Ruminations about stepper motors and Gecko stepper drivers

[richards]

I've spent a few days testing a new Oriental Motor PK296-F4.5A motor and various Geckodrive stepper drivers. The purpose of the test was to find out whether the PK296-F4.5A stepper motor was a viable motor to use with a Shopbot.

The PK296-F4.5A motor has basically identical electrical specifications with the PK296-03AA motor. Specifically, the Inductance rating is the same. In other words, wiring the PK296-F4.5A or the PK296--3AA motor half-coil should give the same basic response.

Tests showed that it worked. I have four PK296A2B-SG3.6 motors. I connected one of them to a Gecko G202 stepper driver with a 33k current limit resistor and I connected the PK296-F4.5A stepper to another G202 stepper driver with a 33k current limit resistor. Both stepper motors responded identically (except for the fact that the GS3.6 motor has a 3.6:1 gearbox attached).

When I changed the connection to the PK296-F4.5A motor from half-coil to bipolar parallel, the motor ran about 1.5 degrees C cooler. When I played around with the resistor, I found that running the PK296-F4.5A drive and the Geckodrive G202 stepper driver with a 100k current limiting resistor gave me optimum performance before the heat rose to an unacceptable level (75C). (I'm also testing whether increasing the current draw through the motor with a higher resistance resistor and then lowering the power supply voltage to allow the motor to run cooler can be done with good practical results.)

Here's what I've learned:

The PK296-F4.5A motor geared 4:1 with a belt drive and allowed to pull about 4.75A produces a little more than 1,200 oz*in of holding torque, or about 75 lb*in, comparing very favorably to the much more expensive Oriental Motor Alpha 7.2:1 motors and drivers, which produces a reported 80 lb*in of torque.

The Oriental Motor PK296-F4.5A motor responded very well to speeds of over 1000 inches per minute, although I have limited my machine to run at not more than 720 inches per minute.

The PK296-F4.5A stepper with a 4:1 belt-drive and a 1.25" diameter spur gear (25-tooth) gives 0.000490625" per step resolution compared with 0.000654167" per step from the Alpha 7.2:1 motor driving a 1.5" diameter spur gear. (Any figure smaller than 0.003 is most likely going to be lost in the "elasticity" of the machine.)

The tests have indicated that an Oriental Motor PK296-F4.5A stepper motor combined with a 4:1 belt-drive transmission and driven with a Geckdrive stepper driver (I personally prefer the G203v) can compare comfortably with the Alpha option.

Further testing may show that reducing the power supply voltage from 35VDC to 28 - 30 V might allow greater torque at a loss of high end speed. Since I've decided that 720 RPM is the maximum required jog speed, it looks like I can safely reduce the voltage (which will allow the motor to run cooler) without sacrificing torque.

REMEMBER that the title of this post is "ruminations". Do-it-yourselfers might find something to play with. The normal 'botter can safely ignore the post.

[rb99]

Hi Mike,

Nice post. How much are these motors? Where would you recommend I look to price them?

Would they be a good choice for new motors for my old PR with my new SB controller?

Thanks,

RIB

[richards]

Richard,
I bought the motor online from Oriental Motor (www.orientalmotor.com) for $139. There was no charge for shipping. I don't know if they will ship to Canada or whether they require that you buy from a dealer.

This is a high current / low voltage motor. To get the most out of it, it needs to be wired bipolar parallel and it needs a stepper driver that can handle 6.3A. The Geckodrive G201, G202 and G203v can all handle the motor, but the auto current reduction jumper on the G201 and the G202 must be set to automatically reduce current when the drive is stopped. The G203v does that automatically without a jumper.

Because the stepper draws a lot of current, it needs a heat sink. On my test bench, I mount four stepper drivers to a strip of 3" X 16" X 3/4" thick aluminum. I would guess that a strip of 1/4" or 3/8" thick aluminum would be a better choice.

This test was run at 35VDC. Using the forumla 32 X SQRT(Inductance) = Max Voltage, I could have used a 39VDC power supply. To keep the heat down, I'm going to try some lower voltages.

Torque is mainly a function of the current that the motor pulls. So the higher the current, the more torque produced. At 6.3A, this motor produces 440 oz*in. With a 4:1 belt drive, that gives us 1760 oz*in or 110 lb*in of torque.

Speed is more a function of the voltage available to the motor. The higher the voltage the faster the motor can turn.

Heat is current X voltage. So reducing one or the other will reduce the heat.

I've set my PRT-Alpha jog speed to 12" per second. Using a 4:1 belt drive and a 30-tooth spur gear, the motor would only have to turn 611 RPM. With a 25-tooth spur gear, it would have to turn 733 RPM. The PK296-F4.5A motor can easily run at 1,500 RPM at 35V, so reducing the voltage to 25-30VDC should still give plenty of speed and less heat.

Avel sells an 18VAC toroidal transformer that would produce about 25VDC and a 22VAC toroidal transformer that would produce about 30VDC. I currently use an Avel 25VAC toroidal transformer on my test bench that produces 35VDC. I'll attach a VARIAC to the input side and vary the voltage to see how the motor runs at reduced voltage.

