Stepper Motor System / Advice

[alistairsam]

thanks, not sure if i can find the inductance.
so if i used say 18v supply on the motor rail, and set my Itrip chopping current to around 1A max, will I be able to drive the 1.8V/3A motor without overheating the driver?
i'm using a variable resistor for the chopper reference voltage so i can vary max current upto 2A (or 1.6A).

[bojan]

Originally Posted by alistairsam

thanks, not sure if i can find the inductance.
so if i used say 18v supply on the motor rail, and set my Itrip chopping current to around 1A max, will I be able to drive the 1.8V/3A motor without overheating the driver?
i'm using a variable resistor for the chopper reference voltage so i can vary max current upto 2A (or 1.6A).

Yes, that is correct.
Chopper should generate the saw-tooth voltage on motor coil, dependent on coil inductance.

[snowyskiesau]

Check out this stepper motor guide from Gecko.
The drives I use are Gecko 250's and G540.

[Bassnut (Fred)]

Yes, no problem driving them at higher voltages with current limiting, tried it, works well. I like the TI DRV8824 microstepping driver your using, very interesting. Is it easy to interface to with a PIC?.

[Bassnut (Fred)]

Originally Posted by bojan

Be careful with this ...
The maximum voltage that can be applied to motor coil is dependent not only on inductance but also on PWM (or stepping) frequency.
The max coil current shouldn't be exceeded, though.

Funny you should say that, I noticed the average current is higher at very low stepping frequencies, the motor heats up quick (5v motor at 12v current limited). Was overcome by limiting lower freq and disabling driver close to stop.

[alistairsam]

Originally Posted by Bassnut

Yes, no problem driving them at higher voltages with current limiting, tried it, works well. I like the TI DRV8824 microstepping driver your using, very interesting. Is it easy to interface to with a PIC?.

hi,

yes, very easy to use. just the usual interface pins
step, enable, sleep, reset, 3 microstep mode pins.
doesnt need a separate logic supply.
only issue is soldering. if you can solder smd's then this is easy.
it also has fault and home pins that go low if there is a fault and home when at home state of the step table.


vid of a position test at 1/32 microstepping. just simple of loop of 6400 pulses each direction.

http://www.youtube.com/watch?v=53tgJ4pwn1w

[Bassnut (Fred)]

Excellent, thanks.

[alistairsam]

not sure if you have heard of Trinamic's motion control chips.
they have stepper controllers and drivers.

the motion control chips are far more complex but have heaps of very cool features.
you dont send pulses, instead send hex commands from the mcu like goto this position, accelerate to this speed, change direction, etc.

it has internal algorithms that calculate a trapezoidal acceleration or deceleration profile on the fly.
also has encoder inputs for closed loop positioning.

also has anti stall features, accurate position, power saving features etc.

the chips are only $7 or $10 in the US, but from element14, its around $60 odd. very good value for money, but bit more complex.

not sure if anyone's used these.



http://www.youtube.com/watch?v=xaCzp..._order&list=UL

http://www.trinamic.com/tmc/render.php?sess_pid=394
http://www.trinamic.com/tmc/render.php?sess_pid=152


been thinking of getting these chips.

[hikerbob (Bob)]

The trinamic IC look very interesting. It's a shame the eval board is so expensive.

If the stall detection is good it would be a big step forward for small stepper systems.

Bob

[hikerbob (Bob)]

I'm working my way through trying to get some of the TMC223 IC's. The one one-line supplier I found only seem to use very expensive shipping options and I'm still waiting on a price from Trinamic. I'm swapping emails with a Chinese supplier to try and work out what it will cost. If it all works I'll see if it's Ok to pass on the details.

I recently picked up a box of 5V 750mA Nema17 steppers from Ebay so the IC's look like a great option for controlling them. Fingers crossed.

Bob

[hikerbob (Bob)]

I've ended up using the expensive shipping option and ordered some TMC223's. I'd negotiated with two other suppliers but after a number of exchanged emails (including links to spec sheets etc) neither actually had the right IC in the SI package.

Now I need to get my head around the I2C instructions to talk to them.

Bob

[alistairsam]

Hi

thats cool. I'll be following this to see what you've managed to do .

the TMC222's are similar and they're available through element14

http://au.element14.com/trinamic/tmc...c20/dp/1214195

did you get the eval board or just the chip?
which mcu are you planning to use to interface?

I2C is easy as its just the master/slave config and addressing that needs to be right. more crucial is sending the right commands and getting the ack back.
also, i think one of these don't do constant velocity, you've got to send a max position, reset and send the max position again and repeat if you want continuous motion.
oh and don't forget the pullup resistors on the I2C bus.

