MC33030
http://onsemi.com
10
If V
Pin
3
should continue to rise and become greater than
V
2
, the actuator will have over shot the dead zone range and
cause the motor to run in Direction A until V
Pin
3
is equal to
V
3
. The Drive/Brake behavior for Direction A is identical to
that of B. Overshooting the dead zone range in both
directions can cause the servo system to continuously hunt
or oscillate. Notice that the last motor run−direction is stored
in the direction latch. This information is needed to
determine whether Q or Q Brake is to be enabled when
V
Pin 3
enters the dead zone range. The dashed lines in [8,9]
indicate the resulting waveforms of an overcurrent
condition that has exceeded the programmed time delay.
Notice that both Drive Outputs go into a high impedance
state until V
Pin
2
is readjusted so that V
Pin
3
enters or crosses
through the dead zone [7, 4].
The inputs of the Error Amp and Window Detector can be
susceptible to the noise created by the brushes of the DC
motor and cause the servo to hunt. Therefore, each of these
inputs are provided with an internal series resistor and are
pinned out for an external bypass capacitor. It has been
found that placing a capacitor with short leads directly
across the brushes will significantly reduce noise problems.
Good quality RF bypass capacitors in the range of 0.001 to
0.1 mF may be required. Many of the more economical
motors will generate significant levels of RF energy over a
spectrum that extends from DC to beyond 200 MHz. The
capacitance value and method of noise filtering must be
determined on a system by system basis.
Thus far, the operating description has been limited to
servo systems in which the motor mechanically drives a
potentiometer for position sensing. Figures 19, 20, 27, and
31 show examples that use light, magnetic flux, temperature,
and pressure as a means to drive the feedback element.
Figures 21, 22 and 23 are examples of two position, open
loop servo systems. In these systems, the motor runs the
actuator to each end of its travel limit where the Overcurrent
Monitor detects a locked rotor condition and shuts down the
drive. Figures 32 and 33 show two possible methods of using
the MC33030 as a switching motor controller. In each
example a fixed reference voltage is applied to Pin 2. This
causes V
pin
3
to be less than V
4
and Drive Output A, Pin 14,
to be in a low state saturating the TIP42 transistor. In
Figure 32, the motor drives a tachometer that generates an
ac voltage proportional to RPM. This voltage is rectified,
filtered, divided down by the speed set potentiometer, and
applied to Pin 8. The motor will accelerate until V
Pin
3
is
equal to V
1
at which time Pin 14 will go to a high state and
terminate the motor drive. The motor will now coast until
V
Pin
3
is less than V
4
where upon drive is then reapplied. The
system operation of Figure 31 is identical to that of
Figure 32 except the signal at Pin 3 is an amplified average
of the motors drive and back EMF voltages. Both systems
exhibit excellent control of RPM with variations of V
CC
;
however, Figure 32 has somewhat better torque
characteristics at low RPM.
