stepper motor voltage rating does not match resistance & current rating

Discussion in 'The Projects Forum' started by hawkoli1987, Oct 7, 2009.

  1. hawkoli1987

    Thread Starter New Member

    Oct 7, 2009
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    Dear all,

    I'm facing a problem with my stepper motor. It was working fine previously. But under exactly the same condition, the new moving pattern becomes a repetition of 4 steps, i.e. after turning from step 1 to 4, it goes back to the step 1 position.

    I'm not sure if it is the problem with the supplied current and voltage. The motor I'm using is RS_441_0401_16HS_012, the datasheet (attached) specifies 36V supply voltage, 0.6amp/phase and 6.6 ohm/phase. But for my previous experiments, 12V worked fine. Now even if I supply 12V, it doesn't work properly again.

    Some additional info:
    1. I'm sending a sequence of signals: 1010, 0110, 0101, 1001 from BasicX to L298N
    2. the circuit is similar to that I've attached.
    3. when driving at a high voltage, say 20V, the power source jumps from 'constant voltage' to 'constant current' mode at the end of each 4-step repetition.

    Thanks a lot in advance!
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Looks like at a particular step, the power supply is being shorted out.

    Check the motor wiring.
     
  3. hawkoli1987

    Thread Starter New Member

    Oct 7, 2009
    4
    0
    Thanks JDT, what do you think is the possible reason for the short at this particular step? One of the switches in the H-bridge is burnt?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I see that you're using 1 Ohm resistors for current sense A and B, but you're not doing anything with the sense voltage. This means that if you're supplying 36v to the L298, with the winding resistance being 6.6 Ohms and Rsense A/B being 1 Ohm, your maximum current per winding is 36v/(6.6+1) = 36/7.6 = 4.74 Amperes.

    If you don't wish to use the PIC's onboard comparators or ADC's to monitor the Rsense voltages (which would take up most of it's processing power), you can use an L297 IC to control the L298 bridge. The L297 has a built-in chopper drive to take care of the current limiting for you.

    You have omitted the reverse-EMF protection diodes from the outputs of the L298. These need to be fast-recovery type diodes. When the current flow from the L298 is turned off, the windings of the stepper motor will generate very high reverse voltages across the bridge, which will sooner or later (probably sooner) destroy it. I'm betting that your L298 has been "zapped".

    See the datasheet for an L297 for how to connect the two IC's, and how the output protection diodes need to be connected.

    Note that the L297 datasheet recommends 0.5 Ohm resistors to be used for Rsense1 and Rsense2. What it doesn't say is that these resistors should be non-inductive. This means that you should not use the typically available wire-wound resistors, unless they are specifically non-inductive. Ohmite makes such non-inductive wire wound resistors; Digikey stocks them.
    Here's a 2-Watt resistor: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=WNCR50FECT-ND
    Here's a 1-Watt resistor: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=WNBR50FECT-ND
     
    Last edited: Oct 7, 2009
  5. hawkoli1987

    Thread Starter New Member

    Oct 7, 2009
    4
    0
    Hi SgtWookie,

    I've made a major modification of my schematics by adding a L297 in the circuit. I followed schematics of the L298N datasheet(attached), but I have a couple of questions which I want to clarify before I make the first try:

    1. If I were to use the normal available type of resistor instead of non-inductive ones, what will be the consequences?
    2. If I were to use normal 1N4001 instead of fast 2A diodes, what will be the consequences? (The fast diodes I got is too big to be mounted on the breadboard.)
    3. Only the direction and speed of the stepper motor is my concern. Therefore, I just connect 'clock' and 'CW/CCW' to my microprocessor; connect 'half/full', 'reset' & 'enable' to 5V and leave 'Vref', 'control', 'sycn' & 'home' floating. Is it correct?
    4. Could you explain to me the function of 'reset', 'Vref', 'control', 'sycn' & 'home' if time permits?
    5. Since my motor (RS_441_0401_16HS_012) has specified 36V, but also 6.6Ohm and 0.6A per phase. The L298N datasheet here also suggested 36V. So after I add the L297, what voltage should I input for L298N?

    Thanks a lot for the help!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    It is preferable that you omit names in your reply, as that may tend to discourage other knowledgeable members from giving you a response.

    If you use inductive wire-wound resistors, the EMF spikes from them may wind up destroying the L297. This is why I specifically warned against using wire-wound resistors. Carbon, carbon-film (thick or thin), metal, are OK - but not wire-wound unless it is specified that they are non-inductive.
    Poor performance right up to the point where the diodes burn up. :eek:
    No. Vref tells the L297 when to switch current on and off in the L298.

    You must provide a voltage to Vref. You can use a resistive divider to provide the voltage to Vref. Vref must be less than 3.5v.

    You want 0.6A per phase, maximum; that's what your stepper motors are rated for.
    Using Ohms' Law, if Rsense = 0.5 Ohms, then E=IR (Voltage = Current x Resistance = 0.6A x 0.5 Ohms = 0.3 Volts.
    You will need to supply Vref with 0.3 Volts.
    SYNC is explained on page 3 of the ST Microelectronics datasheet for the L297. Leave it disconnected.
    CONTROL is explained on page 3 of the ST Microelectronics datasheet for the L297. If you don't want to use it, ground it.
    RESET is explained on page 4 of the ST Microelectronics datasheet for the L297. On power-up, it should be set to logic 0 momentarily, then set to logic high for operation.
    HOME is explained on page 3 of the ST Microelectronics datasheet for the L297. It is an output that does not have to be used, but may be useful during initialization. If you wish to use the HOME output, connect it to Vcc using a 10k resistor. If a RESET has just been issued or the L297 is in the initialized state, you will see Vcc (logic high) on the HOME pin, otherwise logic zero.
    Let's see... the motor is rated for 36v ... the datasheet says 36v ... what to do, what to do... :confused:
     
  7. hawkoli1987

    Thread Starter New Member

    Oct 7, 2009
    4
    0
    Many thanks for such detailed answers. Sorry for not doing enough homework before coming to bother you.

    Anyway, I still have a couple of questions left:

    1. I don't understand the 'control' description in the datasheet, but some other people suggests that when stepper is driven fast we should high 'control', and low it when it is driven slow. Is it correct? If so, what is the speed considered as fast?

    2. My motor specified 36V wroks well at 10V, (except for both the driver and motor are heated up to a temperature of 60C) what is the use of supplying such high voltage? Does it give a larger torque?

    3. I saw many other designs uses UCN5804 to drive unipolar motor. Is that one a chopper-driver? I wonder what's the advantage of using a chopper like L297/L298N combo when it's circuit are so much more complicated than drivers like UCN5804?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    When CONTROL is low, it will act on INH1\ and INH2\ to control the chopper drive. If you want to use INH1\ and INH2\ independently (say, control by your uC) then place a logic 1 on the CONTROL input, otherwise a logic 0 on the CONTROL input.

    That's how I read it, anyway.

    Using the higher voltage helps a great deal to get the current flowing through the motor's windings. Once it's reached the set current (via comparing the voltage across Rsense with Vref) the chopper turns the current supply to the motor off, until the voltage across Rsense drops below Vref; then turns it back on again until it exceeds Vref. The process is repeated continuously.
    The UCN58xx series are simply latching drivers. They only sink current; they cannot source it like an H-bridge. You can't drive a bipolar stepper motor using four current sinks, unless you can use it in a unipolar configuration.

    Bipolar stepper motors are more complex to control because they require a dual H-bridge. Because of this, many hobbyists stick with the far easier to control unipolar stepper motors. However, the penalty for simplicity is lower torque and speed for a given number of motor windings.
     
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