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Thread: Which stepper motor and stepper driver should I use?

  1. #1
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    Default Which stepper motor and stepper driver should I use?

    Over the years there has been a substantial debate going on over which stepper motor (and stepper motor driver) should be used to drive a CNC router. Much has been written. Most of it true, but some information is still being bandied about as fact when the data shows that it is not quite reliable.

    First, let's remember that a stepper motor works because of the principle of magnetism. We all remember, back in grade school, how we played with magnets and how difficult it was to separate two properly aligned magnets. Stepper motors work in basically the same way. When properly sized and properly implemented, a stepper motor can be counted on to move a load a finite distance each time a pulse is applied to the stepper driver. The key words are "properly sized" and "properly implemented".

    Just like we learned, as grade-schoolers, we could pull apart two magnets if we applied enough force. We can also cause a stepper motor to "stall" if we expect it to do more than it was designed to do.

    Some methods have been developed to compensate for those times when we expect a stepper motor to do more than it was designed to do. One of those methods is the feed-back loop built into the Oriental Motor "Alpha" series of stepper motors. The feed-back loop incorporates an encoder (to measure movement) and some kind of timer (to verify that movement took place within a certain time limit). When properly implemented, the "Alpha" system can guarantee that a command to move an axis a certain distance will actually move that distance, or else the controller will enter a fault condition and the operator will be notified that something went wrong.

    On the surface, that concept is a good concept. In practice, it leaves much to be desired. Remember that a CNC router is a multi-axis machine. Typically a CNC router uses four motors, 2 X-axis motors, 1 Y-axis motor and 1 Z-axis motor. Many "moves" require that all four motors work together, at different speeds, to perform a required task. It is not hard to imagine what happens when one of the four motors starts to lag behind the other three motors. The commanded move deviates from the desired path and a "glitch" appears in the work. Depending on many factors, that "glitch" might be inconsequential or it may be large enough to ruin the part.

    The traditional way of controlling stepper motors relied on the fact that the programmer knew how hard and how fast he could move an axis. If he commanded a motor to move too fast or required that the motor move a load too large for the motor's design, the motor stalled and the part was ruined.

    All of this brings us to Shopbot and the design philosophy behind the two controllers they sell.

    Shopbot sells one mechanical model with the option of two different controllers. One controller uses stepper motors without feed-back and the other controller uses stepper motors with feed-back. Of course, the model with feed-back costs substantially more than the basic model. It should cost more because it used technology that costs much more to purchase.

    Finally, with that back-ground, we can look at some of the differences between the two controllers. Both controllers use Oriental Motor stepper motors, which is a good thing. Oriental Motor makes excellent motors.

    The original standard model used a motor equivalent to the PK296-01. That motor is an excellent motor, but it was designed to be used with a stepper driver that could operate with a voltage of about 175VDC. That's a lot of voltage for any stepper driver and more than 2X the voltage that the highly acclaimed Gecko G20x driver can handle. Basically, that meant that the standard motor was not very well matched to the newer stepper driver and that both speed and torque produced by that motor and the stepper driver would be lower than expected. That was not necessarily a design flaw caused by Shopbot, but simply the result of using two components that were not perfectly matched. When Shopbot introduced the 4g upgrade, they allowed users who already had the original Oriental Motor steppers to upgrade to a much nicer controller for a moderate price. Feed speeds were about doubled for a very reasonable cost.

    Those users that needed a higher production machine were steered towards the "Alpha" model which used the much more expensive Oriental Motor "Alpha" stepper motors and drivers. Those motors were tuned to special drivers that caused the motors to deliver both high speed and high torque - at a substantially higher price.

    I bought the PRT-Alpha model and it does run fast and hard.

    Since buying that PRT-Alpha, I've spent more than three years testing various stepper motors and stepper motor drivers to see what could be used to get good performance at a reasonable cost. So, for the "developers" who don't mind building their own electronics, here is what I've found.

