LM628 datasheet(9/24 Pages) NSC

9 / 24 page

Theory of Operation (Continued)

TABLE 1. System Specifications Summary (Continued)

Feedback Device

Incremental Encoder (quadrature signals; support for index pulse)

Control Algorithm

Proportional Integral Derivative (PID) (plus programmable integration limit)

Sample Intervals

Derivative Term: Programmable from 2048/f

CLK to (2048 * 256)/fCLK in steps of 2048/fCLK (256

to 65,536 µs for an 8.0 MHz clock).

Proportional and Integral: 2048/f

CLK

VELOCITY PROFILE (TRAJECTORY) GENERATION

The trapezoidal velocity profile generator computes the de-

sired position of the motor versus time. In the position mode

of operation, the host processor specifies acceleration, maxi-

mum velocity, and final position. The LM628 uses this infor-

mation to affect the move by accelerating as specified until

the maximum velocity is reached or until deceleration must

begin to stop at the specified final position. The deceleration

rate is equal to the acceleration rate. At any time during the

move the maximum velocity and/or the target position may

be changed, and the motor will accelerate or decelerate ac-

cordingly.

Figure 10 illustrates two typical trapezoidal veloc-

ity profiles.

Figure 10(a) shows a simple trapezoid, while Fig-

ure 10(b) is an example of what the trajectory looks like

when velocity and position are changed at different times

during the move.

When operating in the velocity mode, the motor accelerates

to the specified velocity at the specified acceleration rate and

maintains the specified velocity until commanded to stop.

The velocity is maintained by advancing the desired position

at a constant rate. If there are disturbances to the motion

during velocity mode operation, the long-time average veloc-

ity remains constant. If the motor is unable to maintain the

specified velocity (which could be caused by a locked rotor,

for example), the desired position will continue to be in-

creased, resulting in a very large position error. If this condi-

DS009219-11

FIGURE 9. Quadrature Encoder Signals

DS009219-12

FIGURE 10. Typical Velocity Profiles

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