YARF - Yet Another Robo Focuser.

[NickK]

So rather than attempt to rob focus the borg focuser for the time being I have decided to robo focus the 1:1 focuser on the Pentax (no 10:1 knob here on the focuser!). This presents it's own issues - namely static torque and accuracy.

I'm thinking that a bipolar stepper motor with a 100:1 gear box (with backlash) that can move over 32Kg/cm would have enough torque for the 1:1 focuser with a mass of AP equipment attached. Also if the stepper motor is running at a 1.8 degree step, the backlash of 1.5 degree on the stepper motor is acceptable as this backlash would then equate to a 0.018 degree accuracy.

Also I have a 12V power rail and the ardunio motor shield can pump out 2A into each pole circuit (4A max).

Looking at places like active robots etc, the obvious choice is this: 3322_0 28STH32 NEMA 11 Bipolar Gearboxed 100:1 (manufacturer part 28STH32-0674B / 28JXS40K100). The stepper is 1.8 degree, thus 0.018 at the shaft with the added bonus of being 12V and 670mA. It supports sustained 32Kg/cm torque but will do 60Kg at a push.  The stepper will fit with the ardinuo shield in terms of power too.

I can use the manufacturer's data sheet to program the stepper control scheme on the ardino. The ardino is then attached via USB to an ODroid running INDI/Ekos/Kstars. With the OAG, the focus will be static but linked to the focus of the main camera. I'll be capturing the OAG guider images for later processing too so having them in reasonable focus is also useful.

I'll then attach the focuser using a timing belt system.

@Gina - I know you've told me before but where did you get your timing belts and pulleys before the GineRep? 

Later once I have an idea for supporting the additional load on the borg to make it smoother in operation - I'll switch the stepper to the borg instead.

I will leave the temperature compensation until later if I choose to make it compensate focus during capture.

 

[Dr_Ju_ju]

A good supplier of Belts & Pulleys etc..   http://www.beltingonline.co.uk/

 

[ChrisLX200]

I use MotionCo http://www.motionco.co.uk/

The ship quickly, prices are reasonable and I usually keep a stock supply for odd jobs...

ChrisH

[NickK]

Excellent I'll have a look which fit the focuser and stepper shaft.

Once correction is 670mA per pole, so 12V 2x670mA which is reasonable current draw.

Hmm looked at the stepper manufacturer data sheet and it appears it's a 3.8V stepper.. although it's listed on active robots (and phidget) as 12V recommended..  asked them for clarification..

Edit: Ok their tech folk have confirmed, the rated is 3.8V, however when you multiply that by resistance etc the actual voltage used should be 12V (basically the rated parameter is useless by itself).

To add I'll add end limit switches so that if the focuser reaches the the limit in or out the the corresponding switch will be pressed by the moving focuser. This is the cheaper option to an encoder and allows the system to detect the impending limit before the stepper meets with resistance and loses it's location (Steppers sit on a flux wave so once the rotor is displaced it won't reengage until the rotor sits on the flux peak again, unlike a motor).

Reading the Arduino documentation on the Motor Shield R3 - it's recommended to switch from internal power (through the regulator) to extern Vin power by cutting the "Vin connect" track on the underside of the board. No biggie here - I want todo that so I can run direct from the 12V line. The ardinuo shield will give me full stepping (maximum torque) however if I can get away with it also half stepping. If need be I can then switch driver and get a micro stepper controller to give 1/16th steps. I think if the torque required to hold the cameras etc is low enough to use half steps then I can increase the resolution to 0.009 deg for each 'step' through the gearbox (although the backlash would still be 1.5deg).

I think having 1/16th microsteps of 0.001125 deg is a little OTT at the moment! (888.88.. steps per focuser revolution!) considering the micro stepper controller is about £92 by itself. To be honest I'd switch out the gearbox at this point to remove that 1.5 deg stepper side backlash.

EDIT: note the calculation is at the gearbox shaft (1.8 / 16 / 100)

[NickK]

The stepper arrived this morning.. impressed is an understatement. It's solid and for it's size it's heavy!. Even without power I can't turn the shaft so I think it will have no problem with holding the astro kit without power - which means it's very likely to be able to perform with half stepping . The limit switches seem perfect too - small and light. Here's the stepper next the ardunio uno with motor shield that will be controlling it:

I'll now measure up the prospective mounting points, distances and shaft sizes to order the timing belt.

