1979-02.pdf

FEBRUARY 1979

HEWLETTPACKAKD JOURNAL

A High-Quality Digital X-Y Plotter Designed

for Reliability, Flexibility and Low Cost

A new linear step motor design cuts costs and improves

reliability without sacrificing line quality. Microprocessors

and plug-in personality modules provide the flexibility.

by John A. Fenoglio, Bessie W. C. Chin, and Terry R. Cobb

MECHANICAL ADVANCES IN X-Y PLOTTERS

have taken us from dc motors and slide-wire

feedback to the more reliable open-loop rotary step

motor drive systems. With the new HP Model 7225A

Plotter, we are now entering the third generation of

mechanical development. The design objectives for

this new plotter emphasized the need of many users

for a low-cost plotting solution. However, its lower

cost had to be achieved while improving reliability

and maintaining line quality.

The 7225A (Fig. 1) is a dual-microprocessor plotter

that produces high-quality plots on A4-size (SVixll

in) paper. It is available with several different inter

faces to match most customers' applications. Fig. 2

shows examples of its plotting abilities.

connectors and assembly time. To further reduce as

sembly and test time, the plotter is designed to oper

ate without any electrical adjustments.

Cover: Model 7225A Plot

ter is a reliable, low-cost digi

tal X-Y plotter that is expected

to make computer graphics

practical in many applica

tions where graphics capa

bility has been considered

too expensive. Plug-in per

sonality modules adapt

a single mainframe to different input/output

Low Cost

Materials and labor are the primary factors that

determine the cost of a product. Labor is especially

critical because the total labor cost in a product is the

direct labor cost times a rather large multiplier that

includes manufacturing overhead. One way to reduce

the labor content of a product is to invest in tooling.

However, unless there is a prodigious market for a

product, little tooling can be justified because of its

high cost. To circumvent this, we decided to

minimize the number of mechanical assemblies. The

simplicity of linear step motor technology took us a

giant step in that direction. Also helpful was the con

tinuing drop in the cost of electronic components

brought about by technological advances. This al

lowed us to place as much burden on the electronics

as possible to help simplify the mechanics. With this

approach we were able to reduce assembly and test

times for the 7225A by a factor of six compared to

other similar products.

The main emphasis in the electronics development

was to keep the parts count down. This is important

because of the time required to load parts onto printed

circuit boards. The microprocessor selected provides

the RAM, ROM, and latched I/O ports all in a single

40-pin package. The plotter's electronic circuitry is

all located on a single board. This reduces the cost of

Printed in U SA.

Hewlett-Packard Company 1979

Fig. 1. Model 7 225 A Plotter pro

duces high-quality plots on A4-

size (8Vix.11 in) paper. Its linear

step motor design delivers twice

the reliability of older plotters at

about half the cost. Personality

modules adapt the plotter to

different interfaces.

Typically high power and high cost go together, so

another important technique for reducing cost is to

reduce the power requirements. The motor drivers

satisfy this need. They are class D switching

amplifiers that offer substantial power reduction

compared with linear amplifiers. The plotter's power

supply is extremely simple, using a transformer with

a single secondary winding.

quired to repair the item. Repair time can be divided

into the time needed to diagnose the problem and that

needed to correct it. The effort put into reducing as

sembly time directly reduces replacement time, and

Reliability

A true low-cost plotting solution requires not only

that the purchase price be low, but also that the con

tinuing cost of ownership be low. This means high

reliability and low repair cost. Fortunately, the items

that lower the manufacturing cost also improve relia

bility and serviceability, since a mechanically simple

mechanism provides inherent reliability. The only

moving parts associated with the X-Y mechanics are

the ball bearings that support the motors. This has

resulted in a demonstrated life expectancy greatly

exceeding that of previous plotters.

The electronic objective of reduced parts count also

contributes to a lower failure rate. The concern for

power reduction results in lower internal tempera

tures, which help prolong the life of electronic com

ponents. These factors, coupled with a very conserva

tive design approach, produce higher reliability. The

annual failure rate of the 7225A has been reduced by a

factor of two compared to the product it replaces.

Even so, all things must someday fail, and to reduce

repair cost, it is important to minimize the time re

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Fig. 2. Model 7225A draws continuous lines at 250 mm/s

in each axis and draws text at up to three characters per

second. 0.032-mm microsteps result in smooth, visually

continuous lines.

several aids are provided to help reduce diagnostic

time. An external confidence test switch initiates a

test that exercises most of the circuitry and mechanics

to aid in isolating problems. Further fault isolation

assistance is provided by designing the plotter to

support signature analysis troubleshooting tech

niques.

Flexibility

The requirements for a plotter vary considerably

depending on the customer's needs. In the past it has

been necessary to provide several different plotters to

meet all the needs. Plotter characteristics that usually

change are the interface hardware, the command syn

tax and language, and software features. The modular

approach of this new generation of plotters satisfies

these requirements with only one mainframe. A mi

croprocessor in the mainframe controls pen move

ment and monitors the front panel. An interfacing

module called the personality module provides the

needed flexibility. The personality module is a mi

croprocessor-based plug-in card that contains the

hardware connectors appropriate to the selected in

terface. It also contains the language interpreter that

defines the instructions used to control the plotter.

Any extra features and capabilities of the plotter, such

as internal character generation and dashed-line

drawing, reside on this plug-in. A customer can select

a very simple parallel interface with only move and

pen up/down capabilities, and then, as needs change,

replace it with a more complex personality module

that supports the HP-IB (Hewlett-Packard Interface

Bus*), interprets HP-GL (Hewlett-Packard Graphics

Language), and contains a full set of graphics fea

tures. This concept also makes it easy to provide fu-

Quality

Price/performance trade-offs can be made during

the design of any product. The quality of the marks

placed on paper by a plotter is its most important

parameter. A true bargain exists when the price can be

lowered without sacrificing line quality.

