New FAQ section, need ideas

[CoAMarcus]

Hello all, my name is Marcus Freimanis and I work for Geckodrive (which I am sure most of you have figured out). I am writing this post because I will be putting together an FAQ sheet to put up on here, our Yahoo group, and our website to help people out a little bit more. Now, what I need, is simple questions that I can answer in it that are not too involved. What I mean is things like "How do I determine my current set resistor size?" or things like that. I will be collaborating with the rest of our engineers here to give the best answers we can to the questions you guys pose.

Please just respond to this post with any questions you would like to see, and we will decide to include it or not, based on complexity. Thanks a lot!

-Marcus Freimanis

[Bubba]

Marcus,
One that gathers a lot of questions and consternation might be:

How do I tune my servos properly?

The existing "seat of the pants" instructions are rather vague, and I for one, do not want to mess with removing the drives from the heat sinks (nice and sticky with thermal paste ) to remove the cover so I can hopefully attach scope probes in the correct place!

[ckm]

A couple of things I have struggled with that you have generously explained in other threads:

1. Motor sizing
2. AC vs DC (e.g. brush vs brushless)
3. Gearing down of servos
4. Encoder resolution vs precision
5. That list of suppliers you send out...

Chris.

[ckm]

Some links to some of the explanations I was talking about:

http://www.cnczone.com/forums/showpo...8&postcount=14
http://www.cnczone.com/forums/showpo...8&postcount=15

And steppers vs servos would be good too. Actually, some links to appropriate CNCzone threads would work just as well.

BTW, I also think it might be worth while to explain Mariss vs Marcus. I know I am confused....

Chris.

[CoAMarcus]

Okay, I have a preliminary version of the FAQ done. If you guys find any mistakes in it, either grammar or stupidity, please let me know. This is just a rough draft, and I am going to be adding more. If you think of anything I did not address, please just reply here with what I should add. I also need more things to put under the NEVER area on the list. Please read, skim, or do whatever you like to the list and let me know what you think.



Gecko FAQ

SECTION I – GENERAL FAQ

Q.) The Gecko Dos and Don’ts:

A.) Here is a quick list of what you should never do to your gecko:

a. NEVER connect AC voltage to the drive.
b. NEVER plug a stepper drive into a servo motor, or vice versa.
c. NEVER reverse polarity going to the drive.
d. NEVER power a drive with more than 80V and 7A is it is a stepper, or 20A if it is a servo.
e. NEVER unplug the drive with power applied.
f. NEVER connect the COMMON terminal to ground unless the drive is set to COMMON GROUND.
g. NEVER try running more than one motor with a single drive.
h. NEVER expose the drive to any sort of conductive chips or dust, such as from a metal cutting machine. Try to keep the drive isolated in a control box.
i. NEVER expose the drive to excessive moisture.
j. NEVER daisy chain your drives together.

Q.) What size should my power supply filter capacitor be?

A.) Your filter capacitor on your power supply is determined by your power supply voltage and current. Use the following formula to find the optimal value in µF:

(80,000 * I) / V = C

Example:

Using a power supply of 65V and 5A, the equation would look as follows:

(80,000 * 5) / 65 = 6153µF

You would then choose the capacitor value closest to this with a voltage rating of at least √2 times your power supply voltage.

Q.) What are the advantages/disadvantages between steppers and servos?

A.) Step motors and servo motors service similar applications, ones where precise positioning and speed are required.

The biggest difference is that steppers are operated "open loop". This means there is no feedback required from the motor. You send a step pulse to the drive and take on faith it will be executed. Seems like a problem but it's not.

If you have a quartz watch with hour and minute hands, then you have a step motor on your wrist. The electronics generates 1 step pulse per second, driving a 60 step per revolution motor which turns at 1 RPM. It keeps nearly perfect time. Any errors are due entirely to the electronics timing accuracy (quartz crystal oscillator).

Top Ten Stepper Advantages:

1) Stable. Can drive a wide range of frictional and inertial loads.
2) Needs no feedback. The motor is also the position transducer.
3) Inexpensive relative to other motion control systems.
4) Standardized frame size and performance.
5) Plug and play. Easy to setup and use.
6) Safe. If anything breaks, the motor stops.
7) Long life. Bearings are the only wear-out mechanism.
8) Excellent low speed torque. Can drive many loads without gearing.
9) Excellent repeatability. Returns to the same location accurately.
10) Overload safe. Motor cannot be damaged by mechanical overload.