    The AS98 motor used as the basis for the 7.2:1 geared motor on the Alpha models produces about 300 oz*in of torque at 250 RPM. That torque drops to about 250 oz*in at 1,000 RPM. The PK296-03 motor, wired half-coil, produces about 250 oz*in at 200 RPM. That torque drops to about 100 oz*in at 1,000 RPM.

    What is not shown in the charts is the fact that the AS98 motor, when used with the 7.2:1 gearbox, is limited to 80 lb*in of torque and the PK296A2A-SG7.2 motor, with a 7.2:1 gearbox is limited to about 40 lb*in of torque. In other words, looking at the chart, the PK model gives substantially flat-line torque up to the feed speed of about 8-ips when driving a 1.5" pitch diameter spur gear (30-tooth). The AS98 motor gives essentially 80 lb*in of torque up to a speed of about 11 ips, using that same 1.5" pitch diameter spur gear.

    On the surface, it looks like the AS98 motor and driver is the winner for any high-production user.

    Well, if you only had those two choices, then I would agree that the AS98 motor/driver is the optimum motor/driver for a CNC router used with a stepper motor; however, there is a third choice.

    Oriental Motor also makes the PK299-F4.5 motor, which can be wired serial, parallel or half-coil. It is an excellent motor, but it is not offered with a gearbox. My own experience with my PRT-Alpha showed that adding a gearbox was necessary to get acceptable edge quality. I built several models of belt-driven gearboxes and then purchased the 7.2:1 upgrade offered by Shopbot.

    The PK299-F4.5 motor, when coupled with a 3.6:1 belt-drive gearbox develops more than 100 lb*in of torque at more than 20-ips. Those specs are substantially better than either the PK296A2A-SGxx motor or the AS98 7.2:1 motor.

    The PK299-F4.5 motor costs $207 each. The Gecko G203v stepper driver costs $147 each. A homebuilt power supply (45VDC @ 20A) will cost about $200. Belt-drive transmissions for the PK299-F4.5 motors will cost about $200 each, depending on what you make and what you buy. In other words, all four PK299-F4.5 motors with stepper drivers, a power supply and belt-drive transmission will cost about the same as one AS98 geared motor purchased directly from Oriental Motor.

    Resolution between a 3.6:1 belt-drive and a 7.2:1 gear drive will be exactly the same, given the fact that the AS98 uses 1,000 steps per revolution and the PK299-F4.5 uses 2,000 steps per revolution.

    The PK299-F4.5 does NOT have encoder feed-back, but it does have at least 20% more torque than the AS98 motor/driver, so if properly programmed, it can do at least as much as the AS98 motor/driver before it stalls.

    Repair prices for the PK299-F4.5/driver/transmission would be about $200 for any one component. The AS98 is purchased as a complete unit at a cost of well over $1,000 per unit.

    So, with that much conjecture, what is the purpose of this post? It's simply to say that a do-it-yourselfer, who is willing to do a lot of building, can build a very robust electronics package at a substantial savings.

    Do I recommend building your own electronics package? Well, I'm perfectly happy using the Alpha controller that came with my PRT-Alpha. It has worked flawlessly since July, 2004. I don't plan on replacing it until it breaks - then, depending on how much time I have on my hands, I might build my own controller.

  2. #2
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    Hi Mike... Great research.... Great to archive findings here.

    -- Pat

  3. #3
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    I received a few private emails asking what the "secret" is about using a Gecko G203v with Shopbot's SB3 software. There is no secret. Just contact Shopbot and buy a V201 (or newer) controller board and you'll be in business just as quickly as you can crimp or solder together a simple wiring harness between the controller card and the Gecko. Of course you'll need a good power supply and a hefty heat sink for the Gecko G203v stepper drivers if you're going to use the PK299-F4.5 motors wired half-coil with an 80k current limiting resistor (4.5A per coil). The motors will run at about 60C (140F) and the Gecko G203v, with a 5" X 5" X 1/4" aluminum heat sink will run at about 40C (104F). Those temperatures are well within the range allowed. If you let the temperature creep upwards on the Gecko, it will shut itself off at about 50C (122F). In my shop in July, the inside temperature is often at leat 115F, maybe even hotter, but that's the top of the scale on my shop thermometer. With the ambient air at that temperature, heat sinks (and fans) help keep everything in control. Heat has been a problem for some Gecko users, but a good heat sink will cure that problem. (I might do some experimentation with a water cooled heat sink. Some people using a water cooled spindle have reported that hooking up a small fish-tank pump to push a small amount of water through their spindle was all that was required to keep the spindle at ambient temperature.)

