[LUCKY13]

Whats the max frequency or count with quadrature that the 320 can preccess?


I keep hitting a wall without any encoder signal problem and the motors have been tuned on a scope. Not matter what I do it want go beyound this point. The wall seems to be around 110,000 counts per second. I dont know if this is what's causing the wall but there is no noise in my encoder signals, nor do they get week. When I scope both A & B channels nothing gets out of phase and no matter how I tune the motors, or which driver & motor I am tuning it still hits this same wall. The encoder signals are very clean also, it never faults except when it hits this wall.




Jess

[mif73]

Hi Jess, I assume when you say 320 that you mean geckodrive G320 servo drive?
If so, I will try to help you out.
I'm not sure what you mean by hitting a wall, I assume that may mean you get the fault led to light and the drive has to be reset?

The docs that are supplied with the drive specify a max step rate of about 250Khz from the computer etc.
The max position error you can have between command and feedback pulses is 128 pulses, or else the fault led will light.
This can occur because of low power supply capacity, incorrect tuning of the drive, excessive step speed, incorrect accel/decel time constant, or heavy load/weak motor.

Does the problem occur with the motor installed or free running with no load?

What software are you using to provide the step pulses?- have you tried to slow the accel time and or decrease velocity?

I hope this info is on the right track for you , if not let us know.

Good luck.

[LUCKY13]

Hi Jess, I assume when you say 320 that you mean geckodrive G320 servo drive?
If so, I will try to help you out.
I'm not sure what you mean by hitting a wall, I assume that may mean you get the fault led to light and the drive has to be reset?

The docs that are supplied with the drive specify a max step rate of about 250Khz from the computer etc.
The max position error you can have between command and feedback pulses is 128 pulses, or else the fault led will light.
This can occur because of low power supply capacity, incorrect tuning of the drive, excessive step speed, incorrect accel/decel time constant, or heavy load/weak motor.

Does the problem occur with the motor installed or free running with no load?

What software are you using to provide the step pulses?- have you tried to slow the accel time and or decrease velocity?

I hope this info is on the right track for you , if not let us know.

Good luck.

mif73


Yes I am talking about the drive faulting when it hits a wall, and this is on a bench still, not on the mill.


I can back accel/deccel and squeez about 120 counts per second. Of course velocity is directly linked to your rapid speed. If you set velocity to 220, 220ipm is what it wants to run upto.

My system for instance is setup to run a 3 to 1 gear ratio on the motor with a 500cpr encoder mounted on the motor. With this setup at 220imp it has to count 110,000 encoder counts per second (in quadrature). I can squeeze things by slowing down the accel/deccel and get it to run 240imp which will have it counting 120,000 encoder counts per second. This is the very max it will run at no matter how things are tuned. Of course I want more accel than 1 so I chose to run it at 220imp with some accel/deccel.

If I was to set it up with a 4 to 1 gearing ratio on my motors I hit a different max rapid (imp) wall, but it comes down to the same counts per second on encoder counts.

None of these setups will over step the 250khz steps per second limit of the step pulse (if I understand right, I may not). The limiting factor is always the encoder counts per second no matter how I set it up. Lets so I was to run a 2.5 to 1 gearing ratio ( all of these ratios are figured with a 5 pitch screw also) At this setup I would have a max of 260imp (with some accel/deccel). So at 260imp I would figure this


(260/60) = ? x 25.000 = ? The answer would be 110,000 which would be the encoder counts per second . The 25,000 is the counts per inch (BTW I am using Mach3).


No matter what gearing ratio I chose to set the system up with the my rapids limit is controled by the encoder counts per second I can run, 110,000 with steady results and 120,000 with it on the edge. Motor tuning does not effect this, of course i can get to fault at a lower point with the motors out of tune but as long as they are tuned the results are the same. Each motor with all three axis hit the same limit. With the scope all encoder signals are very good, no noise and the signal stays strong. I tried different pulse widths and there was no effect to this limit from 1 to 5us.


Maybe there is something else I am not catching but it seems like its the drive not able to count encoder counts beyound this point. If so a simple change of encoders to a 250cpr could solve it. I would not do this because I would rather have the better ressolution and live with the limits, but I would like to answer as to why there is this limit, or what is causing it. No biggie no matter what is causing it , I just want to understand. One thing I have notcied is others with the same setup as mine hit the same wall if there system is able to push to the max ( big enough power supply and fast enough ability to send the step/direction signals) limits. My system has a 72v 20amp PS and the motors are the HomeShop 850 servos which are rated at 72v. My voltage bever drops below 71.5v on the motor lines. I am running a SmoothStepper which can give very fast step/direction signals but with my 3 to 1 gearing on the motors I only have to run it at 128khz for it to keep up so I don't feel like I am surpassing the 250khz limit of the drive. I can set the SS to 250khz but there is no use with this setup, you only lose res of velocity updates if I do that.


Thanks Jess

[TOTALLYRC]

Max on those motors is 140,000 counts per second based on 4200rpm and 2000 ct/rev. This is optimum based on their specs from the website and the kv ratings on the motors.

Then remember, that when installed, the motors will not go this fast as they will have to move a load.

Remember that 1 encoder count is equal to 1 step from the software unless you are using a step multiplier like the G340.

I think what you are describing as the wall is not the limit of the g320, but the limit of your motor to follow the g320 commands and then when the motor gets more than 128 steps out of position, the drive faults.

