Hello Guest it is October 14, 2019, 07:48:56 PM

Author Topic: Steppers are too slow  (Read 33301 times)

0 Members and 1 Guest are viewing this topic.

Re: Steppers are too slow
« Reply #40 on: December 06, 2011, 06:00:13 AM »
Moving back to Stuart's issues, (Thanks for your reply BTW Sargon). I was just raising a question about his current readings back in post #24. If these readings are correct then something's very wrong. At standstill, the total coil currents should be 4*3.32A*2/3 = 8.85A NOT the 3.615A Stuart records. HOWEVER if Mariss's statement that a chopper drive "draws" current at 20KHz from the PS is correct then I agree with Sargon's comment about using a multimeter to try to read this is not going to give useful results. So are Stuart's motors being current starved - who knows? IF IF IF there was a cap in there we could read the steady DC between the PS and the cap and get a more meaningful reading but I've learned here that I was wrong and that apparantly caps on switched supplies is not advised so..... just glad I use purpose built unregulated power supplies with hunky caps (and after tossing a coin - no dc fuses  ;D).

To figure this out lets take a look at how multimeters (which would be the same as a typical in-line ammeter) work, as well as DC clamp meters. The theory of operation will give us a good clue about what will work in this situation. Time to redeem myself from my earlier stupidity!

For a DC ammeter (or multimeter in DC current mode), the idea is to use a shunt to measure the current. Basically, we have two paths for the current to flow through - one main bypass to move most of the current around the metering device, and a low current shunt to measure the current. This method is actually going to measure the voltage drop across a shunt resistance, and using the value of the shunt, calculate what the current should be. It's important to note that here we are actually measuring DC voltage. In addition to this, there are various ways the internal circuitry can be arranged - the shunt configuration differs from meter to meter so results in this situation will not be same with all meters, but in any case it should not be considered accurate. AC ammeters are built different, measuring an alternating magnetic field by using an iron core with input and output coils. This will not respond to a constant magnetic field, and in turn will not respond to constant current (DC). In short, you're not going to get a good reading on a combination AC/DC signal using either of these measuring techniques.

What will work is a DC clamp meter. These devices will detect the current by making use of the Hall effect. Essentially this type of meter will measure the strength of the magnetic field by creating current in a conductor within a chip that is arranged such that the conductor is at right angles to the magnetic field. The voltage generated by this Hall effect is directly proportional to the strength of the magnetic field and thus current can easily be calculated. It is important to note that this device is not dependant on a reversing magnetic field. It is only looking at the strength of the field. In addition, it will still respond to changes in the magnetic field (caused by an AC component or changes in the DC current) instantly, and therefore will also be able to measure any complex AC that is on the line, or in our case a fluctuating DC signal.

In short, if you want to measure the actual current draw your best bet will not be a multimeter or oscilloscope, but a DC clamp meter. This will give you, by far, the most accurate measurement. That being said, there is always more than 1 way to skin a cat, and there are likely other methods that would work, but this would be the easiest and should be very accurate.
« Last Edit: December 06, 2011, 06:13:12 AM by Sargon »

Offline stirling

*
  • *
  •  2,188 2,188
  • UK
    • View Profile
    • www.razordance.co.uk
Re: Steppers are too slow
« Reply #41 on: December 06, 2011, 06:42:10 AM »
Thanks for this Sargon - definitely sounds the way to go for Stuart to really find out what's going on.

Ian
Re: Steppers are too slow
« Reply #42 on: December 06, 2011, 04:17:36 PM »
For a DC ammeter (or multimeter in DC current mode), the idea is to use a shunt to measure the current. Basically, we have two paths for the current to flow through - one main bypass to move most of the current around the metering device, and a low current shunt to measure the current. This method is actually going to measure the voltage drop across a shunt resistance, and using the value of the shunt, calculate what the current should be. It's important to note that here we are actually measuring DC voltage. In addition to this, there are various ways the internal circuitry can be arranged - the shunt configuration differs from meter to meter so results in this situation will not be same with all meters, but in any case it should not be considered accurate.

