G4 steppers hot

[rhfurniture]

I just got back to work from a short lunch, and found the stepper motors on my recent G4 conversion were pretty good and hot. I could touch them, but it got a bit uncomfortable after about 10 seconds - a bit like a too hot bath. With the old board, they barely got warm. They cooled down a bit once I got going again. Do I need to be alarmed?
Saves on heating bills I guess...
R.

[richards]

Check the current limiting resistors on the G202 drivers. Stepper motors can run hot. (They're rated at 80-degrees C or 176-degree F.) The steppers should have gone into reduced current mode a few seconds after motion stopped.

This is copied from the G202 manual at www.geckdrive.com:

The G202 will accommodate motor winding currents from 1 to 7A and 0.3 to 2A. Use the following equation to calculate the value, (in kilo-ohms) of the current set resistor:
R (in kilo-ohms) = 47 * I / (7 – I) or for the low current range, R = 47 * I / (2 – I)


Here's a chart showing the correct resistor for various currents:

Amps

[rhfurniture]

Mike, thank you for yor reply. I don't think we are quite hitting 80. I guess that when I have a quiet moment I could take my box down and see how shopbot have set it. ie low or high range (2 amps can use either), reducing mode or not - both are jumper options. Is there an advantage to low range?

R.

[richards]

It took a minute to find the reference in the Gecko G202 manual that describes the low range. Here is the quote:

(2) REDUCED CURRENT RANGE: In addition to the normal current range (1A to 7A), the G202 can also operate over a reduced current range (0.3A to 2A). This range is used for motor phase currents of less than 1A. Auto current reduction is not available for this reduced current range.

Because there is NO auto current reduction in the low range, I wouldn't use the low range with a size 34 motor.

[Brady Watson]

Mike,
The problem with using current reduction on a Gecko is that it kicks in too quick. It is NOT something that you want to have kick in while you are cutting a rectangle for instance. One axis is moving while the other is at rest...it could lose position depending on how much force is being exerted.

rh,
I wouldn't be too alarmed with the heat. My tool does the same thing when cutting heavy, fast or rastering over and over again for any appreciable length of time. I got zinged a few times on my forearm by the front X motor...lol!

-B

[richards]

Houston, we have a problem!

Brady, are you saying that the G4 is setup to NOT use automatic current reduction? If so, would it not be better to have an off-line axis constantly toggle +1 step and then -1 step until all axes were idle than to turn off current reduction? That +/1 one step would dither an axis 0.00157 in each direction given that you're using an ungeared stepper motor with a 20-tooth pinion gear - which has a 1-inch pitch diameter. The formula is (1 X 3.14) / 2,000 - 0.00157. Of couse, a geared motor would move that distance divided by the gear ratio. So, a motor with a 3.6:1 gearbox would dither +/- 0.00157 / 3.6 or 0.00044 inches in each direction. In any case, the dither would not be seen in normal woodworking. Worse case, the sum of the dither on an ungeared motor would be less than the thickness of a piece of copy paper.

It seems to me that the CNC control should handle the problem and not the stepper driver, particularly when it was known, before development of the modified control board, that the G202 did NOT have a programmable current reduction delay. (In other posts that I've made relating to the 'chatter' problem with the Alpha, I speculated that it was the current reduction mode on the Oriental Motors Alpha Stepper Driver that contributed to the 'chatter' - particularly when it was discovered that the belt-driven gearbox that I added to my machine eliminated the 'chatter' when the net resolution increase was only 1.5:1, but when the holding torque - in current reduction mode as well as normal mode - was increased 1.5:1).

Adding an axis dithering component to the software would be trivial - and it would eliminate the dreaded 'pointed finger syndrome'. However, if the G202 does not use current reduction (and it if does use current reduction, but only enters current reduction mode when the machine is idle), heat sinks may have to be added to the G202 stepper drivers. (I don't have any idea whether Mariss's requirement for heat sinking is/was based on the assumption that the stepper driver would never enter current reduction mode or whether he based his design on the assumption that a typical stepper is normally run with a limited duty cycle where the stepper driver would spend the majority of its time in current reduction mode.)

[Brady Watson]

Mike,
I didn't design the 4G...I'm in the same boat as you. I do know that the Geckos kick into current reduction mode (if jumper is set for it) rather quickly. So quickly in fact that it doesn't seem appropriate for a CNC tool. The Alphas, as far as I know, do not have this issue. (Take everything I am saying here with a grain of salt...I'm not a ShopBot engineer nor am I an expert on any of this stuff...then again, who is?) The Geckos from what I recall reduces the current to the 25% mark. This means your 305 Oz PRT motor is now a 76 Oz motor. Not quite enough cookies for CNC duty, even holding a single axis while another moves...The Alphas on the other hand, only reduce current to the 50% mark, which equates to about 280 Oz of holding torque on it's 560 Oz motors.

As far as heatsinks on the Geckos at 1.5-2A current, I believe Gecko said that they were not necessary. If they needed sinks, I'm sure SB would have put them on...they're pretty good about that stuff. Those that want to add sinks, just adhere some AL plate to the back of the Geckos with heat sink paste and a zip tie. There...yer sunk
I'm also pretty sure that current reduction is turned OFF (jumper) on the 4Gs to circumvent the '76 Oz' issue when it is enabled. Yep...the motors sometimes get a little toasty. I run on the ragged edge of 80v with my setup (with safety dump/back EMF electronics) and everything runs just fine. In fact, my 7.2:1 geared Alpha gets hotter than the 4G PRT motors do when worked hard. Neither setup has ever shut down because of heat.

