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Author Topic: Steppers are too slow  (Read 33357 times)

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Offline stirling

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Re: Steppers are too slow
« Reply #20 on: December 03, 2011, 09:41:47 AM »
Hi Stuart

Glad things are improving - of course without knowing exactly what's changed it's difficult to comment. It seems that you do indeed have a port/BOB problem on a couple of pins but let's leave that aside for the moment and concentrate on the fact that you have 4 motors running so obviously the other pins are good.

I have a few suggestions. Some I know to be correct, some I think are correct. I'd be grateful for comments where folks disagree.

You say you measured 3.5A. This doesn't sound right. Where exactly did you measure this? If you have each of your drivers set to limit current at 3A (which for your motors you should have) then I would expect the total current supplied from your PS to be around 8A at 24Vdc for the 4 motors.

Am I correct in thinking you appear to have a switched mode power supply? Mariss at Gecko used to recommend adding a capacitor at the output of a switched supply (like you automatically would have in an unregulated supply) but this advice seems to have been removed from their updated stepper guide so I'm not sure if this still holds but it makes sense to me at least.

From your pictures it looks as if you have a mixture of star wiring and daisychain wiring from your PS to your drivers. I'd suggest you make it all star wiring. Again see the excellent blurb on driving steppers on the gecko site.

I see your supply is actually 17A at 24V - wow that is some overkill - shame it wasn't 8.5A at 48V  ::).

Just for clarification, don't worry about measuring current with or without mechanical load on your steppers - it makes no difference to a stepper.

Anyway - just a few thoughts.

Ian
Re: Steppers are too slow
« Reply #21 on: December 04, 2011, 05:09:15 AM »
Hi Ian,
I am fairly confident that my port outage is related to the BOB as I use the same parallel lead from the computer to another combined BOB/ Driver for 3 axis and I am pretty sure this board uses those pins from the computer and it works fine. I am away again with work this weekend but shall be home in the morning and shall confirm then.

I have to admit, the 3.5 amp current rating was taken from a fairly crude analog ammeter fitted internally to another bench top power supply I was trying at the time whilst somebody else manually jogged the 4 motors. I have never really considered how accurate this meter is. I can easily connect my digital multimeter in line with the 4 drivers tomorrow and post an accurate reading. The drivers are set to 3.32 amps from memory.

I have not been made aware of needing a cap fitted across the terminals of the PS. I would be interested to know the theory why. Was the intent to further smooth the output of the DC supply or for some form of protection of the PS from damage.

I have been trying to work out what may have been the cause of my problems as well, and you have just had me thinking about a change I made during the rewire with one of your comments. You will notice the power supply has 3 sets of 24 Volt connections. The way I had it connected before was like this; one set of terminals to 2 drivers, another set to the other 2 drivers and the 3rd set connected to a small printed circuit board with 3 voltage regulators and a couple of filter caps on it. Two of these regulators provide a very stable dual supply of 5 volts to the BOB (The BOB has a couple of jumpers to remove from the board if running dual 5V supply) and the other is a 12 volt regulator to run the cooling fan located in the end of the duct. When I did the rewire I isolated the circuit board with the regulators and provided the 5V to the BOB with a separate single plug pack totally isolated from the PS, and replaced the jumpers. The other change I made was to connect all four drivers to the same PS terminals. (and you are correct, this is done by having 2 sets in series connection joined in parallel at the PS. I can change that to parallel for all 4 without any problem). The other test I will make tomorrow is to see if the three 24 V positive and negative terminals are actually connected internally in the PS. (testing for continuity between terminals with an ohm meter) If they are not, it may be possible to series connect 2 of these to get the preferred 48 volts. Will let you know what I find.

Thanks for your feedback Ian, much appreciated.