 

[NickK]

Next step is to get the ardunio sketch done (I'll adapt one of the existing stepper sketches). The power - I have a AC-DC 12V power to test on so that's good, although the pier power is slightly higher at 13.5V so I will get a 12V regulator to drop down and protect the stepper. Hobby King have some 12V and 5V drop downs that are used in Remote controlled cars etc that will work. 

I'll make a sketch that also detects the two limit switch closures - this would then allow it to calibrate the position. By making the focuser range smaller (and hence the switches closer together) for a known focus camera setup, the startup calibration of steps to one limit then to the other is going to be faster.

Next up - figure out how to attach the stepper to the pentax.. that has a uniquely attachment unfriendly focuser! There is no screws on the underside other than the small gub screws which aren't designed for holding things, see below: 

 

Looks like the only way to attach this is to run a direct coupling (with stepper sticking out) or timing band (stepper sitting above or sticking out) from the stepper mounted attached to the finder scope mount. Alternatively underslung.

I'd like to mount a couple of 300mm lenses above the scope - that sits flush with the top mount clamp blocking the stepper position. Sticking out of the scope makes it prone to collisions. Typically the focuser itself is clamped at an angle so it makes it difficult to have something stuck prone or underslung.. will set the scope up and see what works on the EQ6.

I could make a metal plate that screws into the finder position, bends around the focuser, is clamped into position by the focuser shaft and then it continues to provide a focuser 

 

[JamesF]

I've been very happy with motionco too.

James

[ChrisLX200]

That does indeed look tricky to attach a motor to! However, you do have those finder bracket threaded holes, that might work with a suitable bracket.

ChrisH

[NickK]

6 minutes ago, ChrisLX200 said:

That does indeed look tricky to attach a motor to! However, you do have those finder bracket threaded holes, that might work with a suitable bracket.

ChrisH

Yup - that's the most secure point in my view, it's then what position works for the mount and for the mounting of twin 300mm lenses on a piece of wood screwed above the scope.

One thought that has struck me is - given the weight of the stepper.. it may need additional rebalancing on the mount - so it's probably a good thing to set the stuff up and try it!

[NickK]

Well after much gashing of teeth at the zero motor shield documentation (virtually none), sweating at ardunio IDEs (linux in a VM).. I've switched to the nightly build of the OSX version.. much better.

Here's my stepper test code - you can see my schematic tracings in the comments! Note that although people say "enable 8,9,10,11" in the stepper constructor they're all following blind and don't understand what it actually does.. they're the coils sequences. So for mine with two coils the sequence is in1 and in2 only:

	#include <Stepper.h>
	// Ardunio Motor Shield values (board pins -> L298 -> output)
	const int dirA = 12; //D12 -> P13 in3 -> out3 (B)
	const int dirB = 13; //D13 -> P7 in1 -> out1 (A)
	const int pwmA = 3; //D3 -> enable A (high to provide current, or PWM to provide pulse control)
	const int pwmB = 11;//D11 -> enable B (high to provide current, or PWM to provide pulse control)
	const int brakeA = 9; //D9 -> (dirA e brakeA) -> in2 -> out2 (A)
	const int brakeB = 8; //D8 -> (dirB e brakeB) -> in4 -> out4 (B)
	const int senA = 0; //A0 sense A
	const int senB = 1; //A1 sense B
	const int stepsPerRevolution = 200;  // change this to fit the number of steps per revolution
	                                     // for your motor
	// Original comment:  initialize the stepper library on pins 8 through 11:
	// ^^ this depends on your stepper, the values are coil sequences and that depend on how they're connected driver wise.
	// For the motor shield L298 this is in1,in2,in3,in4. For bipolar I'm using in1 and in2 only.
	Stepper myStepper(stepsPerRevolution, dirA, dirB);
	int stepCount = 0;         // number of steps the motor has taken
	void setup()
	{
	  // initialize the serial port:
	  Serial.begin(9600);
	  // turn on power
	  pinMode(pwmA, OUTPUT);
	  pinMode(pwmB, OUTPUT);
	  pinMode(brakeA, OUTPUT);
	  pinMode(brakeB, OUTPUT);
	  digitalWrite(pwmA, HIGH);
	  digitalWrite(pwmB, HIGH);
	  digitalWrite(brakeA, LOW);
	  digitalWrite(brakeB, LOW);
	// adjust power 1/2 power
	//  analogWrite(pwmA,2 );
	//  analogWrite(pwmB,200 );
	// switch off power
	//  digitalWrite(pwmA, LOW);
	//  digitalWrite(pwmB, LOW);
	  myStepper.setSpeed(35); // max speed is 35.
	}
	void loop() {
	  // step one step but enabling coil power for only the step.
	  digitalWrite(pwmA, HIGH); // this 'enables the power' using a full PWM pulse
	  digitalWrite(pwmB, HIGH); // this 'enables the power' using a full PWM pulse 
	  myStepper.step(30);
	  Serial.print("steps:" );
	  Serial.println(stepCount);
	  stepCount++;
	  delay(10); // to stablise
	  digitalWrite(pwmA, LOW); //  this 'disables the power' using a full PWM pulse
	  digitalWrite(pwmB, LOW); // this 'disables the power' using a full PWM pulse
	  delay(500);
	}
	

