Watlow Optimizing Your Process System User Manual
WATLOW
Optimizing Your Process System
with the Series 988 Controller
AN APPLICATION GUIDE
THE WATLOW
FOR
SERIES 988 FAMILY
Watlow Mission Statement
To Become the Preferred Source of
Industrial Heaters, Sensors and
Controls by Totally Satisfying Our
Customers with Superior Products,
Quick Delivery on Specials, as well as
Standards, and Intelligent Sales
Support.
Copyright Watlow Winona, Inc., © 1994, with all rights reserved.
Introduction
Welcome to the Watlow Series 988 application guide: Optimizing Your Process
System with the WATLOW Series 988. This application guide describes all the
features of the Series 988 and how they can be applied to your system. In
addition, this guide will walk you through the process of determining the optimal Series 988 for your application. Once you have purchased the controller, the Series 988 Users Manual will guide you through installation and setup
of the controller.
Start with Chapter One of this book, where “test drives” show the thought
process used for determining the correct Series 988 feature set for a specific
application. Chapter Two consists of a glossary and a question-and-answer
section that provides more details to help choose the right control. Chapters
Three through Five describe the features of the controller with a sample application for each feature. Chapter Six outlines the controllers’ specifications.
Use this chapter to ensure that the controller interfaces correctly with other
system components. Finally, Chapter Seven walks you through the feature
set selection for your specific application. Once you have used the guide a few
times, you will be able to skip to Chapter Seven and use the previous chapters
for reference.
The purpose of this technical guide is to assist you in the system design process. The final responsibility for the system design must remain with the system design engineer.
Watlow manufactures heaters, sensors and controls. We can provide a complete thermal system. For more information concerning the Series 988, contact your local sales representative listed on the back cover. Literature on the
Series 988 includes:
• Series 988/989 Users Manual
• Calibrating Watlow Process Controls
• How to Use Data Communications with the Watlow Series 988/989
or How to Use Data Communications with the Watlow Process Controls
Disclaimer of Warranty
The information presented here is of a general nature. Because of the diversity of conditions and hazards under which control products may be applied
and because of the differences in components and methods of installation,
representation or warranty of any kind, express or implied, is hereby made,
that the applications discussed herein will be effective in any particular application or set of circumstances, or that additional or different precautions will
not be reasonably necessary for a particular application.
no
We will be pleased to consult with you regarding a specific application, upon
request.
Table of Contents
Table of Contents
Watlow Mission
Introduction
Chapter 1 Test Drives 1.1
Controlling an Extruder 1.2
Drying Grain 1.4
Melting Aluminum 1.6
Mixing Urethane 1.8
Chapter 2 Basic Control Strategies and Terms 2.1
Glossary 2.2
Questions and Answers 2.5
Control Strategies 2.8
Chapter 3 General Software Features 3.1
Auto-tune 3.2
Burst fire 3.3
Communications 3.4
Dead band 3.5
Digital event 3.6
Heater current 3.7
Input filter 3.8
Input linearization 3.9
Ramp to set point 3.10
Remote set point 3.11
Retransmit (master/remote) 3.12
Slidewire feedback 3.13
Chapter 4 Enhanced Software Features 4.1
Cascade 4.2
Differential 4.4
Dual PID sets 4.5
Duplex 4.6
Ratio 4.7
Optimizing Your Process System with the WATLOW Series 988
Table of Contents
Chapter 5 Standard Features 5.1
Alarms 5.2
Auto/Manual 5.4
Diagnostics 5.5
Input errors 5.6
Lockout 5.7
Transmitter power supply 5.8
Chapter 6 Specifications 6.1
Input Table 6.2
Output Table 6.3
Dimensions 6.4
Displays and Keys Chart 6.5
Setup Requirements 6.6
User's Manual 6.7
Product Specifications 6.8
Warranty 6.9
Chapter 7 Select the 988 That Fits Your Application 7.1
Overview 7.2
Input 7.3
Output 7.4
Software 7.5
Standard Features 7.6
Hardware 7.7
Review and Optimize 7.8
Application Worksheet 7.10
Series 988 Model Number Worksheet 7.11
Index 8.1
Prompts, parameters and menus 8.4
Watlow Sales Offices back cover
Optimizing Your Process System with the WATLOW Series 988
Test Drives
Chapter One
Test Drives
Controlling an Extruder 1.2
Drying Grain 1.4
Melting Aluminum 1.6
Mixing Urethane 1.8
How to use this chapter:
This chapter describes four actual applications in which processes are optimized
by using Watlow Series 988 controllers.
