NORD Drivesystems BU0210 - Manual (Page 13)

3 Function description
BU 0210 GB Subject to technical amendments 13
3 Function description
3.1 Introduction
A wide range of positioning and position control tasks can be performed with the positioning function. In order
to facilitate the decision as to which configuration provides the optimum solution for the task, the various
processes for the setting of setpoints and recording of the actual values are described in the following sections.
The setpoint can be specified as either an absolute or a relative position. An absolute position is recommended
for applications with fixed positions, for example with travelling trolleys, lifts, shelf access devices etc. A
relative position is advisable primarily for all axes which operate in steps, especially for endless axes such as
rotating platforms and pulsed compartmentalised conveyor belts. The specification of the setpoint can also be
made via the bus (Profibus, CAN bus, DeviceNet or USS interface). Here the position can be specified as a
value or combination of bits as a position number or increment.
With the use of the optional AS interface, the specification of the setpoint value is only possible by means of a
combination of bits, in a similar manner to control via the control terminals.
If switching between positioning and speed specifications is required, this can be realised by switching
between parameter sets. A position regulation in parameter P600 "Positioning" is parameterised to "ON" in one
parameter set and to "OFF" in another parameter set. Switching between the parameter sets can take place at
any time, i.e. even during operation.
3.2 Position detection
3.2.1 Position detection with incremental encoders
For an absolute actual position, a reference point is needed, with the aid of which the zero position of the axis
is determined. The position detection operates as long as the frequency inverter is supplied with power. The
pulses of the incremental encoder are counted in the inverter and added to the actual position. The resolution
or pulse number of the incremental encoder is set in parameter P301 "Incremental Encoder Pulse Number". By
setting negative pulse numbers, the direction of rotation can be adapted to the installation orientation of the
rotary encoder. After switching on the inverter supply voltage, the actual position = 0 (P604"Encoder Type"
without the option "Save Position") or it is at the value which was present on shut-down (P604 "Encoder Type"
with the option "Save Position").
The recording of the position functions independently of the enabling signal of the inverter and parameter P600
"Positioning". The inverter determines the actual position for as long as it is supplied with power. Changes in
position which are carried out with the inverter switched off do not cause a change in the actual position.
Therefore a reference point run is therefore normally necessary after each "Mains switch-on" of the frequency
inverter.
If the inverter is not operated in Servo- Mode P300 "Servo Mode" the incremental encoder can also be
mounted at another position than on the motor shaft. In this case, the speed ratio of the motor to the
incremental encoder must be parameterised. Rotations of the incremental encoder are converted to motor
revolutions by the inverter with the aid of parameter P607 "Speed Ratio" and P608 "Reduction Ratio".
n
M
: Motor rotations
n
M
= n
G
* Ü
b
/ U
n
n
G:
: Incremental encoder rotations
Ü
b
: Speed ratio (P607[01])
U
n
: Reduction ratio (P608[01])
Example: The incremental encoder is installed on the output side of the gear unit. The gear unit has a ratio of i = 26.3.
The following values are parameterised. P607 Speed ratio: 263;
P608 Reduction ratio: 10
NOTE The direction of rotation of the incremental encoder must comply with the direction of rotation of
the motor. With a positive output frequency (clockwise rotation) the actual position value must
increase. If the direction of rotation is not correct, this can be adjusted with a negative value in
P607 "Speed Ratio".