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General Mach Discussion / Losing Pulses at lower accelerations
« on: April 04, 2013, 02:17:56 PM »
I recently became interested in high speed machining. My homebuilt CNC mill is several years old, and has always worked fine at the lower feed rates that I usually use (1-8 ipm). I started testing what feed rates that I could get out of the system, and got some unusual results.
First, I ran a short program to run the mill back and forth on one axis:
G01 X2
G01 X1
G01 X2
G01 X1
G01 X1.5
G01 X.5
G01 X1
G01 X0
M30
I ran the program using increasing feed rates. I have a digital probe, and set X= zero before the run, then checked it after the run. I was able to get up to 200 ipm with an acceleration of 25 ips/s with excellent repeatability. I didn't go past 200 ipm in my tests.
I ran a similar test on the Y axis. I could get 150 ipm using an acceleration of 10 ips/s before I started losing steps. I found that holding this speed and increasing the acceleration made me lose steps. This all made sense to me.
I then ran a program that moved the X and Y axis:
G01 X2
G01 Y2
G01 X1
G01 Y1
G01 X1.5
G01 Y1.5
G01 X.5
G01 Y.5
G01 X1
G01 Y1
G01 X0
G01 Y0
M30
I found that using a feed rate of 150 ipm for X and Y that had worked OK on the single axis tests made me lose steps. (About 0.070 on both axes) Then things started getting strange. At a value of 50 ips/s, I got no lost steps. I increased to 75 ipm, and started losing steps. At this point, I held the feed rate at 75, and started playing with the acceleration. I found that low accelerations, like 5 ips/s LOST a huge amount of steps. Increasing the acceleration led to less lost steps. At the low accelerations, the stepper motors sounded OK, but the deceleration noise was not a happy one. The higher accelerations cause the stepper motors to stop dead nearly instantly, and really jerks the mill around (relatively - it's a big hunk of iron).
One of the things I thought might be happening is that the stepper voltage could be dropping when two motors are running simultaneously. I monitored the supply voltage with an old analog voltmeter. The voltage is only dropping briefly about 5V on the 70V supply during transitions. I added about 1F of capacitance on the power supply. It didn't change the testing results.
Any ideas?
First, I ran a short program to run the mill back and forth on one axis:
G01 X2
G01 X1
G01 X2
G01 X1
G01 X1.5
G01 X.5
G01 X1
G01 X0
M30
I ran the program using increasing feed rates. I have a digital probe, and set X= zero before the run, then checked it after the run. I was able to get up to 200 ipm with an acceleration of 25 ips/s with excellent repeatability. I didn't go past 200 ipm in my tests.
I ran a similar test on the Y axis. I could get 150 ipm using an acceleration of 10 ips/s before I started losing steps. I found that holding this speed and increasing the acceleration made me lose steps. This all made sense to me.
I then ran a program that moved the X and Y axis:
G01 X2
G01 Y2
G01 X1
G01 Y1
G01 X1.5
G01 Y1.5
G01 X.5
G01 Y.5
G01 X1
G01 Y1
G01 X0
G01 Y0
M30
I found that using a feed rate of 150 ipm for X and Y that had worked OK on the single axis tests made me lose steps. (About 0.070 on both axes) Then things started getting strange. At a value of 50 ips/s, I got no lost steps. I increased to 75 ipm, and started losing steps. At this point, I held the feed rate at 75, and started playing with the acceleration. I found that low accelerations, like 5 ips/s LOST a huge amount of steps. Increasing the acceleration led to less lost steps. At the low accelerations, the stepper motors sounded OK, but the deceleration noise was not a happy one. The higher accelerations cause the stepper motors to stop dead nearly instantly, and really jerks the mill around (relatively - it's a big hunk of iron).
One of the things I thought might be happening is that the stepper voltage could be dropping when two motors are running simultaneously. I monitored the supply voltage with an old analog voltmeter. The voltage is only dropping briefly about 5V on the 70V supply during transitions. I added about 1F of capacitance on the power supply. It didn't change the testing results.
Any ideas?