With all this stuff, bear in mind that an Alpha driver is a superior beast to the Gecko in every way. It is made for CNC duty and was purpose built as such. Nothing against Gecko, believe me...I've got a dozen of them involved in various projects. They are a pretty good value for many projects.

-B

[richards]

Brady,
I think that we totally agree that current reduction on an inactive axis can be a real problem if another axis is active.

After you posted, I scanned the Oriental Motor Alpha stepper driver documentation to learn more about their current reduction. All that I could find is that there is a switch to turn it on or off. I couldn't find a reference to the amount of current reduction, but 50% seems reasonable.

I also rechecked the Gecko data sheet and saw that Mariss specifies a heat sink if current is above 6-amps, which is well above the 2-amps that is advertised for the G4 upgrade.

But, let's get back to the amount of torque required on a CNC router. It must be somewhere between 300 and 600 oz*in, because, until the Alpha came along, that was what was produced with a non-geared PK296 motor and a non-geared PK299 motor. Given the fact that an Alpha motor gives about 600 oz*in of torque (and still retains about 300 oz*in in current reduction mode), it does the job - although I get much better cuts with a geared motor on my Alpha.

I'm beginning to believe that the geared PK296 motors with 3.6:1 or 7.2:1 gearboxes,and the 3.6 to 7.2 increase in torque are the key element when using the G202 drivers. In reduced current mode, a geared PK296 motor is still giving about 300 oz*in of torque. Even your 7.1:1 Alpha is really giving you somewhere around 1,800 oz*in of torque when in current reduction mode.

I'm really rambling, but I'm trying to come to a point.

Given the fact that we need X amount of torque and given the fact that we really get X/4 amount of torque when an axis goes inactive, shouldn't we find a way to keep that axis from going inactive? One way to do that is to do exactly what Shopbot is doing; which is to limit the size of motor to 2-amps and to turn off current limiting on the Gecko driver. Wouldn't a more elegant option be to find a way to defeat current reduction and still allow bigger and better motors to be used? That's why I suggested that a simple dithering routine be added to Shopbot's software. (Any first year computer science student could write a simple routine using AND/OR logic to signal an axis to go into dithering mode when another axis became active.) The net result would be that current reduction would only become a factor when all axes were idle - and in that state, current reduction would be an asset instead of a liability.

Stepper motors produce heat and a way to keep a stepper motor from frying was developed. That method is current reduction. As far as I know, all modern stepper drivers either have automatic current reduction or switch selectable current reduction. If a stepper motor is going to be used in a multi-axis machine, current reduction is going to be a factor unless you severely limit the type and size of motors that you allow to be used on your machine.

Not to change the subject, but there is an elegant way to cross current reduction off the problem chart and still have high torque at standstill. That solution is to NOT use stepper motors and to use servo motors instead. By design, a servo draws almost no current when idle, but if an outside force tries to move the servo off-axis, it immediately draws as much current as necessay to resist that movement. The beauty of DC servo motors is that if a controller can drive a Gecko G202 it can just as easily drive a Gecko G320 servo controller. If a $200 stepper motor is adequate to do the job, a $200 servo would also be adequate to do the job. The fly in the ointment with servos is that they require a gearbox to multiply their torque. (They have plenty of speed so that gear reduction would NOT reduce the speed of the CNC machine.)

Now to sum it up. It looks like we have a number of choices:

1). Limit the size of motors that we use to 2-amps or lower and use G202 with current reduction turned off.

2). Add software dithering to the operation of the controller so that motors up to 7-amps can be used and enable current reduction.

3). Scrap steppers and use servos instead.

It looks to me that if performance is required and if an Alpha is deemed to be too expensive, then a do-it-yourselfer would need to either twist someone's arm at Shopbot to add some dithering code or else dump the steppers and switch to servos.

Well, that's the end of my ramblings - mostly brought on by finding out that Shopbot has turned OFF current reduction on the G202s. As far as I know, no other CNC manufacturer of similar sized machines does that. That shocked me enough to enter my diatribe mode and vent.

[Brady Watson]

"2). Add software dithering to the operation of the controller so that motors up to 7-amps can be used and enable current reduction."

At the moment I think that this is the path of least resistance and backward compatibility for older tools.

Now to add the proverbial fly to the ointment...The G203V drives have NO jumpers inside of them. I am not sure how they handle current reduction, but it does have some otherwise nice features...like being able to jack right into the Alpha board with no mods. So...the dithering routine might be a welcomed addition to the software in the future if the G202s go bye-bye.

I'm pretty impressed with how much collective knowledge & experience went into the 4G setup. There were things discovered (about the 212s for instance) that NO OTHER CNC company or person was able to nail down and resolve. I wonder how many people are running 212s with current reduction turned on, and pulling their hair out!

Yes...the problem with servos is the reduction. I've got a few servos here for experimentation that I want to try out. While I know I can do belt reduction, I'd like to try chain reduction. It's cheap...and with a spring loaded tensioner it should get the job done...although it may not be pretty, it should perform.

-B

[rhfurniture]

To quote from the G202 manual:
SPECIFICATIONS:
Supply Voltage: 24 to 80 VDC
Phase Current: 1 to 7 Amps and 0.3 to 2 Amps (2 ranges)
Auto Current Reduction: 33% of set current, 1 second after last Step Pulse.

I have just bought a cooking thermometer! The stepper runs at 35 - 40 deg cent with continuous small/intermittent movement. I will post temp stood still for a few hours when that occurs (not today) - I would guess that will be 45 ish from experience.
I know this is well within the 80 deg limit. My only concern would be if this reduced long term life and reliability over having it always "cool" like it used to be.

R.