Data Sheet

For example, a rolling robot application may use 64 units, or “ticks” per full circle, mimicking the behavior

or a 32-spoke wheel with an optical 1-bit binary encoder. A fixed-position application may use 360 units,

to move and hold the servo to a certain angle.

The following formula can be used to correlate the feedback signal’s duty cycle to angular position in

your chosen units per full circle.

Duty Cycle = 100% x (tHigh / tCycle). Duty Cycle Min = 2.9%. Duty Cycle Max= 97.1%.

(Duty Cycle - Duty Cycle Min) x units full circle

Angular position in units full circle = ______________________________________________

Duty Cycle Max - Duty Cycle Min + 1

The example code discussed in the Low-Level Programming section below demonstrates angular position

control. It uses this formula along with a scale factor to convert floating-point percentages to integers. It

also includes code to accommodate rotation that crosses the origin, and to compensate for starting

positions other than the origin.

Programming Resources

C example code for low-level position control is available for download, and discussed in more detail in

the next section. It is written for the Propeller microcontroller but should provide an illustration of

programming strategies that can be adapted to other controllers.

Propeller C resources under development include two user-friendly libraries for higher-level control,

servo360 and abdrive360. Both of these libraries support speed-controlled continuous rotation as well as

angular position applications.

The abdrive360 library uses the same functions as the original abdrive library, so example programs that

run on an ActivityBot robot using abdrive will also run using these Feedback 360° servos.

The servo360 library will be similar to the servo library, with enhancements to utilize the Feedback 360°

servo’s Hall-effect sensor output.

Blocks that use these libraries are also in the works for the BlocklyProp graphical programming system, to

be released in Fall 2017.

Low-Level Position Control Programming

This section explains excerpts from this program: Feedback360 Angle Control [Low Level Example].side

This example Propeller C program uses two of the Propeller microcontroller’s cogs (processors), one for

angle detection and another for feedback control. It can all be done in the same loop, but it makes the

code a little easier to digest in separate processors. These two lines launch the feedback and control

functions into other cogs. Control here goes through the servo library, with valid speed values ranging

from -220 (full speed clockwise) to -20...20 (stop) to 220 (full speed counterclockwise).

cog_run(feedback360, 128);

cog_run(control360, 128);

servo_speed(pinControl, -40);