Top Ten DC Servo Advantages:

1) High output power relative to motor size and weight.
2) Encoder determines accuracy and resolution.
3) High efficiency. Can approach 90% at light loads.
4) High torque to inertia ratio. Can rapidly accelerate loads.
5) Has "reserve" power. 2-3 times continuous power for short periods.
6) Has "reserve" torque. 5-10 times rated torque for short periods.
7) Motor stays cool. Current draw proportional to load.
8) Usable high speed torque. Maintains rated torque to 90% of NL RPM
9) Audibly quiet at high speeds.
10) Resonance and vibration free operation.

Top Ten Stepper Disadvantages:

1) Low efficiency. Motor draws substantial power regardless of load.
2) Torque drops rapidly with speed (torque is the inverse of speed).
3) Low accuracy. 1:200 at full load, 1:2000 at light loads.
4) Prone to resonance. Requires micro-stepping to move smoothly.
5) No feedback to indicate missed steps.
6) Low torque to inertia ratio. Cannot accelerate loads very rapidly.
7) Motor gets very hot in high performance configurations.
8) Motor will not "pick up" after momentary overload.
9) Motor is audibly very noisy at moderate to high speeds.
10) Low output power for size and weight.

Top Ten DC Servo (brush type) Disadvantages (besides higher relative cost):

1) Requires "tuning" to stabilize feedback loop.
2) Motor "runs away" when something breaks. Safety circuits required.
3) Complex. Requires encoder.
4) Brush wear limits life to 2,000 hrs. Service is then required.
5) Peak torque is limited to a 1% duty cycle.
6) Motor can be damaged by sustained overload.
7) Bewildering choice of motors, encoders, servo drives.
8) Power supply current 10 times average to use peak torque. See (5).
9) Motor develops peak power at higher speeds. Gearing often required.
10) Poor motor cooling. Ventilated motors are easily contaminated.

Q.) Should I use servos or steppers in my machine?

A.) If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or servo) first and then figure out if it will fit what you need. That is the sign of an amateur or hack.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To separate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but you’re not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531


Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feed rate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The hand crank is about 6" inches in diameter. How much force would you place on the hand crank before you figure you're not doing something right? I hear about 10 Lbs.

10 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) of torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

Q.) When is a G100 necessary?

A.) A G100 is necessary in only several applications, and they are:

1.) When 1350 RPM will not suffice
2.) When a parallel port connection is not possible and it must be run through Ethernet
3.) When more than 4 axes are required
4.) When lots of I/O’s are necessary

If you do not meet these criteria, the G100 will just become an over priced, overly complex breakout board.


Q.) How do I tell what kind of drive I have if I have no cover on it?

A.) See below pictures to determine the type of drive you have without a cover on. Note: The PCB colors may not be the same as those in the picture. The key places to look are the large capacitor, the opto-isolator, and the thru-hole resistors.

I will have pictures of all of our drives here from a top down angle with the cover off, and label what each one is.


Q.) How do I determine if my drive is broken?

A.) To check your drive using a digital multimeter (DMM), follow these steps:

1.) Set the DMM to Ohms and put the negative lead on terminal 1 on the drive.
2.) If the drive is a stepper, put the positive lead on terminals 3, 4, 5, and 6. If any of these shows 0Ω, there is a blown MOSFET. For a servo, do the same test but only put the positive across 3 and 4.
3.) Now take the negative and put it on terminal 2 on the drive. If it is a stepper, put the positive on pins 3, 4, 5, and 6 and follow the same rules as above. If it is a servo, only test pins 2 to 3 and 2 to 4.
4.) If there is a blown MOSFET, the drive must be sent back to us for evaluation. If there is more than one blown MOSFET, then it is not repairable.

SECTION II – STEPPER FAQ

Q.) What is the difference between the G201, the G202, and the G203V?

A.) The G201 is our most basic high end drive. It is meant for experienced CNC users and OEMs who will be wiring it exactly the same way for almost every application. It has no internal protection, and is functionally identical to our other steppers for the most part.