Actually using analogWrite(pwmA, 0); etc instead of digitalWrite(..., LOW); seems to run cooler.

So although the PWM adjustment works, it works by limiting the full power for a duty period. This works on some things but on steppers this equates to a narrower pulse to the coils and so can result in problems in stepping or stalling. I found that I can run at about 90% power (you can hear the difference in the stepping) however below that causes issues with the stepping. 

I also noted that the L289 bridge controller isn't really recommended for steppers, the issue is that the H bridge driver cannot control/limit the amount of current. So for a 2.83V rated stepper the 12V provides the needed flux to perform stepping reliably, however with that 12V comes the additional heating through the current draw. So the chopper driver monitors the current draw during stepping and cuts the current once it reaches the maximum level resulting in a cooler stepper motor. That happens quickly - far quicker than the micro controller can do during stepping.

So the PWM works for non-stepping but not during stepping. The more focusing the more heat build up. Something like this: https://www.quasarelectronics.co.uk/Item/3187-bipolar-stepper-motor-chopper-driver

Well .. I decided to experiment with the input voltage used by the stepper  Reducing to a 5V Vin supply voltage, the stepper works flawlessly and remains far cooler to touch. Infact I would say this is possibly the idea voltage to run it at.. I know this means the flux will be less, however considering the gearbox torque - this may be the way to go.

Using 5V Vin means that the LR style drivers (i.e. the ardino board that supply max current all the time when moving) results in less coil heating. These styles of drivers don't run steppers fast (or at least in the 1000step/sec category) compared to coppers but as I need the accuracy (1:1 focuser) this seems like a good budget trade off. I can run at a 40steps/sec it seems without having a problem.. I may play with this and see if using a combination of PWM to reduce the pulse size and faster cycling results in a faster step.

All this is good for the 'automation project' with the odroid as I'll need to order a set of voltage drop downs soon - so having the option of a switchable 12-5V and 5V on one of them may be good depending on the power needed for stepping (i.e. if I'm using heavier setup I can switch the power up a notch).

Next up is adding the limit switches - these can be attached to the analog expansion port of the ardino to give an all or nothing reading. So all that's needed is then to make a protocol and a sketch that runs as a serial connection (or I2C etc) so that the odroid can command the focuser.

Naturally the hardware side of things too - but it rained today.. so no playing with things outside.

I think a general protocol for the focuser would be:

* ping - provides an application level acknowledgement

* reset - dead man's halt to stepping and abandon the all commands and power down

* calibrate - step to each limit switch twice. Based on the step down here.. this is going to be slow due to it being 0.018 degree step. So I may make the limits very close.

* status - current step position, calibration data and current operation state (or error)

* seekToStepAbolutePosition - start a movement to a specific position (only usable if calibrated)

* stepRelative - step forward/backward

I'm thinking operations will need to work concurrently, so errors could be returned at any time or by calling 'status' to poll the current state. In the past I've seen modal operations but you end up loosing track of the operation vs accuracy in terms of timing - and for this we don't need accurate timing but we do need concurrent behaviour (i.e. an AP wait for the move to complete before taking a picture to prevent vibration from the stepper).

The scope has the option of the borg for find manual focus.. or possible that odroid could work with a phone touchscreen control stick to focus..  so looking happy with it so far

Edit: more experimentation... RPM at 200 works really nicely (400 seems to over drive) so infact the focusing calibration will be reasonably quick but accurate.

