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DRV10983: What are the units for the Closed Loop Acceleration Limit

Don Edvalson49
Prodigy 230 points
Part Number: DRV10983

The Closed Loop Acceleration Limit (SysOpt7, bits 4-6) are described as being specified in units of VCC/s. But I don't think that makes sense. VCC is not changing. If VCC is involved there should still be a unit such as % or Hz/sec. I think it is Hz/sec but I wanted to verify.

Don

  • 0
    TI__Mastermind 18900 points

    Hey Don,

    Unfortunately, we have a few parameters that don't make intuitive sense but make sense in context of what the driver is actually doing. For the case of Closed Loop Acceleration, let's take a step back and see what the driver is doing when it tries to accelerate a motor.

    If we are using a PWM duty cycle percentage to control the speed command to our device, we understand that 100% duty cycle results in 100% speed while 50% duty cycle results in 50% speed for the motor. A 100% speed command means that we are applying V_CC and 50% means we are applying V_CC/2 to the phases of the motor. As a result, we can simiplify the relationship of applying voltage to that motor phases as:

    V_appliedToPhase = %_DutyCycle*V_CC

    As a result, if try to go to 50% to 100% or V_CC/2 to V_CC, the rate at which we accelerate is in V_CC/s. I would agree that it is not intuitive, but since our device can accept a wide range of motors, putting Hz/s would not be realistic.

    In addition, the equation above will not hold true when considering curret limits, AVS functions, and other features that will prevent the device from changing to the target speed command. This includes the Closed Loop Acceleration limit. Remember, you may be at SpeedCmd = 50% --> 100%, but if a slower acceleration is used, the output speed command (SpdCmdBuffer) might output, 60% then 70% then 80% etc, until reaching the target speed. The block diagram showing this relationship is below:

    Let me know if you have any questions.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 230 points

    Hi,

    Perhaps I am being dense here, but it still seems like there needs to be more to the unit. In your example of going from V_CC/2 to V_CC one could say that V_CC increased by 50% or one could say that V_CC increased by 12 volts (assuming a 24 volt supply). So assuming that happened in 1 second would that be 12 V_CC/Sec (volts) or would it be 50 V_CC/Sec (percent) or would it be .5 V_CC/Sec (multiplier). 

    It seems like it needs to be something like V_CC volts / sec or V_CC % / sec.

    Can you tell me which it is?

    Thanks,

    Don

  • 0 in reply to Don Edvalson49
    TI__Mastermind 18900 points

    Hey Don,

    I suppose you can think of the parameter as a %VCC/sec. The actual parameters are 48 VCC/s, 0.77 VCC/s, 0.37 VCC/s, etc. where we understand the device will try to ramp the applied to voltage by 4800% of VCC in one second, 77% of VCC in one second, or 37% of VCC in one second, etc until it reaches the target voltage. We would still have to plug in a VCC value to recieve an actual value for the rate of change, however.

    But, your question began trying to discuss the units, and for the record, I almost fainted trying to do the unit analysis of this problem. As a result, I think I (hopefully) have a better way to address the question you initially asked. We should think of this Closed Loop Accelerate parameter in units of [V/s], with that said, the documentation doesn't deliver this message because it mixes the equation and the units in the description.

    So, the equation for the Closed Loop Accelerate parameter = k*V_CC where k = 48, 0.77, 0.37, etc. This means we get k in units of [1/s] and V_CC in units of [V]. I will try to leverage this post the next time we plan revise the datasheet.

    Let me know if you have any questions.

    Best,

    -Cole