4200rpm*2000steps/rev= 8,400,000 steps/min / 60secs/min=140,000steps per sec. This is the maximum steps per sec that the motor will accept if it runs at 4,200rpm at 72 volts. You are pretty close to this at 120,000. Not every motor will go exactly the same rpm on the same voltages and I would say you are within the tolerance at 120,000 per sec.

3 to 1 * 5tpi * 2000step/rev= 30,000 steps/inch.

The absolute max that these motors will go is 280 ipm with a 3 to 1 on a 5tpi screw at 4200rpm under perfect conditions. If my math is correct and I have been wrong before. As soon as you load the motor with mass and friction from the axis, you will go even slower.

It seems to me that you are at the limits of the motors, not the g320

Mike

[LUCKY13]

TotallyRC, I believe you may be right on this. I picked up a different scope ( one I payed for that has been calibrated lately) that works really good. After redoing my electronics enclosure box so I could get to everything much easier for testing I did a lot of testing to find no other reason for the wall. My encoder signals are just rock solid, and in perfect phase at all times. The step direction signals going to the drives showed good results also. My system is just simply noise free at all times. This may change when I get it mounted up on the mill but for now its as clean on all signals as I could ask.


I did get it to go past the wall with the accel/deccel down to number 1. This allowed it to go the full RPM of the motor which wasnt much more than before really. Still its not a tune that could be used. Running pretty good accel/deccel I can still get a good 220ipm out of the system and I expect to be able to get close to that when mounted on the mill (atleast 180ipm). This is pretty good rapids so I am not going to worry with it as long as the system shows to be working like it is.

The HEDS encoders that are claimed to not work very good show very good signal and this is one thing I expected trouble out of. I have even extended the cable to 8ft now and they still work. I have good isolated power running to them. I don't know if that really helps them or not, but the lines are very clean from noise with this setup. I do plan to try some other equipment I have that will allow me to push more encoder counts just for testing later but I don't expect to find anything. I just want to comfirm for sure that I am not being held back anywhere other than reasons that should be. With these motors having a max of 4200rpm its very possible that it is the limit of the motor as you where talking. This being my first system I didn't realize that a motors RPM limit would react this way and I figured it woudl just top out on RPM and not turn any higher. I guess in a sense thats what they are doing but I didn't expect the drive to loose lock when this happens.


If the Gecko drive step pulse freqency limit is 250khz then 250,000 encoder counts per second should be the upper limits if I am understanding this right. I would like to try a higher count encoder on this system, and a little later I will do so. I guess you can get to fine of a resilution but I would like to try one anyway. Of course right now its pretty high res anyway but I tend to try to get all the performance I can get out of machines no matter what they are. I plan to try a couple of different gearing ratios on my motor belt drives so I will wait until I settle in on a ratio before I go any farther.




Jess

[TOTALLYRC]

Hi Lucky13,

It is nice to see that you are taking a very reasoned approach and not just saying that the drives are no good.
I am happy to see that the HEDS modules that you are using are working for you. All three of mine had to be replaced to get the machine rock solid.

When you said that the motor would top out and you didn't realize that the drive would lose lock, your original post was the first time that I thought it all the way through. It really makes sense because the motor is supposed to run in lock step with the drive at all time and if you get outside of the error window, then it faults.

Out of curiosity, with the 8' cable, were you scoping the signals at the drive where it would be weakest or at the encoder?

Keep up the good work.

Mike

[keebler303]

You can see if you have reached the motors max speed by putting the scope on the motor leads. You should see the PWM signal. As you speed it up, the duty cycle will increase. You are at full speed when the duty cycle is at or near 100%. If your acceleration value is reasonable and the drive faults when the duty cycle approaches 100%, this is almost certainly the problem. You are telling the G320 "faster, faster" and it says "I am going as fast as I can" and then it faults. You are not supplying enough voltage to go any faster than that. Try using half the supply voltage and you should see the drive fault at about half the step rate you are seeing now.

Matt

[LUCKY13]

You can see if you have reached the motors max speed by putting the scope on the motor leads. You should see the PWM signal. As you speed it up, the duty cycle will increase. You are at full speed when the duty cycle is at or near 100%. If your acceleration value is reasonable and the drive faults when the duty cycle approaches 100%, this is almost certainly the problem. You are telling the G320 "faster, faster" and it says "I am going as fast as I can" and then it faults. You are not supplying enough voltage to go any faster than that. Try using half the supply voltage and you should see the drive fault at about half the step rate you are seeing now.

Matt

Thanks Matt.

I feel like we have indead reached the motors limits. I mean lets face it the motors are not going to run any faster than they are spec for. Until I mount the system on the mill itself and see where things fall I am just going to let it be. Although I will run this test just to see what I find. But as long as the motors perform close to what I know they should do there is not much I a can do to help them. I have seen these motors run at 3300rpm on a mill that is bigger than mine so I should be able to reach this level anyway. This would put me at 220ipm rapids with great response out of the table movement so it should be quit good if it does infact reach this. This would be with a 3 to 1 gearing and the motors might just have enough power to run something like a 2.5 or even 2 to 1 gearing which would give me even more rapid speed. It all depends on how it runs though because I want give up any response out of the system for rapid speed. I will just have to wait until I am mounted on the mill to answer these question.



Jess