The G202 has short circuit protection and an internal 470µF capacitor so you do not need to attach one if your power cables are longer than 18”. It is the same as the G201 aside from these two features, and it has a slightly larger footprint (see G202 manual for exact dimensions).

The G203V is protected against almost everything you can throw at it. It has short circuit protection, temperature protection, an internal fuse, common ground, and a plethora of other features. This drive is for the hobby user who needs protection against a variety of issues that could cause problems for their system.

Q.)What is the difference between the G201 (or G202) and the G210 (or G212)?

A.) The G201 and the G210 are functionally identical, except that the G210 has a step pulse multiplier. This step pulse multiplier will only be used if your step pulse signal is very weak or if you have to set the drive to common ground rather than its default setting of common +5V.

Q.) How do I determine the size of the current set resistor?

A.) The current set resistor is determined by taking your motor phase current and substituting it for I in the following equation:

Standard:
47 * I / (7 – I) = Resistor value in KΩ

Low Current:
47 * I / (2 – I) = Resistor value in KΩ

Example:
47 * 5 / (7 – 5) = 117.5KΩ

You then choose the nearest 5% resistor value below that amount.

Q.) What does the standby jumper block do?

A.) The standby jumper block causes the drive to either enter standby mode or not. When current standby is enabled, the current will be reduced to 30% after one second of inactivity. If it is disabled, then the current will stay at full current whether idle or not.

You would keep the current standby disabled on an axis where constant torque is necessary, such as a Z axis. Keeping standby enabled will help reduce motor heat.

Q.) Can I run the G201 / G210 / G202 / G212 / G203V above 80V/7A?

A.) The steppers are rated to 80V/7A maximum, and you can run the voltage and current up to but not past those limits. Anything past either 80V or 7A voids the warranty on the drive.

Q.) When should I use a parallel or series motor wiring?

A.) You should use a parallel wiring on your motor when you need high speed. This will result in a very hot motor which will increase the wear and tear on it. You should use wiring in series when you do not require high speed, which will keep your motor cool.

SECTION III – SERVO FAQ

Q.) How do I determine my optimal encoder line count?

A.) To determine your encoder line count, you must use the following equation:

(Step Pulse Frequency / RPS) / 4 = Optimal encoder line count

To determine your RPS, take 80% of your motors rated maximum no load RPM and divide by 60. Example:

3000RPM max motor, with a step pulse frequency of 45kHz

(45,000 / 40) / 4 = 281.25

You then choose the closest line count below your optimal line count, in this case 275.

Q.) Will my G320 work with a brushless AC motor?

A.) The G320 and G340 are meant for brush type DC servo motors only. Running anything aside from that with the G320 or G340 will result in possible destruction of the drive.

Q.) Why does my motor “sing” when it is idle?

A.) The singing is because the motor is dithering or bouncing between adjacent encoder line counts. The integral term in a PID loop has an infinite DC gain over time and will amplify even the smallest position error. Because encoder feedback can only occur on count edges, the loop is “blind” until it encounters an encoder count edge. It then reverses the motor direction until another edge is found, then the process repeats.


Q.) What is the difference between the G320 and the G340?

A.) The G320 and the G340 are functionally identical, except that the G340 has a step pulse multiplier. This step pulse multiplier is going to have a niche application, and will be required in only two instances: If your encoder line count is very or if your step pulse signal is very weak. You can also use the G902 (the step pulse multiplier in the G340) jumper settings to set the drive to common ground or common +5V.

The G902 has a jumper setting to choose either a multiplication of 1, 2, 5, or 10 times for each step pulse. This means that if you were to have a 2000 line encoder, a setting of 10 on the G902 would make it equivalent to a 200 line encoder. The maximum frequency of the G902 is 200 kHz, so if you have it at a setting of 10 at a step pulse frequency of 45 kHz, it will be limited to going to 200 kHz, not 450 kHz as it would in a perfect world.

Q.) Can I run the G201 G320 / G340 above 80V/20A?

A.) The servos are rated to 80V/20A maximum, and you can run the voltage and current up to but not past those limits. Anything past either 80V or 20A voids the warranty on the drive.

[ger21]

You should throw power supply sizing in there. That question comes up alot!

[CoAMarcus]

Are you referring to the 32 * SQRT(mH inductance) equation? I shall throw that in there. Thanks!

