NORD Drivesystems BU0710 - Manual (Page 13)

3 Functional characteristics
BU 0710 GB 13
3 Functional characteristics
3.1 Introduction
The Posicon positioning card is extremely versatile in carrying out positioning and position controlling tasks.
To make it easier for you to decide which configuration will be most suitable for the application at hand, a
description of the various ways of setpoint setting and actual value recognition is provided below.
Setpoints can be fed to the control system as either absolute or relative position values. Absolute position
setpoints should be preferred in applications where the same positions are approached over and over again,
as e.g. with shifting trolleys, elevators, shelf operating equipment etc. Relative position setpoints are obviously
more suitable where axles operate in an intermittent fashion, especially endless ones driving turntables or
clock-controlled metering belts. Setpoints can also be transmitted over the bus (Profibus, CAN, Interbus, or
USS interface). In that case the position is translated into an encoded value or into a bit combination
representing the number of a position or a position increment.
If the operator wishes to quit the positioning mode and activate speed setting instead, he can do so by
switching parameter sets. While in one parameter set the position control parameter must be set to "On", an
"Off" setting will be required in the other. Switching between parameter sets is possible any time.
Position recognition can be handled using either an incremental or an absolute encoder. With rotary axles
turning in one direction most of the time, only an increment encoder can be used, since with absolute
encoders the range of values is limited to ±50000 revolutions. Using an incremental encoder implies that a
reference point will be needed.
3.2 Actual position recognition
3.2.1 Position recognition with an incremental encoder
The actual position is determined by the Posicon card processor. This calculation presupposes a reference
point which is used to define the zero position of the axle. The position recognition function will be active as
long as the frequency inverter is supplied with a voltage. The pulses generated by the incremental encoder are
counted in the inverter and added to the actual position value. When the inverter supply voltage is switched
on, the device will read out "0" as the actual position if the position saving option in parameter P606 has not
been activated, or else the value which was read out last before the inverter was switched off, if the position
saving option in P606 has been set to ON.
The position recognition function will work whether an enable signal has been fed to the inverter or not, and
also whether the P600 position control parameter has been set to ON or to OFF. The inverter will proceed with
actual position detection as long as it is supplied with a voltage.
Changes of position which take place while the frequency inverter is disconnected have no effect on actual
position indication. That is why a search for the reference point is usually required whenever the frequency
inverter is connected to the mains.
While generally the incremental encoder must be installed on the motor shaft, it may be accommodated
elsewhere if it is not possible to operate the inverter in the servo mode (P300). In that case the transmission
ratio between motor and incremental encoder needs to be parameterized. The inverter will convert the number
of revolutions determined by the incremental encoder into revolutions of the motor shaft proceeding from the
settings made in the multiplication and reduction parameters.
n
M
: number of motor shaft revolutions
n
M
= n
G
* Ü
b
/ U
n
n
G:
: number of revolutions determined by increm. encoder
Ü
b
: multiplication (P607
[1]
)
U
n
: reduction (P608
[1]
)
Example: The incremental encoder has been installed on the output side of the gearbox, the gearbox has got a
transmission ratio of i = 26.34. The following values should be set in the respective parameters:
multiplication: 2634; reduction: 100
On the software level an offset can be parameterized by which the starting point (zero) is transferred to a point
different from the one which is otherwise determined by the point of reference. After the value of incremental
encoder revolutions has been translated into a value representing the revolutions of the motor shaft, the offset
will be added to the actual position value. Accordingly a new offset value must be entered whenever the
multiplication or reduction setting has been changed.