AD2S90

REV. D

–7–

VELOCITY OUTPUT

The analog velocity output VEL is scaled to produce 150 rps/V

dc

± 15%. The sense is positive V dc for increasing angular

rotation. VEL can drive a maximum load combination of

10 k

Ω and 30 pF. The internal velocity scaling is fixed.

POSITION CONTROL

The rotor movement of dc or ac motors used for servo control is

monitored at all times. Feedback transducers used for this pur-

pose detect either relative position in the case of an incremental

encoder or absolute position and velocity using a resolver. An

incremental encoder only measures change in position not

actual position.

Closed Loop Control Systems

The primary demand for a change in position must take into

account the magnitude of that change and the associated accel-

eration and velocity characteristics of the servo system. This is

necessary to avoid “hunting” due to over- or underdamping of

the control employed.

A position loop needs both actual and demand position infor-

mation. Algorithms consisting of proportional, integral and

derivative control (PID) may be implemented to control the

velocity profile.

A simplified position loop is shown in Figure 8.

POSITION

DEMAND

POSITION CONTROLLER

RE-

SOLVER

ACTUAL

POSITION

SERVO

MOTOR

AD2S90

SERVO

AMP

Figure 8. Position Loop

MOTION CONTROL PROCESSES

Advanced VLSI designs mean that silicon system blocks are now

available to achieve high performance motion control in servo

systems.

A digital position control system using the AD2S90 is shown in

Figure 9. In this system the task of determining the acceleration

and velocity characteristics is fulfilled by programming a trap-

ezoidal velocity profile via the I/O port.

As can be seen from Figure 9 encoder position feedback infor-

mation is used. This is a popular format and one which the

AD2S90 emulates thereby facilitating the replacement of encod-

ers with an AD2S90 and a resolver. However, major benefits

can be realized by adopting the resolver principle as opposed to

the incremental technique.

Incremental feedback based systems normally carry out a peri-

odic check between the position demanded by the controller

and the increment position count. This requires software and

hardware comparisons and battery backup in the case of power

failure. If there is a supply failure and the drive system moves,

unless all parts of the system are backed up, a reset to a known

datum point needs to take place. This can be extremely hazard-

ous in many applications. The AD2S90 gets round this problem

by supplying an absolute position serial data stream upon re-

quest, thus removing the need to reset to a known datum.

INCREMENTAL POSITION

+

–

RESOLVER

OPTIONAL

VELOCITY

FEEDBACK

HOST I/O

PORT

TO HOST PROCESSOR

ABSOLUTE

POSITION

HOST

INTERFACE

COMMAND POSITION

SEQUENCER (32-BIT)

POSITION

FEEDBACK

PROCESSOR

(32-BIT)

IN, A, B

AD2S90

DC

MOTOR

DAC

PORT

DIGITAL

PID

FILTER

(16-BIT)

8 – 12

DAC

POWER

AMP

Figure 9. Practical Implementation of the AD2S90

DSP Interfacing

The AD2S90 serial output is ideally suited for interfacing to

DSP configured microprocessors. Figures 10 to 13 illustrate

how to configure the AD2S90 for serial interfacing to the DSP.

ADSP-2105 Interfacing

Figure 10 shows the AD2S90 interfaced to an ADSP-2105. The

on-chip serial port of the ADSP-2105 is used in alternate fram-

ing receive mode with internal framing (internally inverted) and

internal serial clock generation (externally inverted) options

selected. In this mode the ADSP-2105 provides a

CS and a

serial clock to the AD2S90. The serial clock is inverted to pre-

vent timing errors as a result of both the AD2S90 and ADSP-

2105 clock data on the negative edge of SCLK. The first data

bit is void; 12 bits of significant data then follow on each con-

secutive negative edge of the clock. Data is clocked from the

AD2S90 into the data receive register of the ADSP-2105. This

is internally set to 13 bit (12 bits and one “dummy” bit) when

13 bits are received. The serial port automatically generates an

internal processor interrupt. This allows the ADSP-2105 to read

12 significant bits at once and continue processing.

The ADSP-2101, ADSP-2102, ADSP-2111 and 21msp50 can

all interface to the AD2S90 with similar interface circuitry.

SCLK

RFS

DR

SCLK

CS

DATA

ADSP-2105

AD2S90

NOTE:

ADDITIONAL PINS OMITTED FOR CLARITY

Figure 10. ADSP-2105/AD2S90 Serial Interface