Advice on buying servos.

[Agathon]

This is probably a bit premature, but I'm thinking of fitting servos to my Fehlmann Picomax 50 CNC table. Originally it had massive Superior Electric steppers rated at 2.1Nm nominal torque.

I like the Teknic ClearPath integrated servos and I think their CPM-SDSK-3421S-RLN will do the trick - https://www.teknic.com/model-info/CPM-SDSK-3421S-RLN/

However, I like a bargain, so I've been wondering about buying used servos and drivers from eBay. The trouble is I know nothing about servos! Can anyone give me some basic guidance on what to look for and what to avoid?

[Boyan Silyavski]

Hi,
here i organised some info. If something not clear, please ask:

If buying new servos:
-Look for the servos to come with cables if not integrated
-www.jmc-driver.com the cheapest servos around that i know of

If buying second hand:
-cables and connectors, at least the connectors. If servo cables are needed TME.eu sells the cheapest quality cable for servos and generally speaking. Some times a great deal servo without cables could be found, just calculate what will cost you from TME. Otherwise servo cables are extremely expensive. >8-10euro per meter against ~2-3euro from TME
-Samsung and Panasonic servos are the best you could find for the buck on ebay. Light years away from cheap chinese servo, speaking of possibilities. Not that for simple CNc will make any difference, except that manual is better written.
-There are 2-3 reputable sellers from South Corea and Hong Kong/ skmh218, usedparts-pk, and others /. Avoid Yaskawa servos, some are not accepting the necessary signals so no good for us
-Read manuals and make sure you are buying servo rated for around 3000rpm, that accepts Step Dir signal and even better differential signal. Read manual and make sure if motor is general purpose servo not something special purpose
-Absolute encoder is best, but not a must. Just an added bonus
-Look at pictures carefully and buy stuff that only looks newish or not in bad shape
-Servos need or if not needed is best to be geared down 2-3:1. Why? Because gearing down gives better resolution without artificial microstepping, more power from smaller servos $$$ and at the end of the day you probably dont have a machine that could run with a servo spinning at full speed. My Samsung servos are 3000RPm straight line torque rated but could spin to 6-8k RPM without a problem.


-For your scenario 100w-200W servos geared 2:1 will be OK probably.


-Servos need a good controller as more impulses have to be send to them than normal stepper drive. Do not buy servo until you have made all calculations!. 100khz per axis is minimum, normally 400khz and up, depends on machine, feed desired, etc. My Samsung servos for example dont need so fast controller as they are rated


Example:
My 400w samsung servo could have 2000/2048/2500/10000 P/R etc.(Incremental or Absolute type) encoder. It has the 2048 one. Most of chinese servos when say they have 2048 encoder they mean 2048/4=512 Pulse Per revolution. Not so here. The samsung servo has 2048 Pulse per revolution

So i have geared it 3:2 on a 10mm ball screw. That means when once the ball screw turns 10mm distance is traveled. I wanted my machine to have 20m/min real world working speed. It could easily reach 30 or more as my servos could spin to 6-7k RPM but that's another matter. I calculated all based on 3k rpm.

For 20 000 mm/min the ball screw has to turn 20000/10=2000 times as its geared 3:2 / 30 tooth to 20 tooth HTD 5 15mm pulley / if motor spins 3000rpm then i will achieve on theory that 20 000 mm/min. for 3000rpm to happen the controller has to output 3000x2048=6 146000 pilses per minute/60=102.400pulses per sec hence controller should be at least 100khz per channel.

Do you calculation if you have the absolute controller that is 10000 pulses per revolution, you will need at least 400kz controller/ per axis. Now simply said the thing is that most controllers when say the PPR they mean /4 so that means all is multiplied by 4 , so in reality you need a controller in the Mhz not Khz

so for 1mm my motor will make 1/10 turn of the ball screw. ((3/2)* 2048)/10=307.2 pulses per mm which means my machine has electronic resolution of ~0.003mm ( 1mm/307.2 PPR). Not bad for an all purpose woodworking machine. No microstepping or so. Servo drives also support microstepping and internal gearing.



What i am saying get acquainted well with all that mentioned and you will know more or less what are you doing when buying servos

[Clive S]

I like the Teknic ClearPath integrated servos and I think their CPM-SDSK-3421S-RLN will do the trick - https://www.teknic.com/model-info/CPM-SDSK-3421S-RLN/

Are these not just closed loop steppers. They spin a about 1000 rpm A bit like the Leadshine closed loop stepper/servo

[Agathon]

Hi,
here i organised some info. If something not clear, please ask:

If buying new servos:
-Look for the servos to come with cables if not integrated
-www.jmc-driver.com the cheapest servos around that i know of