Optimizing Your Process System with the WATLOW Series 988
1.1
Basic Control Strategies and Terms
Chapter Two
Basic Control Strategies and Terms
Control Strategies 2.2
Questions and Answers 2.5
Glossary 2.8
How to use this chapter:
This chapter explains control terms and
strategies, especially as they apply to the
Watlow Series 988 family of controllers.
It will help you identify issues specific to
your application, and allow you to implement the Series 988 controller in the
most cost effective manner, giving you
optimal control of your specific system.
Many of the decisions for system components are limited by factors not associated with the Series 988. The strategy
presented here is designed to get you
thinking of all the different parameters
associated with your system and how the
Series 988 fits in.
Optimizing Your Process System with the WATLOW Series 988
2.1
Basic Control Strategies and Terms
Control Strategies
Process Control
Process controls are of two basic types: open
loop and closed-loop. An open-loop control
device has no self-correcting feedback information. The closed loop system uses feedback
information from a sensor to adjust the system.
As the process changes, the feedback loop
provides up-to-date status information that the
controlling device uses to make self-correcting
adjustments. The closed-loop control device
provides much better process control.
Control Modes
A variety of control modes offer various degrees
of controllability. The most common modes are
on-off and PID control. The PID control category includes varying degrees of complexity
that provide accurate, stable control under a
variety of conditions.
ON/OFF Control
The operation of the ON/OFF control, as the
name implies, turns the output device full ON
or full OFF. Temperature or process sensitivity
(hysterisis) is designed into the control action
between the ON and OFF switching points to
prevent switching the output device ON and
OFF within a span that is too narrow. Switching repeatedly within such a narrow span will
create a condition of intermittent, rapid switching, known as output “chattering.” The process
is always controlled “about set point,” dictated
by the switching hysterisis of the ON/OFF
control. This form of control action further
dictates that there will always be a certain
amount of overshoot and undershoot. The
degree of overshoot and undershoot will be
dependent on the characteristics of the entire
system.
Time Proportioning
Time-proportioning control provides more
precise control of a process. A time-proportioning control operates in the same way as an ON/
OFF control, when the process is far enough
away from set point to be outside the proportional band. When the process approaches set
point and enters the proportional band, the
output device is switched ON and OFF at the
established cycle time. At the lower limit of the
band, the ON time is considerably greater than
the off time. As the process more closely
approaches set point, the ratio of ON to OFF
time changes: the amount of ON time decreases
as the OFF time increases. This change in
effective power to the load provides a throttlingback effect, resulting in less overshoot.
The ON and OFF cyclic action continues until a
stable relationship is achieved. At that time,
the system will be stabilized such that the
Time Proportioning
Temperature
Set
Point
Proportional
Band
Overshoot
Droop
On - Off
Switching
Sensitivity
Temperature
Set
Point
Time
Profile produced by ON-OFF control
Optimizing Your Process System with the WATLOW Series 9882.2
Time
Profile developed by proportioning control
process is controlled at a point below set point.
The process stabilizes with a resultant droop.
This condition will remain providing there are
no work-load changes in the system.
Integral
If the droop cannot be tolerated, there are ways
to compensate for it. Integral (automatic reset)
is an automatic adjustment to compensate for
Basic Control Strategies and Terms
a droop condition before it exists. An integration function takes place that automatically
compensates for the difference between set
point and the actual process. This integration
automatically drives the process toward set
point. Integration action is prevented until the
process enters the proportional band. If it was
Time Proportioning
With Automatic Reset
Proportional
Temperature
Set
Point
Profile developed by a proportional plus
integral control
Band
Overshoot
Droop
Time
allowed to take place at any point, it would
cause a condition of extreme overshoot. This
function of eliminating the integration outside
of the proportional band is referred to as “antireset windup.”
