# difference between "Control sample time " and "Fundamental sample time " for FOC

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K on 18 Jan 2024
Commented: K on 23 Jan 2024
Hello
while Implementing a drive with a Field-oriented-Control i couldn't get the difference between "Control sample time " and "Fundamental sample time " of the FOC Induction Motor. The documentation doesn't specify anything like shown bellow
I used values from Matlab exmaples that makes sense but I would like to understand their meaning. as I tried to change the "Fundamental sample time " I got an error that it has to be "smaller or equal the inverse of 10 times the switching frequecy" so I guess they depend on each other.
can someone please explain the differance?
one last question: does their ratio (Fundamental sample time / Control sample time) has any special meaning?
regards

Shubh on 23 Jan 2024
Hi,
The "Fundamental sample time" and "Control sample time" are parameters related to digital control systems, such as Field-Oriented Control (FOC) for induction motors.
1. Fundamental Sample Time: This is the base rate at which the system's algorithm computes the fundamental tasks, such as current measurement or PWM updates. It is often set based on the switching frequency of the power electronics. For example, if the switching frequency is 10 kHz, the fundamental sample time might be set to 1/(10×10kHz), which ensures that the sampling is done at a rate ten times the switching frequency. This helps in accurately capturing the waveform for precise control.
2. Control Sample Time: This is the rate at which the higher-level control tasks are computed, such as the speed or position control loops. These tasks typically don't require as high a sampling rate as the fundamental tasks. The control sample time is usually an integer multiple of the fundamental sample time, which means that for every 'n' cycles of the fundamental sample time, the control tasks are computed once.
The error message you received indicates that the system requires the fundamental sample time to be less than or equal to the inverse of 10 times the switching frequency. This ensures that the system can accurately track changes in the motor's operation and adjust the control signals appropriately.
Regarding the ratio of fundamental to control sample time, it often represents the number of fundamental samples per control sample. This ratio is important because it determines how many times the fundamental tasks are executed before a control task is updated. The chosen ratio can affect the performance and stability of the control system. A common practice is to have a power of 2 ratio for ease of computation and timing synchronization.
For example, if the fundamental sample time is 5×10^−6 seconds and the control sample time is 5×10^−5 seconds, the ratio would be 1:10, indicating that for every one control task computation, there are ten fundamental task computations.
These parameters are critical in digital control and should be set according to the dynamics of the system being controlled, the capabilities of the hardware, and the requirements of the control algorithm. It's always good to refer to the hardware specifications and guidelines provided by the motor and controller manufacturers when setting these parameters.
Hope this helps!
K on 23 Jan 2024
Hello
yes that helps a lot thank you
I have one last quastion please: does these two parameters affect the step size (incerement) between the input measurment points?
I inputed my measurments points after interpolating them in a 1D look up table with step size 1 at times 1,2,3,... as shown below
I ended up with weird results, the problem are these semi interval (red plot)
what I would like to achieve is the blue one
so I inputed my measurments points after interpolating them in a 1D look up table with a step size of 0.1 (like at 10.1 then 10.2) and I got the wanted result
so I want to know if there's any corelation between the ratio of Control sample time/Fundamental sample time and the step size I implemented that lead to a right result?
if not how do I choose the right step size between two input points?
regards