Specifications

Position Decoding IP 240

7.2.9 Reference Point Approach

Since incremental encoders cannot indicate the absolute position after a power failure, a re-

ference point must be approached to calibrate a measuring system. The location of the reference

point is determined by the zero mark or reference signal (Z signal) emitted by the encoder during

a preliminary signal. To generate the preliminary signal, you must connect a bounce-free

switching element within the traversing range.

If a zero offset was forwarded to the IP prior to synchronization, the actual value is set to this

value at the reference point. The zero point of the actual value range can thus be offset to the

reference point. If no zero offset was forwarded to the IP, the actual value is set to ”0”.

A complete reference point approach is not absolutely necessary for position decoding.

Preparations for a reference point approach

Connect the position encoder to the IP 240 ( Chapter 4)

Connect the preliminary signal contact to binary input IN.

Reference point approach

Set control bit REFF to ”1” (D 17.0)

Forward the control bits to the IP 240 with control FB 170, function 2 (”Write control bits”).

The channel is now in reference point approach mode. No track comparison is made in this

mode. If the channel was configured with DIGn/8=1 (the IP is to reset outputs when a track is

exited), the active outputs are reset.

• When a ”1” signal is present at preliminary contact input IN, the next Z signal from the

encoder synchronizes the actual value.

The preliminary contact signal must be present for at least t

max.=5 ms ( Fig. 7-10) prior to

the Z signal used for synchronization.

If the duration of the preliminary contact extends over two or more zero marks, synchroniza-

tion takes place with every Z signal.

• When a negative-going edge is detected at the preliminary contact input, the SYNC bit is set

after a delay of t

max.=5 ms, the reference point approach is exited, and the track compari-

son is initiated.

Set control bit REFF to ”0”.

Should you fail to do, so a new referece point approach would be started and the SYNC bit is

transferred. The reference point approach may be initiated only once.

Status bit SYNC

The SYNC bit is set when the reference point approach was correctly terminated with the

synchronization of the actual value.

SYNC is reset

• when a new reference point approach is enabled or

• when an error was detected during position decoding.

Note

Once REFF=1 has been forwarded, the control bits must not be modified until

synchronization has been completed unless you want to abort the reference point

approach.

7-16

EWA 4NEB 811 6120-02a