[hikerbob (Bob)]

alistairsam, I've just gone for the chip's (amd I also got elsewhere a bunch of SMD to Dil boards). I'll drive them from an Arduino, intitally a mega.

As I understand it you are correct about the lack of a constant velocity instruction (it also takes the shortest path to the specified position) and I think that the 222 does not have the stall detection.

I've had some experience with the I2C working with temp humidity sensors in my dew controller but that was mostly minor tweaks to other peoples code.

Bob

[alistairsam]

yes, the 222 does not have stall detection, but if the application is for driving a telescope and it has a slip clutch, will you still need stall detection?

I was planning on getting one of the motor drivers driven by the motion controllers.

I'm eager to know how the acceleration profile works in practice as thats the toughest bit i had to deal with to get the max speed out of a stepper and avoid resonance effects.

I managed to do it with pwm out of a picaxe. works fine to drive a stepper at around 25Khz and upto 1/8 microstepping.

i'll be ordering the 246 driver which has stallguard and the 429 motion controller as it has constant velocity modes.

[hikerbob (Bob)]

alistairsam I'm thinking of focusser control. If I'm understanding it right the 223 could be instructed to drive the focusser in to a stall (fully in) to establish it's position and then could position the focusser exactly at a nominated point. Perhaps temp compensation could be built in as well as focus points for camera and different eyepieces.

Bob

[alistairsam]

Originally Posted by hikerbob

alistairsam I'm thinking of focusser control. If I'm understanding it right the 223 could be instructed to drive the focusser in to a stall (fully in) to establish it's position and then could position the focusser exactly at a nominated point. Perhaps temp compensation could be built in as well as focus points for camera and different eyepieces.

Bob

Hi Bob,

I don't think you need to use stall detection for this although its a good application. you could easily use a limit switch to indicate when the focuser has reached the end of the tube. the mcu can poll that switch or
you could have a simple optical limit switch as well which can raise an interrupt to the mcu.

so on power up, the stepper rotates in one direction until an interrupt is raised, when it stops and sets the step counter to zero.
and then depending on what switch you press, it could go to a preset step number that could be the position of different eyepieces or cameras.

i'll be building this soon as well.

[Bassnut (Fred)]

Originally Posted by alistairsam

you could easily use a limit switch to indicate when the focuser has reached the end of the tube. the mcu can poll that switch or
you could have a simple optical limit switch as well which can raise an interrupt to the mcu.

.

Carefull what sort of limit switch you use. Needs to have accurate repeatibility since focusing can be measured in microns. There are special homing switches, some with diamond tips. Normal optical limit switch accurracy can be affected by ambient light level.

[bojan]

Ditto.

BTW, there is no need for this procedure (limit switch than back to predetermined position).

If stepper motor is de-energised from full step mode, it will stay there (if not moved by external force).
Your software just have to remember the last position (phase) it was in before power down. and it has to restore it before moving to a new position.

[Bassnut (Fred)]

Originally Posted by bojan

If stepper motor is de-energised from full step mode, it will stay there (if not moved by external force).
Your software just have to remember the last position (phase) it was in before power down. and it has to restore it before moving to a new position.

That would be a logical way. My stupid Pyxis focuser (supposedly one of the best) homes on everypower up, increadibly slow, noisy and timesome. The Optec does too, dont understand that.

[alistairsam]

Originally Posted by bojan

Ditto.

BTW, there is no need for this procedure (limit switch than back to predetermined position).

If stepper motor is de-energised from full step mode, it will stay there (if not moved by external force).
Your software just have to remember the last position (phase) it was in before power down. and it has to restore it before moving to a new position.

Hi,

that is possible, but there is always the chance that it gets moved slightly when powered off or at transport, especially if it is friction driven like a 10:1 microfocuser.
so a proper initializing routine will ensure repeatable accuracy.
else other option is on power off, it parks at the home position and then turns off like inkjet printers.

I tested focusing using a bahtinov mask and a 10:1 microfocuser. i took images everytime i turned the microfocusing knob, and noticed that a half turn of the microfocuser was the minimum movement after which a change in focus is discernible. I didnt measure the linear travel though.

this switch comes with 0.25 to 0.5mm sensitivity and has different types of actuators like roller actuators.

http://australia.rs-online.com/web/s...&cm_vc=prev_au

else this reflective optical switch would be better as the electrical ones may have issues with conductivity over a period of time.

http://australia.rs-online.com/web/s...duct&R=2192634