Derivative
As all of the graphs have illustrated, overshoot
occurs with any of the previous control methods. Overshoot may be hazardous to certain
processes. It is preventable with a control
function known as “derivative” (rate).
Derivative is an anticipatory function in a
process control that measures the rate of
change of the process and forces the control
into a proportioning action on an accelerated
basis to slow that change. This action prevents
a large degree of overshoot on start-up and also
functions to prevent overshoot when system
disturbances would tend to drive the process
up or down. A proportioning control with
integral and derivative (PID) control provides
the type of control required for difficult processes with frequent system disturbances or
applications that need precision control.
PID -Time Proportioning
With Auto Reset & Rate
Proportional
Temperature
Set
Point
Profile developed by a proportional plus
integral plus derivative (PID) control
Band
Reduced Overshoot
Time
Control System Tuning
In this phase of making the system work, we
will focus on the process controller as the
primary component of a closed-loop system
that must be adjusted for optimum performance. These adjustments provide a means to
compensate for system problems. For instance,
when the sensor cannot be placed in the most
desirable location because of physical limitations, a PID controller can compensate for the
sensor’s resulting thermal lag problem.
Tuning Methods
Process controls are tuned manually or automatically. Manual tuning requires manually
setting each of the controller’s operating parameters. Automatic tuning, or auto-tuning, is
accomplished by the 988’s digital, microprocessor-based, electronic circuitry. The controller
uses a program inside its memory that will
calculate the correct setting for each of the
controller parameters. For more information on
auto-tuning with the Series 988 refer to page
3.2.
Manual Tuning
The following steps are generally applicable to
most manually set process controllers. Please
take note of a few precautions:
• Take your time in tuning the control system.
If you do it right, it will work a long time
without further attention.
• Do not change more than one control adjustment at a time, allowing the system to settle
Optimizing Your Process System with the WATLOW Series 988
2.3
Basic Control Strategies and Terms
down to a state of equilibrium before making
another change.
• Remember that the time you need to spend
tuning the electronic controller system is
relative to the precision of control you need.
Proportional Band
The proportional band adjustment is the means
of selecting the response speed (gain) or sensitivity of a proportioned controller to achieve
stability in the system. The proportional band
— whether measured in degrees, percent of
range or other units — must be wider than the
natural oscillations of the system and yet not
wide enough to dampen the system response.
The time proportioning output must be set to
switch faster than the natural oscillation of the
system, sometimes called, “system cycle time.”
The tuning procedure is very simple, if you can
use a recorder to monitor the actual process
variable. If a recorder is not available, observe
the process response and record readings over
a defined time period.
If the system oscillates when the proportional
band is at its narrowest setting (not zero), the
adjustment of the proportional band should be
increased in small increments until the oscillation stops. Because the proportional band has
been tuned (adjusted) to have the controller
seek stability of the system, very often an offset
(droop) from the set point occurs. At this point
in tuning the system, the process variable
should be in a state of equilibrium, but not
right on the desired set point.
Integral (Reset)
The reset adjustment is tuned at this point to
correct for the droop caused by the proportional output. While it does automatically
correct for offset errors, automatic reset has to
be tuned to each system. Each system has its
own characteristic response time (system cycle
time), thus, the auto reset time constant
(repeats per minute) must be tuned to match
the overall system response time.
small increments, allowing the system to settle
down (stabilize) before making additional
changes, until the system just starts to oscillate. Then back the setting off enough to reestablish system equilibrium.
Derivative (Rate)
Rate is the last control parameter adjustment
to be made. Rate’s function is to reduce or
eliminate overshoot (excursions above or below
set point). It has a time base (measured in
minutes) that must be tuned to work with the
overall system response time (system cycle
time). The initial setting for rate should be at
the smallest integer of minutes possible (least
corrective action). Increase the integer in small
increment, then after each adjustment increase
the set point moderately. Observe the approach
of the actual process to set point. If it overshoots, continue to increase the rate integer in
small increments. Then increase the set point
moderately until optimum approach to set
point is achieved. If at some point the system
becomes very sluggish or cannot reach the new
set point at all, too much rate (corrective
action) has been adjusted into the control
system. To reduce this “damping” effect,
decrease the number of minutes of rate.
