DriSteem Vapor-logic3 Instrucciones de operaciones descargar pdf (Pagina 45)

second the demand is updated. With an integral gain

term greater than zero and an actual humidity below

set point, each time the demand is updated it is

increased slightly. If the actual humidity is above set

point, the demand will be decreased slightly. The

amount it is increased or decreased is dependent on

the difference between the actual humidity and set

point. (The closer you are to the set point, the smaller

the addition or subtraction.)

When looking at this control scheme, something

interesting is found. The total demand signal for the

humidifier is the sum of the proportional part, the

integral part and the derivative part. As the actual

humidity approaches the set point, the integral

portion makes up the majority of the demand and the

proportional part makes up very little. Once the set

point is achieved and the unit stabilizes, the entire

demand is made up of the integral part because the

proportional part is zero. If the actual humidity goes

over the set point, the integral term will start to

decrease. In addition, the proportional term will go

negative and actually start to subtract from the total

system demand. These two items work in conjunction

with each other to bring the humidifier back to set

point.

The derivative term

A derivative terms basic function in a PID loop is to

anticipate where the demand is going and to help it

get there. Its basic operation is as follows: As the

humidifier is running, the PID loop is generating an

error term. (The error term is the mathematical

difference between the RH set point and the actual

measured RH.) If the actual measured RH is below

set point and is rising, the derivative term will

subtract from the demand in anticipation of the

approaching set point. If the actual measured RH is

below set point and is falling, the derivative term will

add to the demand in anticipation of the need to get

the demand up faster and start climbing toward set

point. The same is true for situations above the set

point. If the actual measured RH is above set point

and is rising, the derivative term will subtract from

the total demand in anticipation of needing to reduce

demand and get closer to set point. If the actual

measured RH is above set point and falling, the

derivative term will add to the overall demand in

anticipation of the approaching set point. In the

majority of control situations, the derivative term is

not needed and is simply set to zero. The

proportional term and integral term will provide very

tight, accurate control without the addition of the

derivative term.

PID setup tips

A large proportional band (10 to 20%) will yield

tighter and more stable control with longer response

times. A small proportional band will produce

quicker response times but control may become

unstable. As a rule of thumb, start with a band of

10%. If the unit does not hunt and a quicker

response is needed, the band can be lowered. If the

unit hunts or the fast response is not needed, increase

the band. This will result in more system stability.

(To increase or decrease the effect of the proportional

band on system performance, the proportional gain

factor may be adjusted. However, for the majority of

systems, the factory default setting of 80 is

sufficient.) A moderate integral gain term (30 to 50)

will also yield tighter and more stable control with a

longer response time. A large integral gain will

quicken the response but may cause the system to

become unstable. As a rule of thumb, start with an

integral term of 40. If the unit does not hunt and

quicker response is needed, raise the integral gain. If

the unit hunts or does not need the fast response,

decrease the integral gain.

Control functions: PID tuning (cont.).

Operation

Operation:

PID tuning

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DriSteem Vapor-logic3 Instrucciones de operaciones Pagina 45

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