
System Descriptions
SORVALL
Centrifuges
3-6
For closed-loop control, one approach is to filter the MC33039's
output with a low pass filter to generate a voltage proportional to
motor speed and feed the resulting signal into the inverting input of
the MC33035's comparator. A signal proportional to desired motor
speed drives the non-inverting input, and the ratio of the input and
feedback resistors R3 and R4 control gain. In this design, low pass
filtering and generating the error signal are combined by using
feedback capacitor, C3.
Ideally, the integrator/error amplifier should produce a ripple free
output even at low motor speeds. To do so at very low speeds
reduces system response time, however. Component values must
be adjusted according to the rotors' load, inertia and friction. In this
particular application the values 2 M ohm and 0.01
µ
F give good
dynamic response and stability.
When motor speed is less than the desired speed, the MC33035
extends the output pulse width to the drive transistors. When
motor speed is greater than the desired speed, the duty cycle de-
creases. However, if the input signal abruptly demands a much
lower speed, the duty cycle could fall to zero and the motor would
coast to desired low speed. Therefore, since the MC33035 has no
provision to dynamically brake the motor and thus control rapid
deceleration, it is best suited for applications which have a large
frictional load or those that do not require a controlled, abrupt
deceleration.
d. Braking
Dynamic braking is used. The three high-side motor drive transis-
tors are turned off and the three low-side drive transistors are
turned on, in sequence. This circulates the back Electro Magnetic
Force (EMF) generated currents through the motor winding for
dissipation of stored energy.
A logic high at pin 2 of the motor drive control connector calls for
dynamic braking.
3-5. Speed Control
Theory of Operation
Speed control is accomplished by a 10k ohm potentiometer used in
conjunction with the Motorola brushless motor control chip
integrated circuit set (MC33035 and MC33039). The wiper of the
potentiometer is the control input to MC33035. Lower speed is
selected when the voltage on the wiper is decreased (by turning
counterclockwise) and higher speed is selected when voltage on the
wiper is increased (by turning clockwise).
Содержание TC6
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Страница 103: ...Illustrated Parts SORVALL Centrifuges 8 4 Figure 8 2 TC6 Chassis Assembly ...
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