Never seen a current meter work as you describe.  A shunt is nothing but a VERY small resistance, on the order of milli-ohms, typically provided by a simple strap of brass or copper, necked down at one point, and trimmed at the neck to provide the required accurate, small resistance.  ALL of the current flows through the shunt, and its resistance creates a small voltage, which is displayed on a moving-coil meter movement with, typically, 50mV full-scale sensitivity.  A DVM works in exactly the same way, except measures the voltage with an A/D converter, rather than an analog meter movement.

Regards,
Ray L.
Regards,
Ray L.
Re: Steppers are too slow
« Reply #43 on: December 06, 2011, 04:50:52 PM »
I'm not sure I understand the question. I don't think I said a chopper would "limit" the current to less than the max setting. That doesn't mean the stepper will always draw maximum current.
This and your earlier comment that load affects current is what I don't get. As I understand it, the chopper by it's very nature is monitoring the current through the motor. ONLY when the current through the motor reaches the set point will the chopper start to chop. (There will of course come the time with motor speed when the voltage simply can't drive the required current because of inductance and ultimately the motor will stall but that's not what we're talking about here). I can see no reason how external mechanical load can affect the current through the motor. Servos yes but steppers no. Maybe I'm wrong.

Ian

That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.  At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance .  The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.

Regards,
Ray L.
Regards,
Ray L.

Offline stirling

*
  • *
  •  2,188 2,188
  • UK
    • View Profile
    • www.razordance.co.uk
Re: Steppers are too slow
« Reply #44 on: December 07, 2011, 06:39:01 AM »
That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.  At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance .  The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.

Ray - I'm not sure where your reply contradicts anything I said. If you check out the attached pic I think I described it adequately for the purposes of the thread. However if we're being Mr. Picky...  ;D

Current limiting will prematurely turn off the output current
Actually current limiting turns off the output VOLTAGE. current decays as a result.

because the current is not turned on long enough to saturate the coil, due to the coil inductance

As above, it's because the VOLTAGE is not turned on long enough to allow the current to rise to saturation because of the effects of inductance. We use a larger than rated VOLTAGE to "combat" inductance by reducing the rise time of the current and hence why we need current LIMITING in the first place.

But enough of this merry banter... now to the bit in your reply which is really pertinent to the point we were discussing... (please read post #33)

What I was asking was... please explain how the LOAD on the stepper motor affects the coil current? and in particular because you've said it here... please explain how a LOW load would result in a lower coil current. You may well be correct - as I said earlier in the thread I've had this stated to me before but no one has ever explained (or been able to explain) why. Personally I don't get it - and that's why I'd LOVE to have someone explain this.

Cheers

Ian
Re: Steppers are too slow
« Reply #45 on: December 07, 2011, 06:54:28 AM »
That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.

I certainly didn't mean to say that choppers don't switch the power at all times. Thanks for pointing that out so others don't get the wrong idea.
Re: Steppers are too slow
« Reply #46 on: December 07, 2011, 07:36:18 AM »
Never seen a current meter work as you describe.  A shunt is nothing but a VERY small resistance, on the order of milli-ohms, typically provided by a simple strap of brass or copper, necked down at one point, and trimmed at the neck to provide the required accurate, small resistance.  ALL of the current flows through the shunt, and its resistance creates a small voltage, which is displayed on a moving-coil meter movement with, typically, 50mV full-scale sensitivity.  A DVM works in exactly the same way, except measures the voltage with an A/D converter, rather than an analog meter movement.

Sorry, I was actually thinking about an alternative measuring method when describing the shunt configuration and it wasn't complete by any stretch of the imagination - you are right about a typical ammeter having the shunt in series with ALL of the current. That being said the main point I was focusing on is that a DC ammeter samples DC voltage to infer the current flow, and cannot provide an accurate reading for this type of power signal.

If you want to find out how much current is being delivered to your motors via chopper circuitry (ie most stepper drivers), the DC Clamp Meter is the way to go - and the only reasonable option that I'm aware of - because it makes use of the Hall effect which responds to DC, AC and pulsed DC, as well as complex combinations. That's the point I was trying to make.

Re: Steppers are too slow
« Reply #47 on: December 07, 2011, 11:30:29 AM »
That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.  At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance .  The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.