The 7225A Plotter has several features that help

ensure good lines. First, there are no cumulative er

rors in the linear step motor approach, nor are there

any pulleys, whose diameter and concentricity toler

ances contribute a major source of error. Also elimi

nated are cables and the errors caused by their stretch

ing when the pen carriage assembly accelerates. Line

quality is further enhanced by adding dynamic veloc

ity feedback to the servo system. Custom hybrid ac-

celerometers sense perturbations in motor movement

and supply correcting error signals. A microstep

drive signal technique is employed to divide the basic

1-mm step size of the motor into 32 equal sub-steps.

This produces smooth, continuous lines at all angles

(see Fig. 2).

â€¢IEEE 488, ANSI MC1.1.

External

Controller

Plug-In

Personality

Module

I/O

Hardware

I/O

Processor

Limit Light

Error Light

l/O-Servo Handshake

Status Bit

Â® Accelerometer

(D Pulse-Width Modulated, Switching Motor Drive

(3) Integrator

Â® Sample and Hold

Fig. the in 7225A block diagram. Two microprocessors, one in the mainframe and one in the

personality module, control all functions. Each motor module contains a specially designed

accelerometer that provides motor-movement feedback.

ture interfaces as new products and customers require

them.

cursor move the front panel is continually monitored

once the X and/or Y motors have reached a constant

velocity.

As the servo processor begins the move, the five-bit

X and Y motor step address is incremented or dec

remented using a vector algorithm. The time between

each step follows a fixed profile during acceleration

and deceleration, and is a constant 125 microseconds

during slew. The processor maintains a step count for

branching from acceleration to slew to deceleration to

a complete stop. At the completion of the move and

any associated pen maneuver, the servo processor

returns to its idle loop and waits for new inputs.

Block Diagram

Fig. 3 is a block diagram of the 7225A Plotter. The

basic mechanism consists of two linear step motors.

Each motor module contains a specially designed

hybrid accelerometer that senses motor movements.

The control system integrates the accelerometer sig

nal to provide velocity feedback.

The remainder of this article discusses the func

tions of the two microprocessors and the various per

sonality modules. The motor and platen designs are

described in the article on page 7. The electronic

design is the subject of the article on page 14. The

servo and accelerometer story begins on page 18.

I/O Processor

The processor in the personality module, if re

quired, is called the I/O processor. It translates com

mands from an external controller to the servo pro

cessor. The I/O processor controls the actions of the

servo processor using eight commands (Fig. 4). At the

start of a conversation between the two processors the

I/O processor first indicates its readiness. The servo

processor responds, after completing any previous

operation, with a status byte on the data/command

bus. The status byte indicates the pen state, a change

in scale limits, the digitize state, the chart hold state,

and whether a front-panel reset has occurred. The I/O

processor accepts the status byte, determines whether

the status affects the next command, and outputs the

command to the servo processor.

The commands force the servo processor to execute

various routines, either outputting data, accepting

input data, or returning to its idle loop. With these

commands the I/O processor can instruct the servo

Servo Processor

The processor in the 722 5A mainframe is called the

servo processor. It directly controls the X-axis and

Y-axis motors, the pen, and the chart hold. It responds

to inputs either from the front panel or from the per

sonality module. It also performs general mainframe

housekeeping chores.

The servo processor is a single-chip 3870 with its

own internal program ROM and RAM and four

eight-bit I/O ports. Two of the ports provide a five-bit

step address to the circuits that position the motors.

Another port is used as a data/command bus to the

personality module. This port is shared with the front

panel, and during moves it provides a compensation

byte to the acceleration-deceleration electronics. The

fourth port, along with extra pins on the first two

ports, provides handshake lines to the personality

module, other front-panel inputs and outputs, pen

control, chart load control, and X and Y limit inputs.

A status line from the servo processor informs the

personality module when a front-panel input changes

the mainframe's status.

The front-panel inputs come from pushbuttons that

are pressed by the operator. The operator can move

the pen over the full platen surface at two speeds, go

to or set either lower left or upper right scaling points,

enter a digitized point, turn chart hold on or off for

paper loading, raise or lower the pen, reinitialize the

plotter, or reset the plotter to a power-up condition.

When a position move is requested either by the

front panel or the personality module, the servo pro

cessor calculates the number of steps required for

acceleration, deceleration, and slew. Before the first

step, the maximum velocity for each axis is deter

mined and used to output a compensation byte dur

ing acceleration and deceleration. Once the servo

processor begins execution of the move, all inputs are

ignored with the exception of a cursor move. During a

Command

Servo Processor Action

Return to Idle Loop

Position and Pen

Accept Five Bytes for Position Move and/or

Pen Maneuver

Initialization

Initialize Plotter to Power-Up Condition

Accept Eight Bytes Setting New Scale Limits

Set P1. P2

Turn on Enter Light and Store Present Position as

Digitize Point when Operator Presses

Enter Button

Enable Digitize

Disable Digitize

Exit Digitize Mode

Output Present

Position and P1. P2

Output Four Bytes Reflecting Position and

Eight Bytes of Current Scale Limits

Output Digitize Point Output Five Bytes of Last Stored Digitized Point

and Pen Status

Fig. 4. I/O processor (in personality module) controls the

actions of the servo processor (in mainframe) using eight

commands. Commands from an external controller are trans

lated by the I/O processor into these eight commands.

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