Dead time delay between axis command (SetPos) and execution in TwinCAT3 with EtherCAT

Question

I want to measure the time between a) setting a target position for my axis on my computer and b) the variable is set on the axis.

I Setup the scope view of TwinCAT3 and displayed the SetPos variable (from the axis) and the setTarget Variable from the EtherCAT slave. I expected a delay of 500us I got 30ms.

The Scope view of TwinCAT3 is showing the SetPos (generated out of GUI the axis functions of TwinCAT3 (reverse sequencing)) and the target position variable inside my embedded system. There is no inertia involved, it is just the variable.

The delay is about 30ms as shown in the screenshot. The EtherCAT Servo is in DC-Mode with a cycle of 500us. The setTarget position variable is wirten inside the Sync0 ISR. The setTarget (PDO) variable is getting captured in 50us.

I am quite sure, that this 30ms delay is coming from TwinCAT itself. Maybe something from generating the SetPoint to transfer it to EtherCAT?

Thanks for your answers! Chris

Answer 1

Maybe it has something to do with the setpoint generator. There are two tasks involved in the setpoint generation: the SVB task and the SAF task. Read more about it here.

The document mentions about the SVB Task:

The SVB task is the setpoint generator and generates the velocity and position control profiles for the entire move of all drives according to the current position, command position, maximum velocity, acceleration and deceleration rates, and jerk of each drive. This task is typically run every 10ms and a change in any of these parameters will result in a new profile for the entire move every 10ms. As such if a drive is at the target position it will still be calculating profiles to hold it at that target position.

Therefore possible causes might be the:

• limits on maximum velocity, acceleration or jerk
• cycle time of this task.

What if you change the limits or the cycle time of this task?