Microchip Technology MCP6 series Manual de usuario descargar pdf (Pagina 11)

 2004 Microchip Technology Inc. DS00900A-page 11

AN900

PID CONTROL FIRMWARE

PID is a well-known, commonly used method of feed-

back control. As seen in the PID algorithm in

Equation 2, PID generates a control signal by multiply-

ing the error, the integral of the error and the derivative

of the error by individual gains and then summing the

results. The proportional term generates a corrective

signal in proportion to the error. The integral term gen-

erates a corrective signal proportional to summation of

the error over time. The derivative term generates a

corrective signal in proportion to the rate of change of

the error. In velocity control applications, the derivative

gain is often set to zero, as PI control is usually

sufficient for achieving well-tuned speed control.

To implement V/f control with velocity feedback, the tar-

get speed, actual speed and speed error are all calcu-

lated as shown in Equation 2. The speed error is

passed to the PID algorithm. Integral error is calculated

in the PID routine by accumulating the speed error over

time. Derivative error is calculated by subtracting the

last error value from the present error value. Since the

routine is called at fixed time intervals, the difference in

the two error values is proportional to the rate of

change of error. In this application, the PWM period

interrupt rate is used to determine the update rate of

the PID calculation.

The PID functions used in this application note are

described in AN937, “Implementing a PID Controller

Using a PIC18 MCU”.

CLOSED-LOOP SLIP CONTROL

In many applications, it is desirable to control slip in

order to optimize for torque, efficiency or power factor

depending upon changing requirements. Figure 8

shows how torque, power factor and efficiency may

vary with the degree of slip for a typical motor. By vary-

ing the amount of desired slip, the motor performance

can be optimized for any of these three attributes. For

example, torque may be maximized by allowing a

higher degree of slip; efficiency optimized by allowing a

lesser degree.

To control slip, the actual motor speed is compared

against the drive frequency to determine the present

slip frequency. The slip frequency is compared to the

desired slip frequency to produce a slip frequency

error. Drive frequency and amplitude are modified in

order to minimize the slip frequency error.

Figure 9 shows how a slip control could be imple-

mented. Identical hardware is used as in V/f control

with velocity feedback. Only the algorithm is modified.

FIGURE 8: TORQUE, POWER FACTOR AND EFFICIENCY VERSUS SLIP

Torque (T)

Slip

00.2

0.4 0.6

0.8 1

Power Factor (PF)

Efficiency (n)

Rated

Slip

Slip for

max n

Slip for

max PF

Slip for

max T

Torque

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 23 24

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