LM628, LM629

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SNVS781C – JUNE 1999 – REVISED MARCH 2013

THEORY OF OPERATION

INTRODUCTION

The typical system block diagram (See Figure 1) illustrates a servo system built using the LM628. The host

processor communicates with the LM628 through an I/O port to facilitate programming a trapezoidal velocity

profile and a digital compensation filter. The DAC output interfaces to an external digital-to-analog converter to

produce the signal that is power amplified and applied to the motor. An incremental encoder provides feedback

for closing the position servo loop. The trapezoidal velocity profile generator calculates the required trajectory for

either position or velocity mode of operation. In operation, the LM628 subtracts the actual position (feedback

position) from the desired position (profile generator position), and the resulting position error is processed by the

digital filter to drive the motor to the desired position. Table 1 provides a brief summary of specifications offered

by the LM628/LM629:

POSITION FEEDBACK INTERFACE

The LM628 interfaces to a motor via an incremental encoder. Three inputs are provided: two quadrature signal

inputs, and an index pulse input. The quadrature signals are used to keep track of the absolute position of the

motor. Each time a logic transition occurs at one of the quadrature inputs, the LM628 internal position register is

incremented or decremented accordingly. This provides four times the resolution over the number of lines

provided by the encoder. See Figure 10. Each of the encoder signal inputs is synchronized with the LM628 clock.

The optional index pulse output provided by some encoders assumes the logic-low state once per revolution. If

the LM628 is so programmed by the user, it will record the absolute motor position in a dedicated register (the

index register) at the time when all three encoder inputs are logic low.

If the encoder does not provide an index output, the LM628 index input can also be used to record the home

position of the motor. In this case, typically, the motor will close a switch which is arranged to cause a logic-low

level at the index input, and the LM628 will record motor position in the index register and alert (interrupt) the

host processor. Permanently grounding the index input will cause the LM628 to malfunction.

Table 1. System Specifications Summary

Position Range

−1,073,741,824 to 1,073,741,823 counts

Velocity Range

0 to 1,073,741,823/216 counts/sample; ie, 0 to 16,383 counts/sample, with a resolution of 1/216 counts/sample

Acceleration Range

0 to 1,073,741,823/216 counts/sample/sample; ie, 0 to 16,383 counts/sample/sample, with a resolution of 1/216

counts/sample/sample

Motor Drive Output

LM628: 8-bit parallel output to DAC, or 12-bit multiplexed output to DAC

LM629: 8-bit PWM sign/magnitude signals

Operating Modes

Position and Velocity

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/fCLK to (2048 * 256)/fCLK in steps of 2048/fCLK (256 to 65,536 μs for

an 8.0 MHz clock).

Proportional and Integral: 2048/fCLK

Figure 10. Quadrature Encoder Signals

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Product Folder Links: LM628 LM629