spindle dc servo / encoder, lathe

[hutchison]

I'm exploring the possibility of using a dc servo motor for my lathe spindle, I have the dc servo drive and the dc servo motor, as far as i know the motor does not have an encoder although i have the option of adding one to it from the manufacturer.

My question is, if my spindle allready has a 200 hole index pulse plate on the spindle plate currently used for the rpm / speed readout in mach3, why would I need an encoder on the dc servo motor?.

and i'm a little confused as to how mach3 connects and picks up information from the encoder as i thought the dc servo drive and motor was a standalone cell and all mach does is put signals 'out' to the servo drive.... wether they are step & direction or PWM.

The servo drive can also use tachy feedback from the motor, what is this?

[keebler303]

The only reason you would need an encoder is if the drive required it. Mach will output either PWM or step/dir to the drive. The drive needs some type of feedback from the motor. This needs to be either an encoder or a tachometer. A tachometer is a small generator connected to the servo which outputs a dc voltage proportyional to its speed. From this, the drive can tell how fast it is going and change the voltage output so that it is going at the speed it is commanded by mach.

Matt

[hutchison]

cheers matt, i see. the tach is for self regulating speed with the drive, by referencing mach's commanded speed and the tach simeltaneously. i wont be using this spindle & drive for a stepped 4th axis type application, yet, but a spindle if possible.

what is the maximum speed/rpm at which mach can read a 500 count encoder?

i also think my 200 hole index plate will be useless as the holes are tiny and the manual says the slots need to be kinda 4 at calculated widths with one larger for main index pulse the rest for position etc. if 4 slots is good enough for the goose.

what do you think of using a 130vdc 12A dc motor for a spindle? its got 382 oz in from stall to 3000rpm, 950ozin peak to 1750rpm. its only a tiny lathe but i want to practice on it first then perhaps transfer the spindle assembly over to something with a proper construction / preloaded spindle and not ball race!

[philserveng]

The reason for a tacho is for closed loop speed control, not "if the drive needs it". Using a dc servo motor for spindle control means you will have a permanent magnet for the field, this will limit the maximum motor speed because the field is fixed. Traditional dc spindle drives have a field winding which can have a variable voltage, this allows the field to be weakened and the spindle can attain higher speeds, with corresponding reduced torque.The addition of a spindle encoder allows the controller to monitor the spindle rpm's and to fix feed per rev or revs per minute. Also, with a marker pulse, will allow for threading and other cutting cycles. If not needed a simple potentiometer can be employed to regulate speed. The encoder is not a tacho and vise versa.

[hutchison]

right so it needs a tacho then!. according to mach i can use a single index pulse to achive feed per rev, so im not seeing the use of encoder anywhere relevant ? thanks for the input phil, copy that the torque graph steeply to 0nm from 3100-3400rpm dead.

[keebler303]

If your servo drive needs the tacho, use that. If it needs the encoder, use that. Different drives use different types of feedback.

The 4 pulses per rev, or even one is all you need for mach for threading. A 500 line encoder would output pulses at 1.5 Mhz at 3000 RPM. Not even close to within the range mach can handle.

I think the motor will work just fine for a spindle. You won't need anything more that 3000 rpm for most purposes and it will be plenty strong. The servo drive will automatically increase voltage to the motor to maintain speed so it does not have to be so big as an open loop spindle.


Matt

[hutchison]

o.k im pretty much set on a tachy feedback set-up, the drive functions with either or both. simeltaneously according to the manual, hence the questions, plus i knew nothing about either, now a little more educated, thanks a lot. I now have to find out if the drive takes step and direction or pwm? i just plug it in and then tune it with mac right? heres a shot of the data :









i'll be guessing for a long time! how does mach plug in?

[hutchison]

im needing a pwm or s&d to 0-10vdc analogue volts convertor, can anyone recommend a good one with opto isolation? its to run this dc servo drive which does not take step or pwm input direct, cheers.

[hutchison]

which is better, a software or loop from mach3 or a hardware loop using the tachy generator for feedback? or are they not similar enough for comparison? mach states:


"Closed loop spindle control, when checked, implements a software servo loop which tries to match the actual spindle speed seen by the index or timing sensor with that demanded by the S word. The exact spindle speed is not likely to be important so you are not likely to need this feature in mach3 turn.

If you do use it then the P, I and D variables should be set in the range 0 to 1. P controls the gain of the loop and an excessive value will make the speed oscillate, or hunt, around the requested value rather than settling on it. The D variable applies damping so stabilising these oscillations by using the derivative (rate of change) of the speed. The I variable takes a long term view of the difference between actual and requested speed and so increases the accuracy in the steady state. Tuning these values is assisted by using the dialog opened by Operator>Calibrate spindle.

Spindle speed averaging, when checked, causes mach3 to average the time between index/timing pulses over several revolutions when it is deriving the actual spindle speed. You might find it useful with a very low inertia spinlde drive or one where the control tends to give sort term variation of speed.
...................................................................................."

where as the tachy generator will feedback upto 100v to the servo drive so it knows what speed its doing.. if the speed drops off from a demanded S word under spindle loading I imagine the servo drive increases power to the motor to compensate?

if i have this type of feedback wit my drive i dont need to use this option in mach right, I just wondered if any or either were better?

[phomann]

Originally Posted by hutchison im needing a pwm or s&d to 0-10vdc analogue volts convertor, can anyone recommend a good one with opto isolation? its to run this dc servo drive which does not take step or pwm input direct, cheers.

Hi,

Have a look at the PWM to isolated 0-10Vdc converters I make and sell.

The DigiSpeed-GX is a basic PWM to isolated 0-10V converter.
http://homanndesigns.com/store/index...products_id=21

The DigiSpeed-XL has a number of extra bells and whistles. It allows a manual speed potentiomer to be added to allow for manual speed control as well.
http://homanndesigns.com/store/index...&products_id=2

As to whether to use the Mach3 PID function or a drive with feedback built in, it is probably best to use the latter, as the PID in the drive will be optomised for it.


Cheers,

Peter.

[hutchison]

excellent, cheers peter, I have heard of your motor drives through the digital machinist magazine for this type of application so i'll take a look thankyou very much!

[hutchison]

Originally Posted by phomann Hi, Have a look at the PWM to isolated 0-10Vdc converters I make and sell. The DigiSpeed-GX is a basic PWM to isolated 0-10V converter. http://homanndesigns.com/store/index...products_id=21 The DigiSpeed-XL has a number of extra bells and whistles. It allows a manual speed potentiomer to be added to allow for manual speed control as well. http://homanndesigns.com/store/index...&products_id=2 As to whether to use the Mach3 PID function or a drive with feedback built in, it is probably best to use the latter, as the PID in the drive will be optomised for it. Cheers, Peter.

any idea how i can get both a positive and negative 0-10vdc to my drive? I can spin both ways without relays if i can incorporate it, I am also chuffed i found about my dc servo drive it takes analogue volts to control it directly / input, and not direct PWm, which is a shame as i could have plugged mach straight in but at least with analogue volts the set-up and monitoring will be easier.

does anyone have an idea which is a better option to use in mach 3 when converting the spindle drive signal to analogue volts, would it better to use PWM or step and direction? i am thinking using mach3's PWM output to volts as perhaps using step and dir would hog the processors resources when processing 35,000 pulses per second , anyone?