If buying second hand:
-cables and connectors, at least the connectors. If servo cables are needed TME.eu sells the cheapest quality cable for servos and generally speaking. Some times a great deal servo without cables could be found, just calculate what will cost you from TME. Otherwise servo cables are extremely expensive. >8-10euro per meter against ~2-3euro from TME
-Samsung and Panasonic servos are the best you could find for the buck on ebay. Light years away from cheap chinese servo, speaking of possibilities. Not that for simple CNc will make any difference, except that manual is better written.
-There are 2-3 reputable sellers from South Corea and Hong Kong/ skmh218, usedparts-pk, and others /. Avoid Yaskawa servos, some are not accepting the necessary signals so no good for us
-Read manuals and make sure you are buying servo rated for around 3000rpm, that accepts Step Dir signal and even better differential signal. Read manual and make sure if motor is general purpose servo not something special purpose
-Absolute encoder is best, but not a must. Just an added bonus
-Look at pictures carefully and buy stuff that only looks newish or not in bad shape
-Servos need or if not needed is best to be geared down 2-3:1. Why? Because gearing down gives better resolution without artificial microstepping, more power from smaller servos $$$ and at the end of the day you probably dont have a machine that could run with a servo spinning at full speed. My Samsung servos are 3000RPm straight line torque rated but could spin to 6-8k RPM without a problem.


-For your scenario 100w-200W servos geared 2:1 will be OK probably.


-Servos need a good controller as more impulses have to be send to them than normal stepper drive. Do not buy servo until you have made all calculations!. 100khz per axis is minimum, normally 400khz and up, depends on machine, feed desired, etc. My Samsung servos for example dont need so fast controller as they are rated


Example:
My 400w samsung servo could have 2000/2048/2500/10000 P/R etc.(Incremental or Absolute type) encoder. It has the 2048 one. Most of chinese servos when say they have 2048 encoder they mean 2048/4=512 Pulse Per revolution. Not so here. The samsung servo has 2048 Pulse per revolution

So i have geared it 3:2 on a 10mm ball screw. That means when once the ball screw turns 10mm distance is traveled. I wanted my machine to have 20m/min real world working speed. It could easily reach 30 or more as my servos could spin to 6-7k RPM but that's another matter. I calculated all based on 3k rpm.

For 20 000 mm/min the ball screw has to turn 20000/10=2000 times as its geared 3:2 / 30 tooth to 20 tooth HTD 5 15mm pulley / if motor spins 3000rpm then i will achieve on theory that 20 000 mm/min. for 3000rpm to happen the controller has to output 3000x2048=6 146000 pilses per minute/60=102.400pulses per sec hence controller should be at least 100khz per channel.

Do you calculation if you have the absolute controller that is 10000 pulses per revolution, you will need at least 400kz controller/ per axis. Now simply said the thing is that most controllers when say the PPR they mean /4 so that means all is multiplied by 4 , so in reality you need a controller in the Mhz not Khz

so for 1mm my motor will make 1/10 turn of the ball screw. ((3/2)* 2048)/10=307.2 pulses per mm which means my machine has electronic resolution of ~0.003mm ( 1mm/307.2 PPR). Not bad for an all purpose woodworking machine. No microstepping or so. Servo drives also support microstepping and internal gearing.



What i am saying get acquainted well with all that mentioned and you will know more or less what are you doing when buying servos

Many thanks for such a comprehensive and detailed reply. It gives me a really good starting point.

Are these not just closed loop steppers. They spin a about 1000 rpm A bit like the Leadshine closed loop stepper/servo

According to Teknic they're a true servo. They certainly sound like a servo when running. Teknic go to some length to make the distinction between ClearPath and stepper motors.

[Agathon]

Apart from the obvious, what's the difference between AC and DC servos?

[Agathon]

I see a few Panasonic and Samsung servos on eBay. However, I notice that apart from the part numbers sellers don't list any specs. Is there somewhere to check specs? Both the Panasonic and Samsung sites seem to be impenetrable.

[Boyan Silyavski]

The servos come on certain series. So you have to find the manual for the series.





https://www.google.es/search?q=Samsu...+drive++manual

[Boyan Silyavski]

Apart from the obvious, what's the difference between AC and DC servos?

The obvious. The AC servo when 230VAC does not need additional transformer. This is best. Then cheaper servos could be 80VAC then you need to feed them 80VAC. But of course depending on price availability sometimes is worth making that PSU.

[Agathon]

The obvious. The AC servo when 230VAC does not need additional transformer. This is best. Then cheaper servos could be 80VAC then you need to feed them 80VAC. But of course depending on price availability sometimes is worth making that PSU.

So it's not the characteristic of the motor itself but just the driver? So are all servos dc motors or are some ac?

[Boyan Silyavski]

So it's not the characteristic of the motor itself but just the driver? So are all servos dc motors or are some ac?

Ahh, the AC does not have brushes. The DC servos with brushes are older motors with older drives that may need Voltage control +-10v, not pulse control. That could make things bad, as this type of controllers are expensive cause their only purpose is to retrofit old machines. Hence thats why i told you to search for drives that support Pulse and Direction