 

[Gina]

+1 for motionco

[NickK]

After looking at the gear centre space for using a belt, it looks like have the same amount of space - either used up attempting to get to the required distance between the two shafts or the length of the stepper and gear box.

 

Option 1: Direct coupling - not that the distance out is actually not too bad because the focuser itself sites inside the scope diameter.

On the mount

 

There are a couple of spots where the stepper could impact the pier but on extremes and it's possible to rotate the scope if needed for targets that low. Star parties where the tripod legs spread out may be a different situation and I suspect I'd have to rotate the scope so that the focuser points out the other side, depending on target. 

 

Option 2: use a timing band - the issue with this is that the two shafts need about 8cm between them - almost the length of the of the stepper.

 

\

 

However due to the positioning it still sticks out about the same distance as the stepper in a direct coupling:

 

 

Whilst outside I had a visitor come and inspect.....

 

 

Summary.

Option 1 is complicated - it requires the mounting bracket to span between three points (the finder, the shaft on the scope and the shaft on the stepper. The contours of the scope and the small 17mm radius cut out where the shaft sits makes it complicated. If the stepper is pulled forward and rotated around - it's still not good for the CoG just different problem. I can use a double sheet of Aluminium (or with a piece f U profile to stop bending) to make it lighter and then some steel for the bend (aluminium is a pig to bend and not become brittle as a sheet) and use rivets to secure together.

Option 2 is easier to fabricate, has less parts (not the aluminium bit) with less issues (although less capable of adjusting the final gear ratios. 

The stepper has 2000 steps per revolution (1:100 with 200x1.8 degree step) then with a scope that has ~80cm of travel on the focuser in just over 2.5 turns (say 2.6 turns), giving 2000 per turn, 2.6*2000 = 5,200 steps for approximately 80mm of focuser travel = 0.0154mm per step. So 1 micron is 0.001mm that's a ~ 15 micron/step. The borg scale you see in the picture is in microns IIRC I'll check with the vernier callipers).

I'm thinking that the option 2 is less problematic with mounting and not flexing under the timing band load. However can I live with a 15 micron/step? The Borg spoils you in terms of focusing..

Oh.. the Borg option..

Lastly the future borg option, the stepper and gearbox is a really nice size for this. With back focus being stolen by the borg, I had to have an adaptor made - the problem is that as the borg focus is rotated the lower section of the borg moves outwards (including the focus dial). I would have to have additional support structs to take the payload weight off the focuser to make it an easy turn (although the stepper has the torque).

[NickK]

Ok.. just to confirm now I've measured it, my mistake the Borg is 0.1mm for each sub-division mark so the 5, 10, 15 are mm and major mark is 1 mm.

>> The stepper has 2000 steps per revolution (1:100 with 200x1.8 degree step) then with a scope that has ~80cm of travel on the focuser in just over 2.5 turns (say 2.6 turns), giving 2000 per turn, 2.6*2000 = 5,200 steps for approximately 80mm of focuser travel = 0.0154mm per step. So 1 micron is 0.001mm that's a ~ 15 micron/step. The borg scale you see in the picture is in microns IIRC I'll check with the vernier callipers).

So this means that the new focuser is higher resolution than the Borg by hand (6.4 ticks per borg sub-interval) in direct coupling mode.

Question is how accurate does it have to be? Looking at the commercial folks, although steps are not a good indication because it doesn't give the final focuser travel..

Lakeside focuser: 4000 steps/revolution.

Astrophysics have two, the more accurate is the PDM with 24,000 steps/revolution (this is used on things like the F3 Riccardi-Honders).

Moonlight give focuser travel per tick as: 0.002032 mm in half step mode (double distance for the coarser full step).

I have no idea about the focusers that simply use motors rather than steppers - they seem a bit like russian roulette. 

 

So we're looking at a good 0.0154mm vs 0.0020 mm probably because this connects to the 10x already in the focuser.

So I can give a 10x belt focus using one or two belts (i.e. 10:1 or 5:1-2:1). Which would then take me to... 20,000 steps/turn - so close to the astrophysics focuser!

The complication is the small 17mm max space between the centre of the focuser shaft and the focuser body. So to have any real 'step up' it would have to be a two band system with a twin pillow block between to hold the intermediate pulley. Reason I'm considering complicated option this is that it makes little sense to do this and then find the focusing is a little better than the borg - actually 60x more accurate. 

