Siemens SIMATIC S7-1200 - Page 680

Technology instructions
10.3 Motion control
S7-1200 Programmable controller
680 System Manual, V4.2, 09/2016, A5E02486680-AK
The MC-PreServo OB and MC-PostServo OB are instances of the ServoOB, and they are of
type ServoOB. These OBs are optional, can only exist if MC-Servo OB is present, and
contain user code. All three OBs (MC-PreServo, MC-Servo, and MC-PostServo) have to run
in the same runtime level, and the same CyclicServoEvent triggers them. The PLC firmware
executes the OBs sequencially, based upon their block number:
OB instance
OB number
Automation System
Object Model (ASOM)
event
Engineering System
Object Model (ESOM)
event
MC-PreServo
67
ServoOB
-
MC-Servo 91 ServoOB ServoOB
MC-PostServo
95
ServoOB
-
Compared to the ASOM, the Engineering System Object Model (ESOM) only links the event
to the MC-Servo OB. The ESOM does not link the event to the MC-PreServo OB or MC-
PostServo OB.
Configuring the axis with an application cycle MC-Servo (OB 91)
Application cycle MC-Servo (OB 91)
You can set the application cycle in which the MC-Servo OB is called in the properties of the
organization block:
● Synchronous to the BUS: You call the MC-Servo OB synchronously with a bus system.
You set the send clock in the properties of the selected bus system.
● Cyclical: You call the MC-Servo OB cyclically with the assigned application cycle.
To avoid disruptions in the program execution on the CPU, set the application cycle
depending on the number of used axes as follows:
Application cycle = number of axes x 2 ms
Number of axes
Application cycle
1 2 ms
2
4 ms
4
8 ms
8
16 ms
The SINAMICS G120 drive updates the drive process image every 4 ms. To improve control,
set the application cycle of the MC-Servo (OB 91) to 4 ms or to a multiple of 4 ms.