DC-servo Question max velocity seem clipped


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Thread: DC-servo Question max velocity seem clipped

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    Default DC-servo Question max velocity seem clipped

    Hi

    quite few year i not play whit kflop and snapamp ,so bit rusty
    have 2 question

    1-) seem that my max moving speed and velocity limited by system ,over some point i cannot increase it
    2-) on PID graph i have power glitch that to sure it normal/like

    so machine hardware is 400W DC motor that drive directly belt X axis , system is build to run a 3m/s so quit fast ,it actually a ex SMT placement machine
    see that i reach only 1/3 of that before Kflop top-up , i think it may related to avoid encoder going faster that FPGA ?

    encoder take me 166.666 cont per millimeter of travel ,so 300,000 cont for 1,8meter must do it normally in ~2 sec
    max output , motion profile V ,A dotn help ,any suggestion ?

    as power out spike it self explanatory

    Best regard
    Marc L.

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    Attached Thumbnails Attached Thumbnails DC-servo Question  max velocity seem clipped-kflop-jpg  


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    Default Re: DC-servo Question max velocity seem clipped

    Hi Marc,

    The KFLOP Encoder inputs handle 1000000 counts/sec and you seem to be only moving at 200000 counts/sec so that is not likely to be an issue.

    Also I would expect total loss of encoder tracking if that was the issue.

    But you didn't show the rest of your parameters so I can't be sure what you are doing.

    I assume these are brushed DC motors?

    Maybe you don't have enough power supply voltage? What is your supply voltage and what is the back-emf constant for your motor?

    Regards



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    Default Re: DC-servo Question max velocity seem clipped

    Hi

    yes DC brushed motor is a PARVEX MD17HSR0001 ,not find lot of information on it

    have try to play whit voltage but not seem to make great difference ,but i agree i let it quite low for avoid costly mistake
    for now i supply it whit 25V and it seem to stay under 2 amp



    WriteSnapAmp(SNAP0+SNAP_PEAK_CUR_LIMIT0,10);
    WriteSnapAmp(SNAP0+SNAP_PEAK_CUR_LIMIT1,10);

    ch0->InputMode=ENCODER_MODE;
    ch0->OutputMode=DC_SERVO_MODE;
    ch0->Vel=400000;
    ch0->Accel=400000;
    ch0->Jerk=30000;
    ch0->P=30;
    ch0->I=0;
    ch0->D=10;
    ch0->FFAccel=0;
    ch0->FFVel=0;
    ch0->MaxI=200;
    ch0->MaxErr=500;
    ch0->MaxOutput=1500;
    ch0->DeadBandGain=1;
    ch0->DeadBandRange=0;
    ch0->InputChan0=8;
    ch0->InputChan1=0;
    ch0->OutputChan0=8;
    ch0->OutputChan1=0;
    ch0->MasterAxis=-1;
    ch0->LimitSwitchOptions=0x100;
    ch0->LimitSwitchNegBit=0;
    ch0->LimitSwitchPosBit=0;
    ch0->SoftLimitPos=150000;
    ch0->SoftLimitNeg=-150000;
    ch0->InputGain0=-1;
    ch0->InputGain1=1;
    ch0->InputOffset0=0;
    ch0->InputOffset1=0;
    ch0->OutputGain=1;
    ch0->OutputOffset=0;
    ch0->SlaveGain=1;
    ch0->BacklashMode=BACKLASH_OFF;
    ch0->BacklashAmount=0;
    ch0->BacklashRate=0;
    ch0->invDistPerCycle=1;
    ch0->Lead=0;
    ch0->MaxFollowingError=50000;
    ch0->StepperAmplitude=20;

    ch0->iir[0].B0=1;
    ch0->iir[0].B1=0;
    ch0->iir[0].B2=0;
    ch0->iir[0].A1=0;
    ch0->iir[0].A2=0;

    ch0->iir[1].B0=1;
    ch0->iir[1].B1=0;
    ch0->iir[1].B2=0;
    ch0->iir[1].A1=0;
    ch0->iir[1].A2=0;

    ch0->iir[2].B0=1;
    ch0->iir[2].B1=0;
    ch0->iir[2].B2=0;
    ch0->iir[2].A1=0;
    ch0->iir[2].A2=0;