Stuart

Offline stirling

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Re: Steppers are too slow
« Reply #22 on: December 04, 2011, 06:21:20 AM »
I have not been made aware of needing a cap fitted across the terminals of the PS. I would be interested to know the theory why. Was the intent to further smooth the output of the DC supply or for some form of protection of the PS from damage.
More savvy folks than me can probably explain it better but as I understand it a chopper drive "draws" PS current at a frequency in the order of around 20KHz. The large cap provides a "reservoir" or "flywheel" for this current "draw". I tend to go on whatever Gecko says and it's allways served me well. Here's the quote from their "original" stepper document.

Quote
The drive works best with unregulated power supplies though regulated linear and switching power supplies may also be used. What matters is the power supply must have a large output capacitor and an unregulated supply intrinsically has one.
(I allways use unregulated supplies)

The other change I made was to connect all four drivers to the same PS terminals. (and you are correct, this is done by having 2 sets in series connection joined in parallel at the PS. I can change that to parallel for all 4 without any problem).
Don't confuse series/parallel with star/daisychain wiring, they're different things. At the moment (from your picture) you have the drivers (in parallel as they should be) BUT divided into two pairs where each PAIR is STAR wired BUT the two WITHIN each pair are daisychained. You need to STAR wire from your PS to each driver separately.

Ian

Offline RICH

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Re: Steppers are too slow
« Reply #23 on: December 04, 2011, 08:23:06 AM »
needing a cap fitted across the terminals of the PS
The cap provides filtering in an unregulated power supply and additionaly acts as holding tank of energy ( sort of like a water tower which feeds the town with water) thus even with a changing demand it has volume to provide constant voltage. A regulated supply as a unit does all this with circuitry to convert ac to dc, filter,maintain the voltage at some current.
I am going to guess that even though a number of terminals are provided they come from a single end source of the ps's output .

Frankly i am not sure on using a cap on the output of the regulated ps since it's electronics already provide the function of the cap. It may actualy affect that ps. Usualy ps will provide a data sheet and additional instructions / information on their use ( which you have not provided ). So i will withhold comment.  Relative to amperage, there is probably a max running amp and also a surge rating ( when it's first turned on) so when measuring you would see the current draw changing. Your ac circuit should address the surge ie; 1.5 to 2x of running
or max surge , but, irrelevant should satisfy local authority / code on it's use

I am sure the electrical guys in here can provide more insight,
FWIW,

RICH
Re: Steppers are too slow
« Reply #24 on: December 05, 2011, 04:50:46 AM »
Hi Ian and Rich,
I understand your reasons for the capacitor across the supply terminals without much problem. I will give that one a bit of thought before fitting one though. The fact that the PS will hold a very stable output voltage leaves me to think that it already does contain capacitors internally as part of the regulation process. Shall try to get some sense from the manufacturer and seek their advice.

Tests today result in the following:

Rich, you are quite correct. There is no way that PS is going to deliver much more than 24 volts. All negative terminals are common as are the positive terminals. The extra terminals just allow for extra terminal space to connect to.

The current draw was quite a surprise. All readings were taken by placing a digital multimeter in line with the 4 drivers and the power supply. ie. disconnected the +ve supply to the drivers and inserted the meter between the PS +ve terminal and the +ve leads to the drivers. The results were: all four stationary  3.615 amps. 1 motor running  2.911 amps, 2 motors running  2.343 amps, 3 motors running  1.726 amps and with all 4 running it was 1.177 amps. I guess this makes good sense as I would expect the current draw to be highest whilst the motor is not able to rotate. I have the ability to hold the peak reading on my meter during the start up, but didn't take it on this occasion. I can do if you are interested in the result.

Ian, I understand your supply star connection. I will run 4 separate figure 8 cables from the PS terminals, with each one dedicated to each individual driver.

Today, I have tried a few other "step per mm" settings and found it to operate quite successfully throughout the range, so hopefully all should be OK now.