Recommended Tuning Reference
There are many reference books on the art of
tuning electronic controllers to the systems
they control. If you are not an instrument
technician qualified to tune thermal systems,
we suggest that you become familiar with the
following reference before attempting to tune
your system:
Tuning of Industrial Control Systems
by Armando B. Corripio
Published by the Instrument Society of America
(ISA)
Member $48.00, list $60.00 (approx.).
Phone: (919) 549-8411
Initially auto reset should be set at the lowest
number of repeats per minute (least corrective
action). Increase the repeats per minute in
Optimizing Your Process System with the WATLOW Series 9882.4
Basic Control Strategies and Terms
Questions and Answers
System Diagram
No matter what your application, you must
start with an accurate system diagram. A
blueprint of the system wiring is typically not a
faxable document. When consulting the factory,
it’s helpful to be able to fax a system diagram
similar to the hand drawings shown in the Test
Drives in Chapter One. This diagram should
include all inputs, outputs, the controller, the
load, any alarms and any connections to other
systems or equipment. This diagram should
show the expected signal types and ranges at
each interface point to allow you to properly
order and set up your 988 controller. For the
advanced user of the Series 988 this will be
second nature. For the less experienced operator, putting together a diagram including all
system components will allow a Watlow sales
rep or factory applications engineer to review
the system for correctness.
Inputs
The 988 controller accepts a wide variety of
input signals and ranges, covering most temperature and process applications. Ask yourself
the following questions about your inputs:
What accuracy is required for each input?
(page 6.8)
• RTD’s will provide the most accurate measurement in a temperature-related system.
• The accuracy of a process sensor for flow, level,
pressure, etc. will depend strictly on the sensor
manufacturers specifications.
• The Series 988 measures the input and updates
the output 10 times per second with one input.
With 2 inputs, each input is measured five
times per second.
What is the wire length required to reach
controller? (page 6.8)
• When designing a system using thermocouple
or RTD sensors, if the lead length is excessive,
errors may be introduced into the system. By
using a signal transmitter to convert the
thermocouple or RTD signal to a process signal
the errors can be avoided.
RTD sensor. However, it almost never pays to
spend less on a sensor, which is arguably the
most important part of the thermal system.
• Process sensors can vary widely in cost. Other
factors, such as accuracy, response time and
durability, must be weighed.
Will this be a grounded or ungrounded application? (page 5.8)
• Although inputs are electrically isolated from
outputs in the 988, you must determine if
other sources of ground loops in the system
may affect the sensor. This can occur, in the
Series 988, if there are two grounded inputs or
if an output is tied to an input, such as using a
transmitter power supply output to power a
thermocouple transmitter.
Is the process relatively fixed or widely varying — what is the range? (page 6.8)
• Make sure you check (and re-check) the ranges
of the input options available in the 988.
Although the 988 can operate outside the ANSI
ranges for specific thermocouples and RTD’s, it
is not recommended and will shorten the life of
the sensor.
Are there concerns over electrical noise
immunity?
• Although the Series 988 is thoroughly tested for
electrical noise immunity, it is paramount that
you follow good engineering practices when
designing the placement of the sensor and
power wires. Refer to the Series 988 Users
Manual and the Watlow Catalog.
Outputs
The outputs command the system to heat, cool,
turn ON and OFF, and also trigger other
actions in this system or related systems.
It is very important that you maintain total
control over the process. External limits or
other devices to disable the outputs can protect
system components and provide an added layer
of safety. Consider the following:
Is cost a factor in the sensor selection?
• Typically, a thermocouple costs less than an
Optimizing Your Process System with the WATLOW Series 988
How frequently will the output need to cycle
to control the process? (page 6.8)
2.5
Basic Control Strategies and Terms
• What is frequent? In order to tightly control a
process, the more frequent the better. Unless
absolutely necessary, mechanical relays should
not be used as the control output. A typical
mechanical relay application cycles ON and
OFF 86,400 times over a period of one month
(30 second cycle time, 24 hours per day). The
mechanical relay option is only warranted for
100,000 cycles.