    Another consideration is pulses per second. To keep expectations in line with the ability of the controller to output a proper pulse train, I use 30,000 pulses per second as the maximum expected from the controller. The PK299-F4.5 motor, if connected to a 3.6:1 belt drive and a 1.5" pitch diameter spur gear (30-tooth), requires about 26,166 pulses per second to move along at 10" per second. 600 inches per minute translates to about 485 RPM, which is still in the high torque range for the PK299-F4.5 motor.

    Those are the "secrets". Shopbot makes it easy. Buy either the PRS-Standard or the PRS-Alpha. Use it until you've worn out the motors and then "hot rod" the machine with a new set of electronics if you're electronically inclined. Just be sure to machine some motor mounts and bearing blocks out of Delrin for the PK299 motors before you wear out your first set of motors, but given the life expectancy of the electronics that ship with the machine, that should give you years and years to make those mounting plates.

  4. #4
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    I wonder where that v204 board is?

  5. #5
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    I've recently received emails from several PR and PRT users who are planning on upgrading their electronics. Because their questions are similar, posting a general answer here might help others who are contemplating doing a home-brew controller.

    ------

    Almost all stepper motors are good motors, meaning that if they are matched with a proper controller, they will do an excellent job of positioning. Most stepper motors have 200 steps per revolution. So, each step equals 1.8-degrees (360 / 200 = 1.8). The stepper driver may be able to change that resolution. The Gecko stepper drivers automatically perform 1/10th step stepping so that it takes 2,000 steps to turn the shaft one revolution.

    Mariss at Gecko is recognized as the premier authority on stepper driver design. After studying his white paper on stepper motor basics, I agree that using 1/10th stepping is the optimum value for a stepper motor. At any rate 360 / 2000 = 0.18 degrees per step. Using a 7.2:1 gear box gives 0.025 degrees per step pulse. In other words, it will take 14,400 steps to move the spur gear attached to the gear shaft one revolution. If you have a 1.25-inch pitch diameter pinion gear, that means that it will take 14,400 steps to move an axis 3.925 inches. (1.25 X PI = 3.925 inches. 3.925 inches / 14,400 steps = 0.0002725 inches per step.)

    That is a lot more resolution than you will ever be able to use on a Shopbot; however, 7.2:1 is exactly what I would use if I were building my own machine.

    Going back to the idea of the proper stepper driver for a motor shows that the most important data item about a stepper motor (pertaining to the stepper driver) is Inductance. In very simple terms, Inductance is AC resistance. The higher the Inductance, the greater the resistance. The greater the resistance, the greater the voltage required to do the job. If you compare electricity to water, it all becomes clear. Voltage can be compared to water pressure and current can be compared to flow. In other words, if you need to get five gallons per minute from a hose, you can use a very small hose at high pressure or a very large hose at low pressure.

    The PK296A1A motors have very high inductance. Inductance is measured in units called Henrys. The A1A motors have 30.8mH inductance when wired bipolar series. The have 1/4th that much inductance when wired half-coil. The formula that Mariss uses to determine the maximum voltage that can be used with a motor is: 32 X SQRT( Inductance ). So, 32 X SQRT( 30.8 ) = 177VDC. If the motors are wired half-coil, 32 X SQRT( 7.7 ) = 88VDC.

    The voltage determines how much power the motor produces. Power is speed X torque. Voltage largely determines the top speed of the motor. It is easy to understand that a motor that can never achieve top speed because it doesn't have enough voltage to push electricity through its coils will never develop full power.