Ray - I'm not sure where your reply contradicts anything I said. If you check out the attached pic I think I described it adequately for the purposes of the thread. However if we're being Mr. Picky...  ;D

Current limiting will prematurely turn off the output current
Actually current limiting turns off the output VOLTAGE. current decays as a result.

because the current is not turned on long enough to saturate the coil, due to the coil inductance

As above, it's because the VOLTAGE is not turned on long enough to allow the current to rise to saturation because of the effects of inductance. We use a larger than rated VOLTAGE to "combat" inductance by reducing the rise time of the current and hence why we need current LIMITING in the first place.

But enough of this merry banter... now to the bit in your reply which is really pertinent to the point we were discussing... (please read post #33)

What I was asking was... please explain how the LOAD on the stepper motor affects the coil current? and in particular because you've said it here... please explain how a LOW load would result in a lower coil current. You may well be correct - as I said earlier in the thread I've had this stated to me before but no one has ever explained (or been able to explain) why. Personally I don't get it - and that's why I'd LOVE to have someone explain this.

Cheers

Ian

Your original post said:  "ONLY when the current through the motor reaches the set point will the chopper start to chop."  This is not necessarily correct, depending on exactly how the PWM is implemented.  The waveform you provided is correct for some PWM power supplies, but NOT necessarily for a PWM stepper motor drive running at a low duty cycle.  As I said, the H-bridge will be switched on and off at a fixed rate, based on the commanded duty cycle that will vary from near zero, to near 100% (again, sinusoidally, if micro-stepping is used).  Each pulse can be shortened if, and ONLY if, the corresponding coil reaches the limit current.  This will likely NOT happen at low duty cycles, as the pulse is not long enough for the coil to saturate.  At some greater duty cycle, limiting will start of occur, but that may not be until the duty cycle approaches 100%, heavily dependant upon the motor inductance, and voltage used.  The whole point of the chopper, as I said, is to modulate the RMS voltage across the coils, which implies a corresponding modulation of the RMS current.  The actual current is not directly controlled (beyond simple peak limiting).  If it operated as you suggest, the ripple in the coil current would be running essentially full-scale all the time, which would do some really serious heating in the motor.  The PWM is there to allow the current to be modulated, in part to reduce ripple, by lowering the peak currents.

For a stepper, I don't believe motor load does affect current, unless there is some small effect related to back-EMF/slip angle, but I'm not sure about that.  It's been a loooooong time since I studied the details of the operation of motors...

Regards,
Ray L.
Regards,
Ray L.

Offline stirling

*
  • *
  •  2,188 2,188
  • UK
    • View Profile
    • www.razordance.co.uk
Re: Steppers are too slow
« Reply #48 on: December 07, 2011, 12:44:25 PM »
Ray - if only I'd said "ONLY when the current through the motor reaches the set point will the chopper start to chop. drive start to limit the current" I really must be more careful. But in the context of the thread surely to goodness it's clear what I meant. It was never about how a chopper works. (have you read the thread?)

The discussion had reached a point where we were discussing whether LOAD affects coil current. I stated... well I guess you'll read it if you're interested.

But then Ray YOU actually re-stated the EXACT point of where we'd got to when you said...

At low pulsewidths, and/or low loads, you'll never reach the limit

I thought EUREKA!  - Ray knows the answer we've been searching for - we're about to get somewhere. BUT you then said in your last post...

For a stepper, I don't believe motor load does affect current, unless there is some small effect related to back-EMF/slip angle, but I'm not sure about that.  It's been a loooooong time since I studied the details of the operation of motors...

So it appears after all this we've made zilch progress - marvelous....

Cheers

Ian
Re: Steppers are too slow
« Reply #49 on: December 08, 2011, 03:06:22 PM »
Gents,

Hope I haven't started a forum war here, but I still have a bit of a problem understanding why a good quality Digital multimeter connected in line with the PS output to the drivers fails to read an accurate current flow compared to a hall effect clamp meter. When i look at the chopwave graph Ian posted at post 44, the current trace has fairly minimal peaks and troughs associated with the switch activity. I read this as the current flow through the coil and not necessarily a waveform of the driver input supply. I would have expected the circuitry within the driver would have provided some smoothing to the input current. Additionally with 4 drivers all connected together I would have expected some further smoothing of this wave form as one goes high and the other goes low.

Is my thinking correct here??

Stuart