• With a solid-state output a variable-time-base
burst fire option can provide the tightest load
control. Make sure the power control is compatible with the burst-fire output. (page 3.3)
What actuator interfaces are required? (page
6.8)
• Typically the output of the Series 988 does not
directly control the load. Instead it acts as a
pilot-duty output interfacing with another
device that actually controls the load. If this is
the case in your application, make sure that
the output of the Series 988 is compatible with
the input of the external device.
What alarm or annunciation outputs are
required? (page 5.1)
• The alarm options in the Series 988 can be
used to alert an operator to a system malfunction. These outputs should not be used as
safety limits to protect system components or
personnel. A separate safety limit device should
be used in any system where damage or injury
could occur due to a system component failure.
How should the alarms operate? (page 5.1)
• The alarm options can be field-configured to
function in any fashion. A process alarm can
be used to warn the operator of impending
equipment damage. A deviation alarm can be
used to warn that the system is operating
outside of the desired specifications. Each
alarm can be set for latching or non-latching,
silencing and reverse or normal logic.
process output (4-20mA, 0-5VDC, etc.). Also, it
limits the minimum on-time of the output.
Does the process require a retransmit output?
(page 3.12)
• Output 3 can be used as either a 4-20 or 020mA output or as a 0-5, 1-5 or 0-10VDC
output to retransmit the process value of either
of the inputs or the set point value. It allows
the controller to function as a master controller
in a master-remote application or to retransmit
a process value to a chart recorder.
What types of monitoring of the outputs is
required? (page 3.4)
• The retransmit option is one way to monitor the
system status.
• A serial communications option can be used to
monitor and/or change all the parameters in
the controller. With the EIA-485 option up to
32 controllers can be monitored from a single
communications port on a personal computer,
over distances of 4000 feet. The RS-422 option
can address up to 10 controllers over 4000
feet, and the RS-232 option can address a
single control over a distance of 50 feet. Make
sure the computer or other monitoring device is
equipped with the correct serial communications card and that the baud rate and other
capabilities are compatable.
Controller Environment
There are several factors to consider concerning the environment that the Series 988 controller operates in:
What are the physical dimensions of the
enclosure? (page 6.4)
• The Series 988 family of controls has a behindpanel depth of 4.06 inches, allowing the control
to fit an enclosure with a depth of 6.0 inches.
When mounting more than one controller in a
panel we recommend a minimum spacing of
1.66 inches between controllers.
Should you monitor the load current? (page
3.7)
• While this option is available as an input, it is
important to note that this feature in the Series
988 is not currently available when using a
Optimizing Your Process System with the WATLOW Series 9882.6
What is the ambient temperature inside the
enclosure? (page 6.8)
• The Series 988 can operate in ambient temperatures as low as 32°F (0°C) and as high as
150°F (65°C). The enclosure requires a non-
Basic Control Strategies and Terms
condensing atmosphere, because the vented
case is susceptible to dripping water. If necessary, include an enclosure heater in your
system to maintain the proper environment.
Is the front panel subject to spray or hosedown conditions? (page 6.8)
• The Series 988 has a NEMA 4X-rated front
panel. This rating allows the controller to be
hosed down directly without damaging the
controller. Two gaskets protect the controller:
one seals the bezel of the control while the
other seals the controller face plate and the
panel cutout. When installing the controller be
sure that these gaskets are not twisted.
What are the agency requirements for the
controller and the system? (page 6.8)
• The Series 988 has a UL 873 and UL 508
recognition. CSA recognition is pending. Also,
the NEMA 4X rating was certified independently by UL under UL 50X.
Controller Operation
Operation of the Series 988 can be broken
down into three concerns: setup; operation;
and maintenance. The questions you ask
yourself should examine each of these areas.
Does the system have any interactive process
variables? (Chapter 4)
• Interactive process variables include ratio,
differential or cascade control or slidewire
feedback of valve position. Also, heater current
measurements can be used to detect system
faults.
Is an event input useful in this application?
(page 3.6)
• Before finalizing the design check out this basic
option. The event input can expand the userfriendliness and/or security of your system.
Is ramp to set point required? On start up?
On set point change? (page 3.10)
• Many systems are susceptible to damage if the
process changes too rapidly. The ramp to set
point feature can minimize system stress.
How should the controller respond to an
error? (page 5.6)
• In the event of an error, system response is
critical. Some systems might require that the
control output turn full ON to provide cooling.