[ger21]

Yes, and you might as well add in your current requirement (1/3, 2/3 motor rating...)

[CoAMarcus]

Here are the updated/new
portions:

Q.) The Gecko Dos and Don'ts:

A.) Here is a quick list of what you should never do to your gecko:

a. NEVER connect AC voltage to the drive.
b. NEVER plug a stepper drive into a servo motor, or vice versa.
c. NEVER reverse polarity going to the drive.
d. NEVER power a drive with more than 80V.
e. NEVER use a motor rated higher than 7A with a stepper.
f. NEVER use a motor rated higher than 20A with a servo.
g. NEVER unplug the drive with power applied.
h. NEVER connect the COMMON terminal to POWER GROUND.
i. NEVER try running more than one motor with a single drive.
j. NEVER expose the drive to any contaminants; keep the drive in a
control box.
k. NEVER expose the drive to excessive moisture.
l. NEVER daisy chain your drives together.

___________________________________________________________________

Q.) How do I determine if my drive is broken?

A.) To check your drive using a digital multimeter (DMM), follow these
steps:

1.) Turn off power to your drive.
2.) Set the DMM to Ohms and put the negative lead on terminal 1 on the
drive.
3.) If the drive is a stepper, put the positive lead on terminals 3,
4, 5, and 6. If any of these shows 0Ω, there is a blown MOSFET. For a
servo, do the same test but only put the positive across 3 and 4.
4.) Now take the negative and put it on terminal 2 on the drive. If it
is a stepper, put the positive on pins 3, 4, 5, and 6 and follow the
same rules as above. If it is a servo, only test pins 2 to 3 and 2 to 4.
5.) If there is a blown MOSFET, the drive must be sent back to us for
evaluation. If there is more than one blown MOSFET, then it is not
repairable.

____________________________________________________________________

Q.) How should I heatsink my drive?

A.) There is not drive-specific heatsink for any Geckodrive, and there
are a variety of ways to heatsink your drive. What you should do is
ensure that your heatsink has fins to increase surface area, and have
air flowing over it. A good heatsink to use is a standard CPU heatsink
with some heatsink compound in between the drive's plate and the
finned aluminum heatsink. The method to determining if it is being
cooled adequately is to feel the drive while it is running. If it is
uncomfortable to the touch, then the electronics are uncomfortable as
well.

_____________________________________________________________________

Q.) What should my settings be in Mach3?

A.) With all drives except for the G203V, set the "Ports and Pins"
setting to "Active Low". If it is a G203V, set it to "Active High". On
all drives, set the step pulse width to 2µS.

_____________________________________________________________________

Q.) Can I use a variable autotransformer with my drive?

A.) No. Do not even think about it, for the following reasons:

1.) The Geckodrive mounting plate is connected to the 'Power Ground'
terminal 1.

2.) The mounting plate hard anodizing is for the benefit of the 8
power MOSFETs, not to insulate the plate from whatever it gets
mounted to. The mounting screws can and will ground the mounting
plate to the mounting surface.

3.) The common anodes ('-' terminal) of the power supply full bridge
rectifier connects to 'Power Ground' terminal 1 of the drive.

4.) If the 'neutral' side of the autotransformer is grounded to the
chassis or the 'ground' wire connects to the chassis, it shorts-out
the rectifier diode whose cathode goes to 'neutral' and whose anode
goes to 'Power Ground' on the drive.

5.) With the rectifier shorted, uncontrolled current flows from
chassis to mounting plate to 'Power Ground' to remaining common anode
rectifier diode back to autotransformer.

6.) This current melts the aluminum plate, over-currents and destroys
the remaining bridge rectifier diodes and causes amusing fireworks
until the circuit breaker shuts down the fun.

7.) The resulting molten aluminum causes some on this list to then
muse about anodizing and its shortcomings.

____________________________________________________________________

Q.) Can I get a step and direction signal from USB?

A.) No. Due to the nature of USB (Universal Serial Bus), it loses all
timing information contained in step pulses. The USB is a serial, or
sequential, port. Step and direction signals require a parallel port
so that no timing information is lost.

_____________________________________________________________________

Q.) Do my motors produce more torque while microstepping or full stepping?

