Page 286 - Programming Microcontrollers in C

Timer Operations 271

prior to the first TOI, plus 65535 times the number of TOIs that
have occurred, plus the time indicated in the input capture register.
With a scheme of this nature, there is essentially no limit to the length
of time that can be measured accurately. The resolution of this
measurement is the time increment of the prescaler output.
A velocity servo is a particularly difficult application. Remember
that velocity or motor speed is the derivative of position. Any derivative
operation is prone to noise, and the successful servo must be carefully
put together. For example, the equation calculated above to determine
the feedback term is essential to the proper operation of the system. An
interesting challenge is to try to duplicate the operation of the system
described here with a normal gain type feedback. Even though the program
seems to be messy, this approach will work, and most other approaches
give you a wildly hunting or oscillating system. The calculation of the
proper feedback value will avoid many of these problems.
Another problem area will be found. If the above FOREVER loop is
allowed to run without any time control, the computer will be calculating
a new feedback value many times before the effect of an earlier
calculation will be seen in the system performance. This operation again
causes the system to become unstable, and usually the system never
finds the final value but hunts over some wide range around it. This
operation can be corrected by slowing the rate at which feedback
corrections are calculated. In particular, several rates have been tested,
and it was found that a stable, accurate system would be obtained with
a new feedback value calculated each quarter of a second. This time
control is shown in the listing below. The main application program is
broken into two parts: the first part is the code that will be executed
whenever the motor speed is changed and the second part is the code
that will alter the PWM_count which is the feedback portion of the
program. The first portion is executed under the control of an if statement

if(old_motor_speed!=motor_speed)
There is no means to change the motor speed in this program, and
this change will be introduced in the next section. Therefore, it is
expected that the portion of the code controlled by this if statement
will execute the first time the program is executed, and it should never
be executed again until the system is reset. The second if statement

if(tick)