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The power supply design PDF said 20 times the rated voltage on the motor. I'm designing a stepper controller myself, and I'm wondering if the geckos have some sort of power supply in them that drops that down or if that 20x the rated voltage is really what the motors see.
The motors do see that voltage, but not constantly. The current to the motors is constantly switched on and off at a frequency up to 20 khz(pwm:pulse width modulation).
Because the coil of a stepper motor reacts slow on current changes, the average curent will be the motor rated current. The high voltage is needed for good performance at higher speeds.
When you are running at high stepfrequencies with a low voltage the current in the coil of the stepper wont have the chance to reach its nominal value before the current is switched off to energize the next coil (high step freq.).
To get a full understanding of this I sugest that you have a look at the following text:
http://www.cs.uiowa.edu/~jones/step/
The detail behind this is complicated, however without going into the detail... the windings are inductive. If you apply a constant voltage across them, the current ramps up exponentially from zero to full current over a period of time. This time period reduces the maximum power you can get into the coils, because time is wasted getting up to the rated current. However if you can apply a higher voltage, this ramp up occurs faster.
With higher voltage, the current can go too high, so current limiting is required. This is where the power is PWM'd to coil, which is achieved by sensing the current going through the coil.
Higher voltage means higher speed. Higher voltage does not necessarily mean more torque though. As slow speeds, it's easy to get the power into the coil and achieve maximum current throughput. However as speed increases, a lower voltage supply cannot get the current into the coil and it stops PWMing the output and runs in voltage controlled mode. So higher voltage doesn't change low speed torque, only the maximum torque at faster turn speeds. If you go too high in voltage, it becomes difficult to manage the PWMing to the coil, because the duty is so low.
Gecko controllers are very sophisticated, as they also incorporate mid-band resonance damping, which prevents the stepper from stalling at high speeds... morphing to full step when required.
I understand all the theory (at least to a certain point) and quite experienced with electronics. I was so impressed with the Gecko G203V's, I dropped my owm design in progress and placed an order.
The big volts are to overcome the inductance of the stepper coils. The inductance resists when you try to reverse the direction the current flows in to get a step. More volts overcomes the inductance faster, if you didn't have the big volts you wouldn't get any kind of speed out of it.
The motor rated voltage is simply the volts you would put across the coil to get the required amps through the coil. Amps=volts/resistance.
To use big volts you need stepper drivers that limit the amps to something the motor can handle without overheating and eventually burning out.
All motors are designed with a power factor rating usually expressed as 0.8pf . You can usually find it on the metal ident plate. The general idea thats simple to understand, is if you run a motor at 80% of its power rating, it will live longer, and not overheat and go into meltdown. Also an electric motor reaches it full potential conversion factor at 80% or its rating, and is able to take sudden surges without overlaoding. The stepper coils allow full voltage to be applied to steppers and servos while limiting current. S0 you want a motor 20% bigger not 20 times, than your controller coils limit. eg. 5 amp coils, 6amp motor.
So if I add a shunt resistor to do current sensing and regulate the current within the range of the motor with pwm I should be okay?
Maybe, it's more complicated than if your doing a bipolar driver and want to do microstepping. Decay modes become important and need to be addressed.Quote:
Originally Posted by sp1nm0nkey
As was addressed by others the reason for higher voltage is to increase the speed attainable by the stepper. If you look at a stepper motors torque verses step rate, you will see it's torque drops significantly as step speed increases. As you increase the motor supply voltage, the rate of drop slows.
Sorry, I'm somewhat of an electronics newbie... I know this may be a bit ambitious. Anyway, what are decay modes and how do I account for them? I wasn't planning on doing micro stepping, but if I did I figured maybe I'd just check the current when one coil is full on (100% duty cycle), and then chop the supply voltage at high frequency based on the sensed current to get it within the motor's range.
I finished a simple stepper driver using tip122s and totally noticed how as the speed went up the current draw went down and the torque went straight down. I guess it just gets complicated sensing current when speeds change fast (can't integrate...) and pulses are short.
Sounds good, you'd probably get nigh on 2/3 the top speed of a Gecko and you'd have complete control of the coils from your software.Quote:
Originally Posted by sp1nm0nkey
I went the commercial driver route when I had a mains voltage surge and all 3 axes failed the smoke test simultaneously :eek:
If I was determined to do it myself again I might rummage that ghastly auction site for some of these...
http://www.national.com/mpf/LM/LMD18245.html :D
Oh hey... http://www.stepperworld.com/Tutorial...rostepping.htm
That was a huge help.