 

Lastly I noted that the rack & pinion focuser on the pentax has a 'decent' amount of play between the gear and the pinion, so that will likely mean the system will need to perform focusing and then having to perform a backlash adjust so that the rear weight is sat on the engaged gear and pinion.

The shafting and CNC machined grips could be made into a support frame for the borg - allowing 4 steel shafts to support the 30mm or so movement would make it heavier but far more accurate.

 

[ChrisLX200]

This is the great advantage offered by a 3D printer - the ability to design a bracket of such complexity that you would never dream of attempting to fabricate by any other method. This was very helpful for the motor-unfriendly design of the FeatherTouch focusers. Don't forget that you can micro-step a geared motor and reach much finer resolution.

ChrisH

[NickK]

25 minutes ago, ChrisLX200 said:

This is the great advantage offered by a 3D printer - the ability to design a bracket of such complexity that you would never dream of attempting to fabricate by any other method. This was very helpful for the motor-unfriendly design of the FeatherTouch focusers. Don't forget that you can micro-step a geared motor and reach much finer resolution.

ChrisH

True - the phidget micro stepper that you can get from active robots (chopper driver too) for £92 can do 1/16th steps. However that would only be 8x the resolution when it's half stepping already and I could do that with just a single band pulley for quite a bit less! What you do get is better energy performance and thus the drive can be driven faster. Another option is to switch controller to something that supports micro steps but then it needs that current control to implement sub-stepping.. I'm not a great electronics person (software and mechanics - ok).

Just looking at the largest pulley I can get onto the pentax focuser (17 mm radius max flange) considering it has a 7mm (possibly 8mm I'll recheck) shaft (compared to the stepper's 6mm gearbox shaft).

Having just said that.. I've just realised that 17mm is radius not diameter.. which means much more choice!

For example: Aluminium 3M Pulley, 32T, 8mm Bore, to suit 9mm Wide Belt. (I have a suspicion that the shaft is 0.25" thus 8mm) which has a 34.4mm flange diameter... below the 36mm limit

If need be I can use a idler pulley on the back of the belt to make the pitch of the belt ensure that the teeth engage better.

Looking at the diameters, the limit of the space for the flange means to go more than 34.38:11.5 diameters (2.98:1) either - extend the shaft out and fit a larger pulley with a pillow block needed to keep the shaft from bending. Or use a twin belt - again needing pillow blocks but at the same time giving space in an easier design.

So to get the 10x that intermediate can be smaller than the stepper: 34.38:9.5 (3.6:1) can then be backed onto 10/3.6 => 2.76x9.5=26.49mm mimum. There's a 26.74mm pulley.

So 1 rotation on the stepper 9.5:26.74 = 0.3555 on the intermediate. Then switching to belt two 0.3555 on the intermediate 9.5 to the focuser 34.38 (9.5:34.38) = 0.3555/3.6 = 0.098 or.. 0.1 rotation on the focuser.

The beauty of this design is that the belts can be positioned with the intermediate to one side of the focuser and the stepper to the other. The result is a flatter design (like a V but horizontally). Even better that the stepper has a smaller gear so it can be mounted close to the focuser (belts allowing!).

 

[ChrisLX200]

You could try adding a third tube ring and using that as a base for the motor...

The electronics to build a focuser (Arduino UNO/NANO, DRV8825) costs about £10 - that gives you micro-stepping down to 1/32 if you need it.

ChrisH

[NickK]

13 minutes ago, ChrisLX200 said:

You could try adding a third tube ring and using that as a base for the motor...

The electronics to build a focuser (Arduino UNO/NANO, DRV8825) costs about £10 - that gives you micro-stepping down to 1/32 if you need it.

ChrisH

I have the UNO so connecting that up is straight forward it's the additional bits around the drivers for managing current etc. that I would have an problem with.

edit: just had a look at the DRV8825 .. it seems both cheap and very easy to setup. I'll have a look at some of the breakout boards. I can see it's the same step-direction interface with M1-3 providing the step selection (i.e. down to 1/32).

That may be a more cost effective (read that as I can easily adapt the ardunio code to control that) unless there's someone that's written a driver for it already. I'd still add limit switches in - given the mount of torque on the gearbox.

Thank you

I could make something based on their design: https://www.pololu.com/product/2133

[NickK]

Ok, depending on where you get the DRV8825 it's between <£6 and to be credit card secure.. £12 on amazon.co.uk.