Other systems would require that the control
outputs be turned OFF. The design of some
systems might allow the control to continue
operating in the manual mode without operator
action. All of these options are available with
the Series 988.
Are you shipping products overseas? (page 2.3)
• In some countries and in certain markets in the
U.S. the default units may be either degrees C
or degrees F and the PID parameters might use
proportional (in percent of span), integral and
derivative or proportional (in degrees), reset
and rate. The 988 can switch between these
default sets to suit the user's preference.
What level of operator security do you need?
(page 5.7)
• The Series 988 provides numerous levels of
both software and hardware lockout. Refer to
page 4.5 for more information on lockout.
What do you do if a system component fails?
(page 5.5)
• When investigating a system failure, the Series
988’s Diagnostics menu can be used to monitor
inputs and selectively activate outputs. The
menu also shows the software revision and the
I/O types.
What about field calibration?
• All Watlow microprocessor-based controls can
be field calibrated with the right equipment.
The calibration manual for the Series 988
family of controllers is available upon request.
Putting It All Together
You should now have a good idea of what types
of questions to ask when designing your system
with the Series 988. We suggest that you read
through the rest of this chapter and Chapters
Three through Seven for more detailed information on the features available in the Series 988.
If you are already familiar with the 988, skip to
Chapter Seven to select the 988 that fits your
application.
Optimizing Your Process System with the WATLOW Series 988
2.7
Basic Control Strategies and Terms
Glossary
annunciator — a device that uses pilot lamps to
indicate the former or existing condition of a
system being monitored.
ANSI — American National Standards Institute.
burst fire — output that switches full AC cycles
ON and OFF repeatedly. Zero-cross burst fire
switches only at the zero-voltage point of the
AC sine wave. Variable-time-base burst fire
would switch ON and OFF 30 times a second to
achieve a 50-percent power level with a 60cycle AC power supply. Also see “zero switching.”
calibration offset — adjustment to the actual
process input and to the process values the
Series 988 uses for display and control.
cascade — control algorithm in which the output
of an outer control loop is the set point for an
inner loop. The inner loop, in turn, determines
the control action.
closed loop — control system that uses a sens-
ing device for process variable feedback.
derivative — anticipatory action that senses the
rate of change of the process, and compensates
to minimize overshoot and undershoot. Also
see “rate.”
differential control — With enhanced software,
the Series 988 controller can control one
process in relation to the difference of a second
process. A set point is added to the measured
value of the second process. This sum is used
as the set point for the input 1 process value.
DIN — Deutsche Industrial Norms, a widely
recognized German standard for engineering
units.
droop — difference between the set point and
stabilized process value.
duplex control — With enhanced software,
duplex control splits a single process output
into two individual outputs. For example, a 420mA output is split into a 4-12mA direct
acting (cooling) output and a 12-20mA reverse
acting (heating) output, thus allowing one
control output to function as two.
cold junction — point of connection between
thermocouple metals and the electronic instrument.
cold junction compensation — electronic
means to compensate for the effective temperature at the cold junction.
current transformer — a transformer, designed
for measuring electrical current, with its
primary winding connected in series with a
circuit carrying the current to be measured.
dead band — The dead band setting determines
the amount of interaction between heat (reverse
acting) and cool (direct acting) control outputs.
default parameters — the parameters, or pro-
grammed instructions, permanently stored in
the microprocessor software.
Optimizing Your Process System with the WATLOW Series 9882.8
duty cycle — percentage of load-ON time relative
to total-cycle time.
external transmitter power supply — a DC
voltage source of power for external devices.
filter — as applied to the Series 988, a low-pass
filter designed to minimize display or process
input fluctuations.
form A — single-pole, single-throw relay that
only utilizes the normally open (N.O.) and
common contacts. These contacts close when
the relay coil is energized. The contacts open
when power is removed from the coil.
form B — single-pole, single-throw relay that
only utilizes the normally closed (N.C.) and
common contacts. These contacts will open
when the relay coil is energized. The contacts
will close when power is removed from the coil.
