Cascaded Current Loop in Motor Control

[PPhil]

‎08-09-2018 11:51 PM

Hi There,



My questions is in regards to the use of a cascaded controller model where by the output of the velocity controller sets the setpoint of the current controller. What I do not understand about this is when velocity tracking and a disturbance is encountered the current controller aims to reduce the output voltage to maintain the current and the velocity controller aims to increase the output voltage (via the current controller) to maintain speed. Due the response of the system this causes an exaggerated tracking error when compared to direct velocity control.



Is there something I am missing here? It would make more sense for the current controller to increase the current setpoint when the current measurement is increased.



Any guidance would be much appreciated.

 

[KlausST]

Hi,

A quite vague and confusing description.

We don't know which type of motor you are using.
Let me assume it is a brushed DC motor
Then a "current controller" is not useful for speed control.
Use a "voltage controller" instead.

A current controller is good for"torque control". Where you want the torque to be constant, but the RPM changes a lot with varying mechanical load

A voltage controller will keep the RPM way more stable when you change the motor load.

Klaus

 

[PPhil]

Hi,


A current controller is good for"torque control". Where you want the torque to be constant, but the RPM changes a lot with varying mechanical load

A voltage controller will keep the RPM way more stable when you change the motor load.

Klaus

Hi Klaus,

I apologise for the confusing description.

This is what results suggest but many of the papers I have read propose a cascaded loop architecture with an inner current loop. The primary motivator for having the inner current loop is that it makes it simple to implement current limits without have to switch modes or simply turn the motor off.

Thanks for your help.

 

[FvM]

If it involves a velocity tracking error, the controller uses either an inappropriate topology or wrong parameters. Usually a cascaded controller topology implements feedforward paths to make setpoint variations become effective without delay. In so far it may be a topology problem.

To avoid misunderstandings, you should show a diagram of the controller topology, e.g. a simulation model.

 

[PPhil]

I guess I am trying to find out if it is an inappropriate topology for velocity tracking despite be used in servo controller such as http://elm-chan.org/works/smc/report_e.html.
Even with Feedforward terms the current controller still seems to degrade overall tracking performance. When encountering disturbance torque the current controller aim to reduce the motor voltage to maintain the current setpoint while the velocity controller aims to increase it. As the current controller is running faster and is generally more responsive the motor voltage is reduced before the velocity controller increases the current setpoint to maintain velocity. I have attached simplified block diagram. I can do a more complex version if required and I may need to model it up in Simulink to check the theory.

Thanks again for your guidance.

 

[FvM]

How's the motor driven, current or voltage?

 

[PPhil]

How's the motor driven, current or voltage?

Sorry, I don't understand the question. It depends on whether one uses current control or not. It is driven ultimately by a pwm and therefore voltage.

 

[FvM]

It is driven ultimately by a pwm and therefore voltage.

That's what I'm asking for.

 

[KlausST]

Hi,

It is driven ultimately by a pwm and therefore voltage.

I recommend one half bridge continously grounded, the other 0..99% PWM´d with slow decay mode.

Klaus

 

[FvM]

I recommend one half bridge continously grounded, the other 0..99% PWM´d with slow decay mode.

Implementation details are irrelevant here, question was about the control loop topology, clarified now.

 

[KlausST]

Hi,

This is exactly why I mentioned it: The control loop will behave different if the PWM is in slow_decay or fast_decay mode.
With slow_decay mode the regulation loop is more stable.

Klaus

 

[FvM]

I don't see it directly related. Slow versus fast decay (more commonly known as unipolar and bipolar PWM scheme) doesn't affect the small signal driver behavior, both are driving the motor with a controlled voltage. The main effect is on pwm ripple current.

 

[KlausST]

Hi,

with slow decay mode the motor acts like driven by a DC voltage: V_DC is about V_supply x duty_cycle.
There is an about linear duty_cycle to RPM relationship.
In case of rapidly reduced duty_cycle the motor will act as generator and the RPM is actively decelerated. (pushing electrical energy back to the power supply - this may cause overvoltage problems)

With fast decay mode the relationship is far from being linear. It is very weak with low duty_cycle and becomes more stiff with high duty_cycle.
It acts like torque controlled at lower duty cycle and it acts like RPM controlled with higher duty cycle.
The regulation loop needs to compensate for this "unlinearity". It will be difficult to run the motor with low RPM and varying torque.

Klaus