They also have some flexible shaft couplers (like the ones in your picture) that will couple the gearbox drive straight onto the pentax focuser shaft.

I have a bracket for the stepper and I have some scrap aluminium & steel to make amounting bracket. Just need to find some titchy screws too.

I'm thinking that some form of water proofing of the stepper motor (although it needs cooling) would be good. I don't normally foresee condensation issues but given I've been out and come back in with everything soaked through dew point..

In terms of wiring - I can see how todo that.

I'll also order a couple of SBEC/UBEC (one 5-12V 4A switchable with 5V4A secondary) - this allows me to select the voltage for the stepper (5-12) and also power the ardunio etc and a couple of 5V4A for the power for the automation project.

 

[ChrisLX200]

Got mine for about £2 each https://www.amazon.co.uk/gp/product/B010NMJJXI/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1  Although you do need to buy 5 on that deal (considering I melted one I thought having a few spares seemed a good idea...) I have tried printing some 'flexible' shaft couplers but in either PLA or ABS they turn out too stiff. I have yet to try printing with a more flexible material. Make sure some air can get around your motor as they can get warm, and in a sealed container might get very warm. Mine are all open to the elements and have out in some very dewy conditions with no ill effects.

ChrisH

[NickK]

9 hours ago, ChrisLX200 said:

Got mine for about £2 each https://www.amazon.co.uk/gp/product/B010NMJJXI/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1  Although you do need to buy 5 on that deal (considering I melted one I thought having a few spares seemed a good idea...) I have tried printing some 'flexible' shaft couplers but in either PLA or ABS they turn out too stiff. I have yet to try printing with a more flexible material. Make sure some air can get around your motor as they can get warm, and in a sealed container might get very warm. Mine are all open to the elements and have out in some very dewy conditions with no ill effects.

ChrisH

Yep saw those and some of the warnings but the main consideration at the moment is how much heat will the stepper make. With 12V it was too hot to touch after a short time. With 5V it seemed fine so I'll run a simulation today - do a simulated step calibration and then a do a set of exaggerated focusing with 5 minutes between. I'll wind some fishing braid with a weight on the end and clamp it to the table to simulate a bit of load.

The UBEC I will get for the focuser/ardunio power has a controllable output including if I did just go down the belt route I could switch the power off using the ardunio to the stepper motor (it would cut the power to the driver rather than disconnect the stepper that would destroy the driver). Even with 1/32 stepping using current control, the power could be switched off during exposures.

I'll see how warm it all gets - I think that will be the deciding factor.

[NickK]

I've left the stepper on full PWM after doing a full step of moves - the idea is that this demonstrates the heating of the coils if the current remains on whilst the stepper is holding focus.

Ambient in the room is about 20degC. The stepper temp is around 29degC which is cold for a stepper. The L289 driver on the ardunio motor board is about 50degC without a heatsink (PowerSO20 style) but that's ok up to 130degC at the junctions.

So I think that answer it in terms of running at 5V. The stepper can run all day and not be in any danger of overheating compared to 12V and possibly has additional headroom for running at a slightly higher voltage if needed for the micro-stepping.

Interesting there's a difference in sound between + and - stepping, possibly some backlash on the gearbox when - stepping at 200 speed. I may need to find something that works nicer for -steps.

So I'm happy with that. I think I'll order the 5x DRV8825 pack and I like that it's fulfilled by amazon themselves. You never know when you need another stepper  and I want to robofocus the twin 300mm lenses - probably using the HiTec standard HS-311 servos I have already as they have very little vibration although I have noted ripple current wobbles when at rest in the past depending on what's driving them.

I see that the DRV8825 board needs a 100uF capacitor over the motor supply. I'm assuming that if I'm using between 5-12V supply then a 12V cap with 100uF capacity is going to work? (this is outside my knowledge - although I understand how it works, the charging of the external cap sinks the initial spike). I want to order this at the same time.

Edit I think I understand this now - C=Q/V. Ie 100uF = Q/12V, Q=1200. So if I was using a 12V capacitor with 5V the capacitor would only charge to 5V (as that the supply) but the storage would then need to be Q (1200) / 5 be an equivalent of a 5V 240uF.

The voltage is the max that the cap can take so it's good to give it head room. Say 15V although the supply will never charge it to that much. The net impact when running at 5V is that the initial 5V spikes with the inrush are held, as the Q(1500=15V*100uF)/5 = 5V 300uF, three times as long as the 100uF? Not a biggie as the time this occurs is faster than the ardunio system boots. 

