[brian257]

Hmmm, tried this in a different forum with no response. I will try again here.

I have a Kelling (Automation Technologies) KL34-180-72 DC servo motor connected to a Gecko G320X drive. I am using the Kelling 72V, 20A power supply. I have it temporarily mounted to the knee on my Bridgeport to do some testing to see what ratio to gear the pulleys before I build the final mounts. Right now it is set up with a 2:1 ratio. I know that ratio is a bit low, but I want to see what I can drive with the motor then gear it a little more. With the smallest pulley I can get on the motor and still have enough meat on the hub to tighten the set screws I can get a 3:1 ratio and still have the lead screw pulley smaller than the dial so I can still use it easily in manual mode.

When I first tried this I connected the motor directly to the power supply without the Gecko. Moving the table up it moved slower than it did going down, but it still had plenty of torque to truck it right on up.

After connecting the Gecko and running it with Mach III to see what max speed I could get I expected that it would still have the same torque available and I would not have a problem until I exceeded that max speed going up, but I was very surprised that I could not get the table to go up at any speed. It moves down just fine, but going up it sometimes just does not move at all and sometimes it starts to move and goes just a little way then stops.

I have not done the PID tuning on the Gecko yet, but I did set the pots to the recommended positions and I have the max current cranked all the way up on the drive. I ran through some troubleshooting with Gecko on the phone and all the other settings seem to be fine. I have tried with the jog speed set very slow and faster all with the same results.

Any ideas what to check next? I have not put a meter on the motor direct to the power supply since I don't have a DC meter that will go over 20A, but it is a 20A power supply and the motor is not stalled so I am making the assumption that it is not the 20A limit on the Gecko that it keeping the motor from running. Is it normal to be able to get more torque out of the motor when connected direct to the power supply than when the Gecko is driving it?

[will gilmore]

Try checking the voltage at the motor.

[gus]

It sounds as if there is not enough current in the driver.

Bridgeport used 3 15 amp drivers and did nothing as heavy as the knee

[derek]

The acceleration might be too high in your servo tuning. I have a servo motor driving my knee. It's one of the old Pac sci tread mill motors. I'm running it at 120V through a Rutex drive and it will labor to move the knee up. I added gas springs to either side of the knee in order to make it more neutral. It zips right along now.

[Karl_T]

I also added air cylinders to my knee. that's the key.

May be a bit overkill, but i used a ball screw to replace the acme screw and added a brake to the servo. This allowed use of a small servo and small drive.

[brian257]

I just got two air cylinders yesterday. I bought two of them with a 2" bore and 14" stroke from Automation Direct for about $200 total with the end brackets and fork ends and got a $20 5 gallon air tank from Harbor Freight. That will give me 628 pounds of lift at 100 PSI. I should have a chance to mount them up sometime this or next week. Should make a huge difference.

[keebler303]

It is possible the servo motor was drawing more than 20A when it was connected directly to the power supply. It is a fact that you will lose some voltage when you run through the servo drive. At locked rotor torque, where you were at, the current is dictated by the resistance of the motor. If the resistance is too high, the drive cannot get 20A through the motor. In any case, you don't want to be operating at the maximum possible limit of the motor and drive. I think the air springs will make a big difference.

Matt

[Al_The_Man]

I just got two air cylinders yesterday. I bought two of them with a 2" bore and 14" stroke from Automation Direct for about $200 That will give me 628 pounds of lift at 100 PSI. I should have a chance to mount them up sometime this or next week. Should make a huge difference.

My Excello mill is fitted with a similar set up, but has a constant pressure regulator, this prevents pressure changing in the cylinder, when in the down direction the cylinder pressure can climb due to compression.
Al.

[brian257]

I did the calculations. The two cylinders together are 88 cubic inches. the tank is 1,350. With 100 PSI in the tank with the cylinders fully extended it will only increase to 106.5 PSI with the cylinders fully compressed. It would be a much different story with no tank or a very small tank.

[brian257]

I finally got my air cylinders mounted on the knee today. I didn't have the motor connected so I did some hand turning to see what kind of difference it made. I was very surprised to find out that with only 80 PSI of air that it is now easier to crank the table up with the air assist than it is to crank it down without it. 80 PSI gives me about 500 pounds of lift. I figure that the table and knee are around 900 pounds. With the small pulley on the machine I can grab the pulley and crank it up. Could barely do that before the cylinders. I will try hooking up the motor and running it on Mach 3 tomorrow, but looks like problem solved!

It is surprising how much friction there is with the acme screw. I would never have guessed that providing extra lift on the table makes it easier to go down also.