I didn't give you a direct answer to your question about whether the motors could be used with your new SB controller. Shopbot has used the G202 drivers, then the G203v drivers, and (I believe) a slightly modified G203v driver that had an internal resistor changed to work better with the higher inductance of the motors that most 'botters had on their machines. I believe that their newest PRS-Standard controller uses RBK motor/controllers without Geckodrive products. Oriental Motor recommends their RBD245A-V stepper driver for this motor, which (I believe) is the driver that comes with the new PRS-Standard.

I have had no experience with the RBD245A-V stepper driver. On paper, it looks very good, but it also costs 2.2X more than a G203v without giving any additional torque or speed.

In the past, before I started using Geckodrive products, I used Oriental Motors and drivers exclusively. During the fifteen years or so that I furnished those drivers and motors to my customers, I only had one stepper driver fail. I've used about fifteen Geckodrive drivers without any problem, so it looks like they are both excellent products. Using all Oriental Motor products would make ordering and servicing easier. Using an Oriental Motor stepper motor with a Geckodrive stepper driver would save you $732 on a machine that used four motors without sacrificing torque or speed.

If the goal is to find a motor that gives basically the same performance as the motors supplied with the Alpha machines, the PK296-F4.5A with a 4:1 belt drive would produce almost identical torque and almost identical speed. The limiting factor might be your tolerance for heat. A stepper motor is rated to run at 100C, which would burn your skin if touched. That heat won't hurt the motor, but I prefer maximum motor temperatures closer to 60C.

(The PK299-F4.5A motor, which costs $208, when geared 4:1 can produce 2X the torque of this motor. In the past, that has been my preferred motor, but this smaller motor looks like it can do everything required in a smaller, cheaper package.)

[rb99]

My controller still has geckos, i believe the 203s.

Thank you for taking the time with so much detail.

Can you tell me where i would get the belt drives?

The PK299-F4.5A motor sounds like a good deal as well for only $70 more. I could use these for the x and y.

Does the 63v power supply i have present a problem?

RIB

[richards]

Richard,

XL-Size timing belts and pulleys can be found at WW.Grainger and McMaster-Carr (www.mcmaster.com). I would recommend the 18-tooth for the motor and the 72-tooth for the shaft. That will give you a 4:1 ratio. You'll have to have the bore opened up to fit the motor and the shaft. If you space the pulleys about 3.55 inches apart from each other (center to center) a 170XL belt will fit. In the motor mounting plate, slot the mounting holes to allow the belt to be properly installed and "snugged" up.

The PK299-F4.5A motor is an excellent motor. Running it half-coil with a 35 to 45VDC power supply and limiting the current to about 4.5A with an 84k 1/4-watt resistor will give excellent performance and only moderate heat.

The 63v power supply is too high for these motors unless you wire them bipolar series. Bipolar series has excellent low speed torque, but it quickly looses torque as the speed increases. You would be better off if you wired the motors bipolar-parallel or half-coil if you want speed.

Bipolar parallel gives you 1.4X the torque when compared to half-coil, but it also draws 1.4X more current and produces 1.4X more heat (if you use the same voltage). The PK299-F4.5A motor has more than enough torque when wired half-coil, so there would be no need to wire the motors bipolar-parallel.

I'm experimenting with wiring the motors PK296-F4.5A motor bipolar-parallel and then running the motor near its full rated current but reducing the voltage until the heat is acceptable. That should give full torque and acceptable speed without frying my fingers every time I touch the motor.

(The 63VDC power supply was used to accommodate some of the motors supplied by Shopbot in the past whidh came from the factory with only four wires meaning that they had to be wired bipolar-series. Those motors also had relatively high inductance. Because of the higher inductance and the biploar-series connection, a higher voltage power supply was necessary to get better performance from the motors.)

[rb99]

Those belt drives are not pre made?

Do you have a picture of what I would have to make?

Thanks,

RIB

[richards]

Richard,
Sorry, but you have to build your own belt-drive transmissions. Dirk Hazeleger posted some photos on his blog (http://cncshare.blogspot.com/) that show a side view of his belt-drive transmission. I've posted a few photos on the Shopbot Forum of some of my initial designs.

Basically, you're going to need an 18-tooth XL size pulley on the motor, a 72-tooth XL size pulley on the shaft, a 25-tooth or 30-tooth pinion gear, a 5" or 6" shaft, two bearings to fit the shaft, an XL-size belt to connect the two pulleys together, a motor mount plate and some spacers.

I've built several different sets of models out of plastic, one out of 1/2" baltic-birch and one out of 1/2" aluminum. The last set was cut out of Delrin. Everything was cut on my Shopbot, (although I used my vertical mill to make bearing holders for some of the early models).

Depending on the material that you use, each belt-drive transmission will cost about $150 to build.

[rb99]

Thanks again Mike. My initial thought is they are more work and expense than I am prepared to undertake at this time. Maybe I will feel differently after awhile...


RIB