DRV11873: Two questions about Closed Loop Control

Hello Went Cao,

Let me address both of your questions below:

2. How to realize closed loop control ? could you please me to describe the theory ? Thanks a lot.

Before we talk about commutation or the 150 degree algorithm, closed loop control, in context of the DRV11873, refers to the ability to change the speed of a motor. The PWMIN pin will detect a duty cycle percentage and the DRV11873 will attempt to expose that percentage of VCC to the motor. In a very simple example, if the duty cycle on the PWMIN pin is 50% and VCC = 10V then the average voltage seen at the phase of the motor will be 5V. As a result, if the user changes the duty cycle percentage, the voltage exposed to the motor will change by that same percentage. We know that the change in voltage results in a change in current and this will interact with the motor constants (e.g. Kt or Ke) and change speed.

Note, that this is not a speed loop. Speed loops need a target speed reference to compare against. The FG pin reports the speed but the user would have to use some other logic (e.g. MCU) to determine whether FG is reporting a speed that is faster or slower than the desired speed.

The feedback element that is associated with a closed loop system (and our closed loop control) comes in the form of measuring the motor’s Back Electromotive Force (BEMF). Note the COM to Phase Comparators in our block diagram. The BEMF gives us enough information to commutate the motor. Specifically, you can think of the architecture as a state machine. For example, if U, V, and W are "x, y, and z", then transfer to state "B".

The “state machine” makes up the sensorless proprietary BEMF 150 degree commutation control algorithm. You may have seen 180 degrees or sinusoidal or 120 degree or trapezoidal algorithms. A traditional trapezoidal algorithm has 6 states over one period where the 150 has 12 states over one period. The increased states mean an increased resolution in the shape of the current waveform. If sinusoidal commutation has phase current shaped like a sine and trapezoidal commutation has phase current shaped like a trapezoidal then 150 degree is somewhere in between. Take a look at the reference pictures in the datasheet or measure them yourself to see how the current almost looks sinusoidal.

The more sinusoidal the current, the more sinusoidal the torque will be. If the BEMF is also shaped like a sinusoid, then the BEMF will not fight against the torque resulting in lower vibration, acoustic noise, and efficiency.

1. Closed Loop Control is used for control speed by limit current ?

In context of the answer above, we do have a current limit that limits the current during each electrical cycle. If we limit the current, the speed will be limited as well because of the motor torque constant (Kt). But, load will also affect speed so the current limit is usually used to limit power dissipated through the motor or for protection. If speed wants to be controlled, users will usually implement a speed loop like mentioned above.

Let me know if you have any questions.

Best,

-Cole