I see that a 5V spike can be as high as 40V without the additional capacitor. So do I need to spec for 40-50V or simply 15V for example?

Any particular type? I know not ceramic and with a high ESR. So a normal 15V 100uF electrolytic can?

 

[ChrisLX200]

I used a 16v 100uF can for mine. Note that the DRV8825 needs a minimum of 8.2v to operate!  However, you can adjust the output current using that trim pot to suit the motor. Just connect your meter between the driver output and one coil to measure current (under static load with the coil energised) and adjust the pot so the current is around 75% of the rated load. Don't turn it all the way anti-clock as I did (I think that was the direction) as it very quickly overheats, the solder melts, and then the chip falls off... LOL ;-) With the current set correctly you should be able to drive even a 5v motor @ 8.2v without it getting too hot.

ChrisH

[NickK]

14 minutes ago, ChrisLX200 said:

I used a 16v 100uF can for mine. Note that the DRV8825 needs a minimum of 8.2v to operate!  However, you can adjust the output current using that trim pot to suit the motor. Just connect your meter between the driver output and one coil to measure current (under static load with the coil energised) and adjust the pot so the current is around 75% of the rated load. Don't turn it all the way anti-clock as I did (I think that was the direction) as it very quickly overheats, the solder melts, and then the chip falls off... LOL ;-) With the current set correctly you should be able to drive even a 5v motor @ 8.2v without it getting too hot.

ChrisH

Good catch. The UBEC I was looking at outputs 5/6.3/7.2/12V selectable.. grr.. I read that you need to put the ammeter between the driver and the coil when doing that to get the correct current measurement.

So really I want something that's 8.2 or more likely 9V.

My desk AC-DC can output 5/6/7.5/9/12/13.5/15V so I can test the heating effect of running at 9V. Let me retry the heating test with 9V instead of 5V.

Edit: certainly noisier and more 'positive'. Currently it's sat 'holding' focus for the 5 minute simulated exposure after the simulated calibration and focusing moves. ... 55degC atm.. slow rise.. 56.. 57.. seems to hold steady at about 57degC which is perhaps too warm for continued operation like that. The L289 is at almost 80degC case temp..  however the L289 supplies max current during this test and the PWM is not set to disable the output from the bridges. Hence full heating.

 

[ChrisLX200]

You may not even need to leave the coils energised with that high ratio box, it likely wouldn't slip though clearly it's the better option. 60deg is well within spec for these motors, I hate to think what my NEMA17 motors run at inside the 3D printer - the bed sits at 110deg and the extrusion motor is right next to the hot end at 240deg ;-) Yes, there are small fans but I wouldn't want to touch the motors!

ChrisH

[NickK]

The stats on this stepper are:

Recommended 12V by phidgets (although the rated is 3.8V)

Rated current 670mA

Phase resistance 5.6 ohm

Inductance 4.2 mH

Additional manufacturer specs:

Step Angle 1.8

Step Angle Accuracy 5% (full step, no load)

Resistance Accuracy 10%

Inductance Accuracy 20%

Temperature Rise 80OC Max.(rated current,2 phase on)

Ambient Temperature -20OC~+50OC

Insulation Resistance 100M Min. ,500VDC

Dielectric Strength 500VAC for one minute

Shaft Radial Play 0.02Max. (450 g-load)

Shaft Axial Play 0.08Max. (450 g-load)

Max. radial force 28N (20mm from the flange)

Max. axial force 10N

Rotation CW( See from Front Flange)

 

Compared to the 43-135 ohm on some of the uni-pole steppers I would say that the stepper is low resistance bipolar. Hence the suggested £92 chopper controller to spike the power to the recommended 12V and keep the current under 670mA. The L289 is a constant voltage, constant current (although you can use the PWM to reduce the step pulse length or simply switch on/off in a digital form).

The DRV8825 is a constant voltage, varying current (stepping cycle) with the maximum current output being set by the variable resistor. Also with the enable, it can be disabled during times of non moving although re-enabling means getting the two phases enabled simultaneously (perhaps tying the enable pins together then cycling enabling) if the gear box can hold the load without assistance of the stepper motor even better.

EDIT: correct the specs with the motioncontrolproducts (manufacturer) specs from the data sheet.

So with 3.8V with 0.67A it sits at 80degC.