Help with design of stepper drive

[DrewSmith007]

I'm looking into driving this motor:

http://www.eminebea.com/content/html...pm42l048.shtml

with this IC:

http://www.allegromicro.com/en/Produ.../2916/2916.pdf

I'm not exactly sure how it works though, i.e. what pins on the IC are step, direction, or if it even operates this way. I plan on running this drive from a microcontroller, probably a cubloc 405.

Any help with the inputs, current sensing and PWM would be greatly appreciated.

Thanks
Drew.

[nhatson.elec]

I'm looking into driving this motor:

http://www.eminebea.com/content/html...pm42l048.shtml

with this IC:

http://www.allegromicro.com/en/Produ.../2916/2916.pdf

I'm not exactly sure how it works though, i.e. what pins on the IC are step, direction, or if it even operates this way. I plan on running this drive from a microcontroller, probably a cubloc 405.

Any help with the inputs, current sensing and PWM would be greatly appreciated.

Thanks
Drew.

To Drew
UDN2916 just is, Dual Full-Bridge PWM Motor Driver, you need tranlator to control stepper motor

Pin I0,I1 to control current motor coil.
Pin Phase to control current direction.

Best regards

[DrewSmith007]

Thanks for your input.
I don't guess I know what a translator is. Care to elaborate or point me in the right direction? I don't mind reading...

Thanks

Drew

[MechanoMan]

For your FWB chip there, your uC will need to tell which side of the bridge is high and which is low. It would help to have a uC with an actual module for this.

A "stepper driver chip" usually means it takes in a step pulse and direction signal and itself calculates what the state of the FWB should be. The driver may or may not actually have the FWB power output stage on the chip.

[DrewSmith007]

Thanks, but I think I'm going to go with this self built driver. I saw it on instructables, and digikey puts the price about 25-30 bucks for 3 axis. It doesn't have any special features like PWM, but it does take step/direction and should work for what I have in mind.

http://www.instructables.com/id/Easy...and-Driver-ci/

^^ for those interested... I'll let you know how it turns out.

Drew.

[irving2008]

Theres something very strange with that data sheet.... I think their coil resistance for bipolar operation is wrong.. if its 60ohm unipolar it should be 30ohm bipolar parallel and 120ohm bipolar series - it could never be 7ohm...

That motor will need to be run in Unipolar mode on that driver. On a 24v rail @ 60ohm coil resistance it will take 400mA per phase and should not require the dropper resistor (bulb) if the motor spec sheet is to be believed...

According to the spec sheet the motor generates about 1W of power output (roughly 0.08Nm at 100pps = 2rps = 125rpm) which at 80% efficiency is 1.25W input - however the rated input is 24v * .4A = 10W, these motors clearly aren't that efficient!

It should work OK, but dont expect high performance from the motor.... what do you plan to drive with it?

[DrewSmith007]

It's just going to drive the axes of a small engraving machine. probably 2"x2"x.5". Dremel tool spindle, .015 depth of cut.

[irving2008]

It's just going to drive the axes of a small engraving machine. probably 2"x2"x.5". Dremel tool spindle, .015 depth of cut.

what are you engraving, what sort of cutter on the dremel?

[DrewSmith007]

soft aluminum rabies tags. Currently my dad wastes at least an hour a day doing it by hand, and if I build a cnc engraver his receptionist can do it.

[irving2008]

soft aluminum rabies tags. Currently my dad wastes at least an hour a day doing it by hand, and if I build a cnc engraver his receptionist can do it.

A worthwhile cause then

Some questions:
1/ What are you using for linear motion, slides or what?
2/ What leadscrews (I suspect just allthread).
3/ Is this a moving XY table or moving YZ gantry... how much weight (worst case) needs to be moved (table + Y axis if moving table, all Y + Z axis components if gantry)
4/ How fast do you want to cut
5/ Am I right to assume a burr head engraving tool, say 0.02" (.5mm) dia?

