield the signal pulse-width, in microseconds, currently output by the internal microcontroller to an axis servo. This level of control signal precision was selected for mathematical precision in repeated operations, and to exceed performance requirements if very high precision servos are installed. (It is noted that for many R/C servos, the response dead band to control input variation is typically specified to be say about 8 microseconds, but it may be as low as about 1 microsecond for very high precision models.)