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work in open-loop system DRV8308

Aleksei Sporyagin
Prodigy 200 points
Other Parts Discussed in Thread: DRV8308EVM, DRV8308

. Hello. My task is to provide high-repeatability precision keeping one instantaneous speed = 3000 ob/min brushless DC motor; on shaft of motor is big moment of inertia for smoothing rippling of speed; load is ventilation; brushless DC motor Maxon 118889(no-load speed 11000 ob/min, number of pair poles = 2); acceleration is smooth (1minute – 3000 ob/min); brake action is running-out; reverse is not necessary. Configuration systems: control from computer GUI, DRV8308EVM, brushless DC motor Maxon 118889, moment of inertia= moment of inertia motor. Motor is controlled from computer GUI v1.2 in open-loop system (open-loop PWM or speed register). The speed of motor is increased in open-loop system (open-loop PWM or speed register) when the speed of task is constant (3000ob/m). Because? How is avoid this? LOCK is settled. Why LOCK isn’t reflected change of speed of motor? The task is controlled instantaneous speed of motor. Can this task realize DRV8308 (SPEEDTH, LOCK,…)? I read datasheet DRV8308 and I think   SPEEDTH is half ripple amplitude speed of motor, but explain please on which algorithm realize control of speed motor (SPEEDTH, LOCK,…); what is SPEEDTH? Which are parameters DRV8308 recommend to tune for realization high-repeatability precision keeping one instantaneous speed motor(ADVANCE, PWMF,…)?

  • 0
    TI__Mastermind 21320 points

    Aleksei,

    I am not quite sure I understand the issue.

    LOCK will assert when the speed loop is locked. The speed loop is only utilized in the Clock Frequency mode (closed loop control). The PWM and Register control are open loop modes, LOCK will not assert in these.

  • 0
    TI__Mastermind 21320 points

    Aleksei,

    Correction to my previous post. nLOCK will function in the PWM and SPEED Register modes as well. It is looking a speed consistency.

    Can you clarify what you mean by instantaneous speed of motor? No motor can respond instantaneously to a speed change. There will always be some period of ramp up and down.

  • 0 in reply to Nicholas Oborny
    Prodigy 200 points
    Hello.Thank you on answers. Please answer on all questions topics.
  • 0
    TI__Mastermind 21320 points

    Aleksei,

    There are a number of parameters that must be met for LOCK to be achieved. These are detailed in section 8.3.6.

    MINSPD - Minimum Hall_U period for to be set

    SPDREVS - Minimum number of revolutions at set speed

    SPEEDTH - Speed variation tolerance

    The accuracy of the speed control will depend heavily on how well tuned the system is.

  • 0 in reply to Nicholas Oborny
    Prodigy 200 points

    Please answer on  questions: because the speed of motor is increased in open-loop system (open-loop PWM or speed register) when the speed of task is constant (3000ob/m);How is avoid this? LOCK is settled. Why LOCK isn’t reflected change of speed of motor?Can controlled accuracy maintaining of speed with help  DRV8308 (SPEEDTH, LOCK,…)?IS needed accurate controlled accuracy maintaining of speed;what is SPEEDTH(I read datasheet DRV8308,but interpret simply)?which algorithm realize control of speed motor (SPEEDTH, LOCK,…)?Which are parameters DRV8308 recommend to tune for realization high-repeatability precision keeping one instantaneous speed motor(ADVANCE, PWMF,…)?

  • 0 in reply to Aleksei Sporyagin
    TI__Mastermind 20985 points

    Aleksei, datasheet Figure 13 on Page 24 shows the parameters that control LOCKn.  The motor needs to reach a consistent speed for the number of cycles equal to SPDREVS, within a variance of SPEEDTH, and with a HALL_U period larger than MINSPD in order for LOCKn to go low.

    For instructions on tuning, please refer to the User's Guide in Section 3.

    Best regards,
    RE