Basic Control Strategies and Terms
form C — single-pole, double-throw relay that
utilizes the normally open (N.O.), normally
closed (N.C.) and common contacts. The user
has the option of wiring for a form A or form B
contact. Refer to the form A and form B above
for more information.
hunting — oscillation or fluctuation of the
process between the set point and process
variable.
hysteresis — in ON/OFF control, the process
change necessary to change the output from
full on to full off.
input linearization — For thermocouple and
RTD inputs, the process variable is automatically linearized. Certain flow transmitters
generate a non-linear signal corresponding to
the flow being measured. The square root of the
signal is calculated to generate a linear signal.
integral — control action that automatically
eliminates the offset, or “droop,” between the
set point and actual process value. Also see
“reset.”
isolation — Electrical separation of sensor from
high voltage circuitry. Allows use of grounded
or ungrounded sensing element.
JIS — Joint Industrial Standards. Also Japanese
Industrial Standards Committee (JISC). Establishes standards for equipment and components.
open loop — control system without sensory
feedback.
output — action in response to difference be-
tween the set point and process variable.
overshoot — condition in which the process
exceeds set point due to initial power up or
process changes.
P control — proportioning control.
PD control — proportioning control with deriva-
tive (rate) action.
PDR control — proportional derivative control
with manual reset is used in fast responding
systems where the reset causes instabilities.
With PDR control, an operator can enter a
manual reset value that will eliminate any
droop in the system. The option requires
enhanced software.
PI control — proportioning control with integral
(auto-reset) action.
PID control — proportioning control with inte-
gral (auto-reset) and derivative (rate) action.
process variable — regulated system parameter,
such as time, temperature, flow, humidity, etc.
proportional band — span of the process from
the set point within which time proportional
control action takes place.
linearization — the extraction of a linear signal
from the non-linear signal of a flow transmitter.
Also see “input linearization.”
NEMA 4X — a front-panel rating that certifies
the control as washdown capable and corrosion
resistant.
ON/OFF control — control of a process by
turning the output full ON below set point and
full off above set point.
Optimizing Your Process System with the WATLOW Series 988
proportioning control — See "time proportion-
ing control."
rate band —
where the rate (derivative) function begins. The
Series 988 rate band centers on the set point
and is twice the width of the proportional band.
ratio — application in which the flow of an
uncontrolled stream is measured and used to
maintain the flow of a controlled stream at a
ratio to the uncontrolled stream.
a thermal control band that defines
2.9
Basic Control Strategies and Terms
reference junction — synonymous with cold
junction. Also see “cold junction.”
retransmit — an analog signal representing a
control variable, either the process values or
the set point values.
RTD — resistance temperature detector. Resistive
temperature-sensing device that displays a
positive temperature coefficient.
slidewire feedback — closed-loop, valve-actuator
control using a potentiometer to indicate valve
position.
switching sensitivity — in ON/OFF control, the
process value change necessary to change the
output from full on to full off.
thermal system — a regulated environment
consisting of a heat source, heat transfer
medium, sensing device and a process variable
control instrument.
thermocouple — temperature-sensing device
constructed of two dissimilar metals that
generates a measurable, predictable voltage
that corresponds to its temperature.
supply power to external signal conditioners,
transducers or transmitters. With internal DIP
switches, the user selects between 5, 12 or
20VDC at 30mA ratings.
zero-cross — Action that provides output switch-
ing only at the zero-voltage crossing points of
the AC sine wave. Also see “burst fire.”
thermocouple break protection — fail-safe
operation that assures output shutdown upon
an open thermocouple condition.
three-mode control — proportioning control
with integral (reset) and derivative (rate). Also
see “PID.”
time proportioning control — action that varies
the amount of on and off time when “close” to
the set point, i.e., in the proportional band.
This variance is proportional to the difference
between the set point and the actual process.
In other words, the amount of time the output
relay is energized depends on the system
process value.
transmitter power supply — When Option “T” is
ordered for output 2, 3 or 4, the Series 988 can
Optimizing Your Process System with the WATLOW Series 9882.10
General Software Features
Chapter Three
General Software Features
Auto-tune 3.2
Burst fire 3.3
Communications 3.4
Dead band 3.5
Digital event 3.6
Heater current 3.7
Input filter 3.8
Input linearization 3.9
Ramp to set point 3.10
Remote set point 3.11
Retransmit (master/remote) 3.12
Slidewire feedback 3.13
How to use this chapter:
This chapter describes the software
features that are available in Watlow
Series 988 controllers. Enhanced software options provide additional features
that are described in Chapter Four.