[DrewSmith007]

A worthwhile cause then

Some questions:
1/ What are you using for linear motion, slides or what?
2/ What leadscrews (I suspect just allthread).
3/ Is this a moving XY table or moving YZ gantry... how much weight (worst case) needs to be moved (table + Y axis if moving table, all Y + Z axis components if gantry)
4/ How fast do you want to cut
5/ Am I right to assume a burr head engraving tool, say 0.02" (.5mm) dia?

I haven't really decided what the design will be. That will depend on my group members. I'm doing this for a senior design project if it's approved. I suspect we'll use allthread, and probably brass or delrin bushings on steel rods for the linear motion. we might be upgrading the steppers to some I pulled out of a copier, if this driver can run them.

I had planned on a moving y table, and x-z gantry. Seems most compact and stable. I'm thinking 5-10 IPM. If the burr head engraving tool is the round ball on a shaft, that's what I was thinking of. I guess a v-bit could be used as well.

[irving2008]

I haven't really decided what the design will be. That will depend on my group members. I'm doing this for a senior design project if it's approved. I suspect we'll use allthread, and probably brass or delrin bushings on steel rods for the linear motion. we might be upgrading the steppers to some I pulled out of a copier, if this driver can run them.

I had planned on a moving y table, and x-z gantry. Seems most compact and stable. I'm thinking 5-10 IPM. If the burr head engraving tool is the round ball on a shaft, that's what I was thinking of. I guess a v-bit could be used as well.

Toby,

Since this is a project I wont give it all away , but the following might help...

The way to calculate required torque is shown below:

The required motor torque to drive a leadscrew assembly is the sum of three components: static friction torque, torque to resist the cutting forces and inertial torque, T = Tf + Tc + Ta

- Tf: Torque to move load against friction: The torque to move a certain load is a function of the lead and efficiency of the leadscrew assembly plus the friction of the linear bearings

Tf = Load x Lead x u/(2π x e) Nm
where:
Load = N e.g. mass of gantry x 10 (for gravity)
Lead = metres
u = friction coeff (oilite bearings is roughly 0.3-0.5, linear/ball bearings is 0.1, delrin on steel about 0.2)
e = efficiency of screw (0.8 for ball, 0.2 for acme or trapezoidal, maybe similar for delrin nut on allthread)

- Tc: Torque to resist cutting forces

Tc = CF x Lead/(2π x e) Nm
where:
CF = cutting force in N (this can be estimated from power of cutting tool in Watts/(2π x revs per sec x tool dia in mm)
Lead = metres
e = efficiency of screw (as above)

- Ta: Inertial torque:
Ta = (ls + IL + Im) x α Nm
where:
Is = The inertia of the screw in kgm^2 = π/32*density * length * diameter^4
(density in Kg/m^3 = 7850 for steel, length and diameter in metres)
IL = effective inertia of load (kgm^2) = Load x (Lead/(2π x e))^2
Im = motor rotor inertia in kgm^2 (typically 2 - 5 x 10^-5kgm^2 for a NEMA23/34 motor, less for a small one)
and
α = Angular acceleration (rad/s^2)

A typical acceleration for a small to medium machine is 2000rad/s^2 which allows it to follow a cutting path without under or overshoot.

Note: if geared/belt reduction (i.e. not 1:1 drive) then:
(Tc + Tf) is adjusted by the ratio
(Is + Il) is adjusted by the square of the ratio

Additional torque associated with driving and supporting the leadscrew must also be considered but can generally be accounted for by adding 5% assuming supporting ballraces and good quality couplers.

These torques are at the operating speed of the motor, not the holding torque. So once you have worked the torque T out you must work out the actual motor revs at the required cutting speed (depends on lead) and from the torque curves factor this back to the required holding torque (or use the fact that typically torque at cutting speed = 40% holding torque)

A working margin of 1.5 - 2 times is considered normal (so if 1Nm is the result of the above a 2Nm stepper would be selected)