    Best regard

    DC-servo Question  max velocity seem clipped-pavex-jpg



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    Default Re: DC-servo Question max velocity seem clipped

    Hi Marc,

    It says something about 117V and 3000RPM. So 25V would produce maximum:

    25/117 x 3000 = 641RPM

    You didn't say your encoder counts/rev so we can't tell what RPM you are running at.

    Regards



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    Default Re: DC-servo Question max velocity seem clipped

    Hi good point need ~16 turn for make full travel of 1,8M so ~300000 cont

    this make something like 17,000 cont encoder ?? normally encoder is 5000/turn so something not match

    whit PSU straight on motor i have decent speed ;-)

    tomorrow will try whit a different encoder for see

    Best regard

    p.s. if encoder read 20,000 cont this make 16 turn for 300000 cont so may make much more sense
    so how did snapamp read it as quadrature so 5000 turn into 20,000 ?



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    Default Re: DC-servo Question max velocity seem clipped

    Hi Mark,

    Yes SnapAmp counts quadrature transitions.

    SnapAmp PWMs only go to 230/256 so the motor will only see max of 22.5V.

    200000 counts/sec / 200000 counts/rev = 10rps = 600RPM would make sense for that motor and supply voltage.

    You might measure the voltage on the motor. It's not straight forward because it is switching at 33KHz. One trick I do is put a low pass RC filter (1uF 10K = 10ms) then measure the capacitor voltage with a meter or scope. If it is reaching 22.5 V then you know that ithe is at the limit for the supply.

    I wouldn't expect changing encoders to make any difference.

    What RPM do you get with the 25V on the motor?

    Regards



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    Default Re: DC-servo Question max velocity seem clipped

    Hi at naked eyes ,it near double ,on final design i target 40V for motor ,so i may help

    any clue for the big power output spike on mid of acceleration ? ,that is what worry me the more
    i expect to have power spike at start of moving due to big inertia ,but not once it almost reach full velocity ?

    i found that the jerk value seem quite delicate

    Best regard and Thank for help



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    Default Re: DC-servo Question max velocity seem clipped

    Hi Marc,

    any clue for the big power output spike on mid of acceleration ? ,that is what worry me the more
    i expect to have power spike at start of moving due to big inertia ,but not once it almost reach full velocity ?
    The way a DC motor works is the ability to generate torque decreases with speed. For example assume for discussion your motor coil has 1 Ohm of resistance. At zero RPM the 22.5V supply can cause 22.5Amps of current to flow to cause a large torque (motor torque is proportional to current). At 600 RPM the motor is acting like a generator that is generating 22.5V (back-emf). The 22.5V can not cause any current to flow into the motor to generate any torque at all. Even though little torque is required at that region of the motion the motor can't supply it. The servo commands an ever larger output current but that doesn't do anything because the voltage is already saturated at 22.5V maximum. Later when the trajectory slows slightly the servo is able to regain control and come out of saturation.

    By the way the maximum current command for SnapAmp is +/-1000 counts. You might set the Max Output to 1000 to see more clearly when the current command saturates. Although this has little to do with your current issue which I believe to be voltage saturation.

    Plotting Velocity may help you understand what is happening. Also in the future if you save and post the raw data then we could plot the data however we wish to see it.

    HTH
    Regards



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    Default Re: DC-servo Question max velocity seem clipped

    Hi ok so in case of voltage saturation it also mean that i near of loss the servo lock-up

    will resume and make more test in 2-3 week ,have to disassemble the machine for integrate Z and A axis

    but will use the original 40V supply so normally it will better

    thank allot



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DC-servo Question  max velocity seem clipped

DC-servo Question  max velocity seem clipped