I think I'll just have a comparator on a shunt between the steppers and ground and a pot connected to a reference. I'll put a buffer between the fets and the uC and switch the buffer with the comparator. Quick and easy fast decay. It may be loud and inefficient but it should work :D
My first attempt turned the power on when the clock went low then turned it off when the it reached the desired power. My second attempt used D type flops, I clocked them at 20kHz and put the output from the comparators on the data in lines. That way everything had a chance to settle down before I read the sense resistors and I no longer suffered from transients.
An old style moving coil meter is a real bonus when it comes to adjusting the current, these new fangled digital meters can get really confused when the line is clocking :D
decay modes are not as simple as some literature put it. an oscilloscope is a good help when playing with them. on my previous job, I was designing DC to AC inverters and it's hard to debug hardware problems without a scope.
I am using PWM controllers (as they already have much of the needed hardware) and HIP4081 when designing my own controllers.
the supply voltage can actually go anywhere from 5x to 20x or even more. they are rough estimates on what can give you good performance on your motor. you can even use a non-regulated supply, but performance will vary depending on current consumption of the load. if you are already using the controller, try to measure (full rated current on stopped motor) the winding voltage and you will see the rated voltage of the motor.
using LC filters before the motor windings sometimes helps in reducing motor heating. they reduce HF AC current into the winding thus reducing eddy currents.
Doesn't this kind of defeat the purpose of using a high voltage supply? The reason for using a high voltage is to gain more rapid change in current. An LC filter is going to reduce the rate of current change.Quote:
using LC filters before the motor windings sometimes helps in reducing motor heating. they reduce HF AC current into the winding thus reducing eddy currents.
the LC filter is much smaller compared to the inductance of the winding. Unitrode published an old application note about doing so (I believe it's for UC3717A Stepper Motor Driver).Quote:
Originally Posted by jeffs555
the LC filter does reduce rate of current change but not that much if sized properly. I've done this to old stepper motors and heat generated while stopped and enabled is significantly less. the filters would be a little pain though if you have to do microstepping.
there are also other ways to make the winding see less HF AC ripple, mostly in the methods of recirculation.
Perhaps the LC was for a snubber network, to dissipate the back EMF spikes and prevent them from being absorbed by the motor.
I've been using shunt resistors to limit the current to the stepper motors and have had good success. It waste power but it was a quick fix. I would find chopper power supply's from surplus houses most of them were 5 volts at 100 amps so I would find 3 and series them for a total voltage of 15 volts and then get a heavy duty series resistor normally it was wire wound to handle the current and place that in series to limit the max current seen by the stepper motor. My stepper motors were quite large to power my 3hp router head. (www.brokenhammer.com)Quote:
Originally Posted by sp1nm0nkey
I just have not had time to detail my setup but you can see pictures of the control circuitry on brokenhammer.com
good luck and keep on experimenting.
My next project on Brokenhammer will be a cnc plasma cutter or maybe the humvee project . ha
virg
Sorry just could not help noticing your icon. I competed back in the 70s at an autocross with my formula v that I had designed against an austin healy that had a 327 in it with formula racing tires. Needless to say he captured fastest time of the day. ha www.brokenhammer.comQuote:
Originally Posted by jeffs555
Virg
hi guys, wouldn't the higher voltage increase the EMI and heat up the motor due to the higher eddy current?
After 2 hours of running the motors would be warm but not hot. It's choosing the right shunt resistor. You want the motors to step gingerly and not stall out and not exceed the specification of the motor. The big fear is damaging the permanent magnets in these old steppers by creating to much magnetic flux. So choose the right shunt resistor. I'm not saying this is right. It's just a poor man's way of experimenting.
I did have a smaller motor for my z axis and just used the same shunt resistor that I had used for the larger x,y motors. It got very hot and I just added heatsinks to the outside of the case. The motor (cross your fingers) is still working today. What I mean by hot (never measured it) was that I could hold my hand on it for about 4-5 seconds before it became uncomfortable. Not very scientific is it. ha
Take care and keep experimenting
I used a cap and large shunting diode at the motor to protect my mosfet drivers.
motor heating may be caused by the eddy currents mainly caused by the ripples. as what Unitrode suggested in their application note, a small LC filter will reduce EMI and lower the ripple seen by the motor (I believe Gecko is using another scheme to lower ripples but not filters) thus reducing heating.
EMI would be caused by higher rate of current rise. filters may help diminish them. the best way to reduce them is using common mode chokes on the wires to the stepper motor. I also put the drivers as near as possible to the motor so that the long wires may not act as antenna.