    The formula assumes that you are willing to let the motors run at their highest rated temperature, or about 85-degrees centigrade. That's too hot for me, so I de-rate the maximum voltage by at least 10%. The means that the A1A motor, if wired half-coil, can be used with a Gecko stepper driver. (The Gecko products are limited to 80VDC.) The problem with wiring the A1A motor half-coil is that many of the motors supplied by Shopbot have only four leads, Black, Green, Red, and Blue. Those leads are connected to the ends of each of the two coils, meaning that those motors can only be wired bipolar series. If your motors have six leads, Black, Yellow, Green, Red, White, Blue, then you can wire the motors half-coil by using the Black/Yellow leads on the A coil, and the Red, White leads on the B coil.

    That's a lot of background information, but it was required to explain why I would use the A2A motors which have 1.5mH Inductance when wired half-coil. That Inductance would allow me to use a 35VDC power supply and still get all the torque that I need from that motor. The Gecko controllers, including the G540, can all handle 35VDC. The A2A motor draws 3A of current. All of the Gecko products can force 3A of current through the motors at 35V. So, the A2A motor would work perfectly for my application.

    The PK296AxA-SG3.6 motor has a gear box that is rated at 22 lb*in of torque, or about 350 oz*in. That is not much torque. It will limit the depth and/or speed of your cuts. The 7.2:1 motor's gearbox is rated at 44 lb*in of torque, or about 700 oz*in of torque. That is a little more than the PRT-Alpha that was supplied with the 1:1 motors. That will allow you to make reasonable cuts.

    Keep in mind that it is the gearbox that is the limiting factor, not the motor. If you used the motor and built a 3.6:1 belt-drive transmission (similar to what several of us did for our PRT-Alphas - before Shopbot changed to a 7.2:1 Alpha motor), you would get about 1,100 oz*in of torque out of a PK296-01AA motor (which is a motor that has the same electrical specs as the PK296A1A-SGxx motor). The PK296-01AA motor would have to be wired half-coil and it would have to be driven by a Gecko G203v because you would need 70V to 80V to drive that motor. (A much better motor would be the six wire PK296-03AA motor or the eight wire PK296-F4.5A motor which could be wired half-coil and connected to a 35VDC power supply. Those motors draw 4.5A, so they would have to be driven by the Gecko G203v stepper driver.) By the way, the PK296 size motor with a 3.6:1 belt drive develops 1,100 oz*in of torque, which is very near the performance of the 7.2:1 Alpha motor which is rated at 1,280 oz*in of torque. Resolution between a Gecko driven 3.6:1 motor and an Alpha driver 7.2:1 motor is exactly the same because the Alpha controller has 1,000 steps per revolution and the Gecko has 2,000 steps per revolution. Keep in mind that the SG gear box on a PK motor is not the same as the gear box on an Alpha motor. The Alpha motors have gear boxes with a lot less backlash.

    (There is a reason why the Alpha series costs as much as it does. Quality and close tolerance comes at a price. You get what you pay for. However, the belt-drive transmission has no backlash if the belt is properly adjusted. But, designing and building a belt-drive takes time and effort and is bulky when compared to a gear box. Again, there is no free lunch. The choice is yours: Big and bulky belt-drive with no backlash with a cost determined by your ability to build your own belt-drive transmission, small compact unit with very good backlash data at a substantially higher price, or small compact unit with adequate backlash data at a low price.)

    The ultimate motor would be the PK299-F4.5A motor attached to a 3.6:1 belt drive transmission. That motor, when wired half-coil and driven with a 45VDC power supply produces about 2,200 oz*in of torque.

    The Gecko G540 is not easily adapted to the Shopbot controller. However, it can be plugged into the parallel port of a machine running Mach3 software. You would have to run your Shopbot supplied A1A motors at a maximum of 50V, which would limit their power. The problem is not the motors or the controller; it is using that controller with those motors. That controller was not designed to drive high inductance motors.

  6. #6
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    what?

  7. #7
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    Jeff,

    At our meeting next week, you and I need to spend an hour just playing with stepper motors while everyone else is talking about other things. At the end of the evening, you and I will have at least had some fun.

  8. #8
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    Write on!

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