Thread: Gecko G540 - local source
Well i didnt get away with the tax so thats another £43 to add to the £189 for the Gecko , so if id checked around and wanted the hassel of conecting drivers and break out board up , the price wouldnt have been much differant than buying the gear from Zap in the first place. As Gary says i dont think the performace will suffer as im going to use 48v 7.3 amp power supply , its just that youv,e paid for the motor holding torque and would like to use the full spec of the motors. Its for a mini mill so i think it will make enough speed for what i want ..... time will tell .
Remember that below the motors corner speed the torque is essentially flat except for resonances (which is where the gecko scores over other drivers which can show marked troughs in the torque at low revs). Above the corner speed the torque is inversely proportional to speed less the detent torque. A bigger motor will have a higher detent torque. the corner speed is related to inductance and drive volts and is independent of drive current. Once above the corner speed a smaller motor at higher voltage can produce more torque than a bigger motor driven at the same voltage as the bigger motor will often have higher inductances - this is despite the fact the bigger motor might have a higher holding torque.
The one particular stepper I'm using is Pacsci rated at 845 oz in at the rated current of 3.3 amps which is what I set my drive at. It is a 4 wire series wound and has a phase inductance of 8.3 mh.
Experts seem to all agree that low inductance is required for the best performance and I have found that this is not the case.
Unipolar drives are also said to be inferior, I have been using such a drive on a small lathe set up for a production contract and had no problems at all.
Also, I have never been able to get the type of rpms from a stepper that are discussed on forums either using high or low inductance motors, unipolar or bipolar, series, parallel. I'm talking rpms that that are actually useful to drive anything.
I have purchased high end drives and quality new steppers over the years and never been able to get they speeds I read about, then again i'm a microstepping fan.
The fact of the matter is that a higher inductance motor has a lower corner speed and therefore you are more likely to be in the downward torque slope region than the flat region at operating speeds. How that plays out depends a lot on the motor and what it being asked to do. Even two seemingly identically spec'd motors from different suppliers can behave very differently under load.
Having said that, you are right that there is often a big difference between the theoretical and the practical. It all depends on the situation so there is no 'one right answer'. I have some motors that according to their spec sheet should go to 900rpm. In practice I have never been able to get them above 300rpm even on a 48v supply and a range of drivers. I think a lot of people underestimate the impact of inertia and acceleration in their designs. Big leadscrews (20mm+ diameter) of any reasonable length have huge inertia and this is rarely factored in most designs leading to stalling and/or lost steps well below the revs that might have been expected. Of course, turning down the acceleration may allow you to creep up on those speeds... The other key issue is resonance. Most designs have no idea of their resonant frequency (its not easy to work out) and often a motor will stall out well below the expected revs simply because torque has vanished at that point and all the power is being absorbed by the resonance. Adding damping mass to the motor shaft can, non-intuitively, increase the maximum revs or sometimes lightly gearing the motor (1.1:1 say) to allow it to turn slightly slower can produce a better overall result without unduly affecting other parameters.
I have come to the conclusion that no matter what the drive, within reason, a look at stepper manufactures torque/speed curves is the most reliable indicator of what can be expected and they all look pretty much the same.
I have several Parker OEM 750 drives with many features yet they do not do any better that an inexpensive Allegro SLA 7044m based uniplolar drive I've used for several years.
I have not found steppers inaccurate or too slow, but I have never been interested in super fast G00's or air machining
Well in the end I opted for a local driver, coupled with some nice steppers and a home brew power supply. All in all a made to measure solution
In case anyone else stumbles upon this thread when searching for Gecko G540 info, here's an update to a previous post;
At the time of writing this (September 2011), the UK, (European?) distributors of Gecko gear, including the G540 is listed on the Gecko web site as being;
6 Hayland Industrial Units.
St Leonards on Sea.
East Sussex. UK. TN38 9NN.
Phone: +44 (0)1424 850660
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