Technical data

ManualsBrandsDanfoss ManualsMedia ConverterVLT 5000 FLUX

VLT 5000/5000 FLUX SyncPos Quick-Setup

MG.10.Q1.5B – VLT is a registered Danfoss trade mark12

AV is the actual velocity

AP is the actual position (calculated from encoder

feedback) in qc (Quad Counts).

CP is the current Setpoint position in qc.

CV is the Setpoint velocity in qc/ms.

(Position deviation) is calculated by CP–AP.

NB!

In SYNCV mode the PID controller is working

with speed deviation instead of position

deviation. Speed deviation is calculated by CV–AV.

The controller in the SyncPos card utilizes two

control strategies at the same time:

1. An open-loop feed-forward control. Since the

asynchronous motor inherently has a good

open loop performance the feed-forward con-

trol is a very important part of the controller in

most applications. Benefits from using feed-

forwards control is a very fast and accurate

response to changes in the setpoint reference.

2. A closed-loop PID control. The PID controller

monitors the difference between the actual

position and the setpoint position. Based on this

information it calculates a control signal to

minimize the position deviance. Thus the

SyncPos option is able to compensate for

changes in load or friction. The PID controller is

also necessary to compensate for any position

deviance caused by inaccurate setting of the

open-loop feed-forward controller.

In short: The feed-forward control is used to handle

changes in the setpoint reference (especially impor-

tant in synchronization applications), while the PID

control is used to handle changes in load conditions

or inaccuracies of the feed-forward control.

Proportional factor KPROP (11)

The

Proportional factorProportional factor

Proportional factor is multiplied with the posi-

tion deviance and the result is added to the control

signal (the internal speed-reference to the VLT).

Since the calculated control signal is proportional to

the position deviance (or error) this kind of control is

called proportional control. The behavior of the

proportional control is similar to that of a spring – the

further the spring is extended the stronger the

counter-force it produces.

Influence of the

Proportional factorProportional factor

Proportional factor:

KPROP too small large position deviation due to

non-compensatable load and

frictional moment;

KPROP larger quicker reaction, smaller steady-

state deviation, larger overshoot,

lesser damping;

KPROP too great heavy vibrations, instability.

Derivative factor KDER (12)

The

Derivative factorDerivative factor

Derivative factor is multiplied with the derivative

of the position deviance (the 'velocity' of the position

deviance) and the result is added to the control

signal. The behavior of the derivative control is

similar to that of an absorber – the faster the

absorber is extended the stronger the counter-force

it produces. Thus using the

Derivative factorDerivative factor

Derivative factor

increases damping in your system.

Influence of the

Derivative factorDerivative factor

Derivative factor:

KDER small no effect;

KDER larger better dampening, lesser over-

shoot; if KPROP is increased

simultaneously: faster reaction to

control deviation at the same

level of vibration;

KDER too large heavy vibrations, instability.

Integral factor KINT (13)

The sum of all error is calculated every time the

control signal is updated. The

Integral factor Integral factor

Integral factor is then

multiplied with the sum of all positioning errors and

added to the overall control signal. Thus in case that

steady-state position errors occurs in your

application, make sure you use the integral part of

the controller. Steady-state errors will be levelled out

as the summed error increases over time until the

control signal eventually matches the load.

It is possible to set a limit for the control signal

generated by the integral part of the controller (anti-

windup).

Influence of the

Integral factorIntegral factor

Integral factor:

KINT very small steady-state position deviance is

very slowly regulated to zero;

KINT larger faster regulation towards zero

steady-state position deviance,

larger overshoot;

KINT too large heavy vibrations, instability.

Integration limit KILIM (21)

The

Integration limitIntegration limit

Integration limit sets a limit for the control sig-

nal generated by the integral part of the controller.

This helps to prevent the so called "wind-up" pro-

blems which typically occurs in applications where

the overall control signal (the internal speed-referen-

ce) is maxed out for long periods of time.

10 steps to optimise the PID loop