How to determine encoder cpr needed?

[radioactive]

Based on reading on this forum I'm trying to determine what encoders to purchase for my mill retrofit. Here are my numbers, let me know if it sounds correct or not?

Servo motors are rated at 140V and 2400 rpm, however will be driving them with Gecko 320's at 80V. So the max rpm should be reduced proportionally to 1714. Figuring rated RPM as 80% of max rpm I come up with 1371 (round to 1400)

I'm looking at using Renco encoders 500 cpr linky. I believe this means that the geckos will see them with a resolution of 500 x 4 = 2000 steps per rotation?

the ball screw is a 5 TPI model. Servo's use a 2:1 belt reduction. I am going to be using Mach 3.

I will get 500 cpr x 4 x 2 x 5 TPI = 20,000 SPI (steps per inch) with the encoder directly mounted onto the shaft of the servo.

I can get 1400 RPM x 4 x 500 cpr = 2,800,000 SPM (steps/Minute)

max feed will be: 2,800,000 / 20,000 = 140 IPM.

My max velocity will be:

140 x 20,000 / 60 = 46,666 SPS (steps / second)

Note: max feed was 170 IPM using a servo rpm of 1700, but wasn't sure if I need to derate them from max speed for loaded conditions.

Given the numbers above I don't see any issues with Mach 3, the smooth stepper, or the drives using 500 cpr encoders?

Also, not knowing what is above or below normal speeds, the 140 IPM seems just fine to me.

2.3" per second - seems just fine

[H.O]

Hi,
Yep, all numbers seem to be correct, good job! Mach3 should handle it nicely on a fairly quick computer and since you mention the Smoothstepper you shouldn't have any problems at all reaching that speed.

One small note though, the Geckos are rated for 80V, you might want to have some margin there. If you really want to press the last bit out of them make sure you fit a powerdump circuit to the supply so the voltage doesn't go above the limt for the Gecko. They are rated 80V and will/should work at 80V but not much above that.

[SmoggyTurnip]

Originally Posted by H.O Hi, If you really want to press the last bit out of them make sure you fit a powerdump circuit to the supply so the voltage doesn't go above the limt for the Gecko.

I didn't know such a thing existed. Where would you find one?

[mc-motorsports]

Better check those Renco encodors... I think they pull 150ma which will blow your Gecko drives.
Check US Digital Encodor, you need something that pulls less than 50ma, otherwise the drive will fail immediatly.

[radioactive]

I will be buying a power supply as well, so I will have to find something in the 75 volt range. Anyone know of a good place to look for one?

The specs on the Renco encoders list them as pulling 55 mA max... I would think these should work, otherwise I can use a seperate 5V power supply for the encoders and not worry about it.

[Al_The_Man]

Originally Posted by radioactive The specs on the Renco encoders list them as pulling 55 mA max... I would think these should work, otherwise I can use a seperate 5V power supply for the encoders and not worry about it.

The ones I picked up draw 50ma, they have a push-pull output stage.
Al.

[mc-motorsports]

I had some from Renco that drew 150ma, blew my drives. Figured I would give a heads up and save someone some money and time.

[Jason3]

Sounds like a worthwhile project

You might try Antek for power your power supply: http://www.antekinc.com/ps.html - they're nice, heavy duty units. You can have them with a second 5v (and third, if you like) regulated output for your encoders or whatever else too - just email John and ask, he should be able to set you up!

Regards,

Jason

Edit - should point out there's a range of voltages available for the regulated outputs, not just 5v. I get them with three fused LED indicator outputs for the servo drives, and an additional 12v output for the Granite Devices' drives logic circuits...

[mc-motorsports]

Originally Posted by Jason3 Sounds like a worthwhile project You might try Antek for power your power supply: http://www.antekinc.com/ps.html - they're nice, heavy duty units. You can have them with a second 5v (and third, if you like) regulated output for your encoders or whatever else too - just email John and ask, he should be able to set you up! Regards, Jason Edit - should point out there's a range of voltages available for the regulated outputs, not just 5v. I get them with three fused LED indicator outputs for the servo drives, and an additional 12v output for the Granite Devices' drives logic circuits...

Antek is good stuff. And I want to say that voltage is linear to servo RPM? Here's where Al chimes in...
What would be better for servo's... 50v at 20a or 80v at 10a?

[Jason3]

More current = more torque. More voltage = more RPM.

I better leave it to Al to give us the definitive word though, I'm no expert

Regards, Jason

[Al_The_Man]

Remember that the typical torque curve for servo's is similar to Steppers, maximum at zero rpm, with servo's, a gradual flattening off slightly up to max rated rpm so if you want rated torque, you need a supply that will meet that current, if you want the rated rpm, then the drive has to be capable of supplying the necessary voltage.
BTW, I usually purchase Antek Toroidal's and wind on any auxiliary winding I might need, e.g. 5v, 24v etc.
Al.

[radioactive]

Al,

Regarding the servo drives amperage usage. My existing servo's are SEM 30M4-59, rated on the nameplate as 2400 rpm, 142 volts, 30 pulse amps. Since I don't think lightning fast speed will be required for what I'm doing, I settled on Gecko's which as you know are 80V 20A drives. So my speed will be reduced by roughly 45% which isn't an issue for me. How will the peak amperage be affected? Is it also reduced by 45% due to the lower voltage and as a result have less torque? Or will it still require the same peak current and continuous current?

I'm thinking in my head that somehow you should be able to control how much current the drive requires (i.e. don't accelerate the servo's as fast). Is this a parameter setup in Mach3 as far as how fast to output the step signal?

I need to determine how beefy of a power supply I will need...