3.1Optimizing Your Process System with the WATLOW Series 988
General Software Features
TL
Overview:
The auto-tune feature allows the controller to
manipulate the process and calculate PID
values based on the process response. This
relieves the operator from the tedious task of
manually tuning the PID parameters to match
the characteristics of the thermal system.
Auto-tune
During an auto-tune the controller monitors
how fast the process increases and decreases,
and from this information calculates proportional band, reset and rate values and automatically enters them into non-volatile
memory.
The point at which the auto-tune takes place is
determined by the auto-tune set point
parameter. It is adjustable from 50 percent to
150 percent of the current set point, with 90
percent being the factory default setting. If the
auto-tune set point is 90 and the current
setpoint is 300 degrees, the control goes into
ON/OFF mode of control at 270 degrees (300
deg. x 0.9). The displayed setpoint is unchanged and the bottom display information
alternates with tune
at a one-second rate
until the auto-tune is completed. Any changes
to the set point during an auto-tune will cause
the auto-tune sequence to start over, based on
the new set point.
To abort an auto-tune, set the auto-tune
prompt
to or press the AUTO/MAN
key twice, or cycle the power OFF and ON. The
previous PID parameters will be restored.
Requirements
The Series 988 can be ordered with two complete sets of heat/cool PID sets. This requires
the enhanced software option. A single set of
PID parameters is available with the standard
software option.
350°
315°
auto-tune
complete
temperature →
time →
Oven
heater
input 1
A
TL
W
PROCESS
L1 L2 L3 L4
DEV
% OUT
SERIES 988
DISPLAY
AUTO
MODE
W
MAN
output 1
limit
limit
sensor
Sample Application:
In this example the oven needs to hold at 350 degrees F. To complete the
tuning sequence set the set point for 350 degrees and auto-tune by setting
the auto-tune prompt to
3.2
Optimizing Your Process System with the WATLOW Series 988
.
temperature
sensor
General Software Features
TL
Overview
Variable, time-base burst firing from the 988
provides a command signal to an SSR or SCR
firing card that translates into a burst of AC
cycles. The output is zero-cross fired and
always allows at least one AC cycle to pass
within the variable time base. The fact that we
are zero-cross switching the power device
means we enjoy the benefits of low radio
frequency (RFI) noise. Burst firing is the preferred mode to control resistive loads.
The burst fire time base in the 988 varies from
a maximum 1.66-second time base (1-percent
output) down to a 33.3-millisecond time base
(50-percent output). The examples below show
how the time base varies depending on the
percent output.
Burst Fire
Requirements
The 988 family has built in zero-cross detection
circuitry. Therefore special firing circuitry is
normally not required for triggering SCR’s. To
enable burst fire the 988 must have an open
collector or solid-state relay output. The feature
is enabled by selecting burst fire
cycle time prompt for the appropriate output.
You should note that the short time bases used
by burst fire makes it incompatible with the
heater current feature (see page 3.7). The
heater current option requires a minimum of
300 msec. ON-time to get a reading.
Only the 988 and 989 can use the burst fire
feature.
cannot use burst firing.
at the
The low-voltage units (986 and 987)
50% Output100% Output
10 ON, 0 OFF 1 ON, 1 0FF
10% Output
1 ON, 9 OFF 2 ON, 1 OFF
input 1
A
TL
W
PROCESS
L1 L2 L3 L4
DEV
% OUT
SERIES 988
W
DISPLAY
AUTO
MAN
MODE
output 1
QPAC
with
QCD
input
66% Output
Semiconductor Oven
limit
limit
sensor
Sample Application
The 988 is being used to control a heated platen in a semiconductor oven.
Previously it required a power control requiring a 4-20mA signal to implement
burst-fire control. We have replaced the power control with a Watlow Loyola
QPAC with a QCD card that accepts a signal directly from an open-collector
output of the 988. This gives smooth control at a lower overall system cost.
temperature
sensor
heated platen
3.3Optimizing Your Process System with the WATLOW Series 988
Loading...