A.) It's a give and take kind of situation:

1) For the same peak current, a microstepped motor will have 71%
(1/sqrt 2) the holding torque of a full-step drive. This is because
motor torque is the vector sum of the phase currents. Advantage: Goes
to full-steppers.

2) Most people want motors to turn, not just 'hold'. As soon a
full-step driven motor turns, its torque drops to 65% of its holding
torque. Where did the missing torque go? To resonating the motor is
where. Motor manufacturers sometimes specify 'dynamic torque'; this is
specified at 5 full steps per second. It is always between 60 to 65%
of holding torque. Not mentioned is the horrible racket the motor
makes at 5 full steps per second.

Microstepped motors do not resonate at low speeds, so no torque is
invested in resonance. Microstepped motors keep all their holding
torque while turning slowly. 65% for full-steppers, 71% for
microsteppers. Advantage: By a hair (6%), goes to microsteppers.

3) Things get a little dicey as speed increases. Microstepping ceases
to have any benefit above 3 to 4 revolutions per second. The motor is
now turning fast enough to not respond to the start-stop nature of
full steps. You can say the step pulse rate is above the mechanical
low-pass frequency limit (100Hz or so) of the motor. Motion becomes
smooth either way.

Simple drives persist in microstepping anyway above this speed. This
means they still try to make the motor phase currents sine and cosine
past this speed. A little problem with that and it's called 'area
under the curve'. The area under the sine function (0 to 180 degrees)
is only 78% of a square wave (full-step). Advantage: Goes to full-step
again.

More sophisticated drives transition from sine-cosine currents to
square-wave quadrature currents about then. Same as full-steppers.
Advantage: Draw.

4) As speed increases even more, another really big problem crops up;
mid-band resonance. This is the bane of full-steppers and
microsteppers alike.

Maybe you have experienced it; the motor is turning 5 to 15 revs per
second when you hear a descending growing sound from the motor and
then it stalls for no good reason at all. Faster it's OK; slower it's
OK, but not OK in that range. All you know is there is a big notch in
the speed-torque curve. This is mid-band instability or parametric
resonance.

Simple drives have no defense against this except to try not running
the motor in this speed range. Better drives have circuitry to
suppress this phenomena and it involves rate damping.

This is the equivalent of shock absorbers (rate dampers) on a car,
without them a car bounces repeatedly. Imagine a washboard road
surface in sync with this bounce; there would be sparks flying from
the undercarriage in short order. With rate dampers the 'bounce' is
suppressed to a single cycle. Mid-band compensation does the same with
steppers.

5) More than any other type of motor, step motor performance is tied
to the kind of drive connected to it. More than any other type motor,
a stepper can be driven from very simple drives (full-step unipolar
L/R) to very complex ones (microstepping full-bridge bipolar
synchronous PWM mid-band compensated).

____________________________________________________________________

Q.) How hot is too hot for a step motor?

A.) The maximum heat for most steppers is around 100ºC (212ºF), but it
is generally never good to have the motor heat go above 85ºC (185ºF).

_____________________________________________________________________

That is what I have added so far, in addition to fixing all
grammatical errors. Please keep the suggestions coming, as I am adding
most of them into the FAQ!

I will also be writing a "CNC Machine Setup Basics" white paper that
we will have on the site. All this will do is describe what the
different parts are for the machine on the control side, going from
computer to drive shaft. If there are any other papers you would like
to see written, please let me know.

In the process of redoing the website, I am also going to be giving
every piece of literature a major overhaul. If you have any complaints
about any of it, please let me know on here or email me. I will be
fixing things such as the "6 pin header shown in the manual, when
there is an 8 pin header on the board" problem. Thanks a lot for the
great replies!

-Marcus Freimanis

[jseiler]

Why is there a 1350 rpm limit w/o a g100?

[CoAMarcus]

There is a limit of 1350 with the old version of Mach3. Its maximum output was at 45kHz, which at 2000 steps per revolution worked out to 1350 RPM. Mach now has a 100kHz output, which brings the speed up to around 3000 RPM. 1350 is a conservative number because I do not know what version you may be running, or if your computer can do 100kHz.

[machining fool]

Can you tell me if I can conver my Bandit Control stepper motore to simple power feed? I am tired of buying cards that cost 400. a shot. Bob