Stuart
Re: Steppers are too slow
« Reply #25 on: December 05, 2011, 05:38:34 AM »
Hi Ian and Rich,
I understand your reasons for the capacitor across the supply terminals without much problem. I will give that one a bit of thought before fitting one though. The fact that the PS will hold a very stable output voltage leaves me to think that it already does contain capacitors internally as part of the regulation process. Shall try to get some sense from the manufacturer and seek their advice.

You cannot determine the stability of a supply with a meter. Meters, especially digital meters, tend to average out the voltage, and you will be unable to detect spikes unless they are quite drastic. A decent oscilloscope will give you a much more detailed and accurate idea of what is happening. Generally speaking, switched supplies almost always have relatively low capacitance (ability to store the charge and resist voltage changes). Note that the important difference in supplies in this application is not regulated vs unregulated, but switched mode (using chips - ie a chopper circuit) vs linear (transformer, bridge and caps). A regulated linear supply may also have additional circuitry to limit current and possibly other protection features. The problem with using switched mode is that you are driving an inductive load which by it's nature is trying to maintain constant current - therefore it will fight the supply, which is trying to maintain constant voltage (inductors want constant current, capacitors want constant voltage).

I personally haven't ever heard of adding capacitance causing a problem, but that being said I haven't needed to add any to my machines either (48V switched mode supplies). I am, however, planning to eventually replace them all with linear supplies (much, much better for inductive loads, like motors). Switched supplies are much cheaper to produce (cost is in copper and winding the large transformer) and excel at driving non-inductive loads, such as electronics (ie computers).

The current draw was quite a surprise. All readings were taken by placing a digital multimeter in line with the 4 drivers and the power supply. ie. disconnected the +ve supply to the drivers and inserted the meter between the PS +ve terminal and the +ve leads to the drivers. The results were: all four stationary  3.615 amps. 1 motor running  2.911 amps, 2 motors running  2.343 amps, 3 motors running  1.726 amps and with all 4 running it was 1.177 amps. I guess this makes good sense as I would expect the current draw to be highest whilst the motor is not able to rotate. I have the ability to hold the peak reading on my meter during the start up, but didn't take it on this occasion. I can do if you are interested in the result.

This is indeed quite expected. You will probably also find the current drop when you provide a load to the motors - it changes the slip angle in the stepper and results in better power coupling within the motor, reducing current requirements. Creating torque without moving is much more difficult for the motor to maintain.

Offline RICH

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Re: Steppers are too slow
« Reply #26 on: December 05, 2011, 06:02:31 AM »
Just to clarify, the surge i mention was on the upstream  side of the ps and not on the downstream side ( ie; feed to the stepper).
Like a motor, when running the circuit may be fine, but on motor start-up the surge will be much greater and thus you need a higher rated circuit.

RICH
Re: Steppers are too slow
« Reply #27 on: December 05, 2011, 06:23:35 AM »
Thanks Rich,
I can measure that, but I think a few of my other workshop toys give the power meter a bit of a tickle in the ribs more so than this at the moment.

But I'll take your comment on board, thanks.

Stuart

Offline stirling

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Re: Steppers are too slow
« Reply #28 on: December 05, 2011, 06:47:36 AM »
This is indeed quite expected. You will probably also find the current drop when you provide a load to the motors - it changes the slip angle in the stepper and results in better power coupling within the motor, reducing current requirements. Creating torque without moving is much more difficult for the motor to maintain.
Hi Sargon - can you explain this a bit more please. I just don't get how a current limiting device (a chopper in this case) would LIMIT current to LESS than it was set to. (Not to be confused with not being ABLE to drive the "set" current as the motor approaches stall speed).

Ian
« Last Edit: December 05, 2011, 07:24:38 AM by stirling »

Offline Tweakie.CNC

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Re: Steppers are too slow
« Reply #29 on: December 05, 2011, 07:37:42 AM »
Just a note Guys,

Do not fit a large capacitor across the output of a switched mode power supply it affects the sense response.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.