Stepper motor controller current limiting question
in [Basics]
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From: Tim Wescott on 17 Feb 2010 13:00 On Wed, 17 Feb 2010 13:19:04 +0000, Bob Masta wrote: > On Tue, 16 Feb 2010 15:57:04 -0800 (PST), HC <hboothe(a)gte.net> wrote: > >>Hello, all. I'm trying to build my first stepper motor controller and I >>have two questions. In trying to learn about stepper motors and >>controllers I read the document AN907 from Microchip among other >>documents and sites: >>http://ww1.microchip.com/downloads/en/AppNotes/00907a.pdf >> >>In that document the author writes about current limiting a stepper >>motor that is purposefully driven at a voltage that is higher than its >>designed voltage in order to get the current up more quickly (overcoming >>the inductance of the coils) and then using current limiting to prevent >>the motor from being run at higher than maximum current. Several >>methods are discussed including PWM and using a current-sense resistor. >> >>I think I understand the reasons why a current-sense resistor is not >>ideal (cost and heat dissipation). PWM sounds great and I've used a PWM >>circuit for controlling the speed of a DC motor in the past. However, >>let's say the motor is rated at 5V and I choose to drive it with 10V so >>I'd need to limit the duty cycle to 50%, what happens when the winding >>is first energized? > > I'm no stepper motor guru, but I think the above is suspect. If you > double the voltage (on a purely resistive load, at least) the power > dissipation is *squared*. the power dissipation goes up by *two squared*. > The equivalent duty cycle to maintain the > same dissipation would thus be only 25%, not 50%. > > But that's what you'd want the long-term average to be. Presumably, you > would get much higher instantaneous power when starting up, which would > then taper off to something like 25%. > > Or am I missing something here? The inductance of the motor, if the PWM frequency is fast enough. -- www.wescottdesign.com
From: George Herold on 17 Feb 2010 14:36 On Feb 16, 6:57 pm, HC <hboo...(a)gte.net> wrote: > Hello, all. I'm trying to build my first stepper motor controller and > I have two questions. In trying to learn about stepper motors and > controllers I read the document AN907 from Microchip among other > documents and sites:http://ww1.microchip.com/downloads/en/AppNotes/00907a..pdf > > In that document the author writes about current limiting a stepper > motor that is purposefully driven at a voltage that is higher than its > designed voltage in order to get the current up more quickly > (overcoming the inductance of the coils) and then using current > limiting to prevent the motor from being run at higher than maximum > current. Several methods are discussed including PWM and using a > current-sense resistor. > > I think I understand the reasons why a current-sense resistor is not > ideal (cost and heat dissipation). PWM sounds great and I've used a > PWM circuit for controlling the speed of a DC motor in the past. > However, let's say the motor is rated at 5V and I choose to drive it > with 10V so I'd need to limit the duty cycle to 50%, what happens when > the winding is first energized? Let's say that I'm using a frequency > of 31kHz or so, it seems that, when the winding is first energized, > the voltage may need to be applied longer than the 50% pulse to get > the current to the maximum the motor is rated at, and then run it at > the 50% duty cycle once the current is at the maximum. If you look at > the document you will see that Figure 21 shows a longer-than target > pulse at the beginning of the power cycle and then short pulses (lower > duty cycle) to maintain the target maximum current in the winding. > Therefore, I must conclude that the circuit is requiring some kind of > current detection in the winding. And if I rely soley on calculated > duty cycle for the PWM, obviously I would have to be careful of the > duration of the pulses (50% duty cycle at 0.5 hertz would give 1 > second of full power to the winding, so pulse duration can impact the > current across the motor). That said, it would seem that a way to > monitor the current in the winding would be important. > > So the first question is: what method would be best to measure the > current in the motor windings considering that I am using a PIC (I > have several to choose from including the PIC16F630, PIC16F684, > PIC16F54, and some PIC16F84's) to control the motor? My test motor is > a 6-wire unipolar and I intend to power the common junctions with > positive voltage and just switch the negative with a MOSFET (I have > some TIP41C's I will try). I am good with the PWM control but I am > not sure how to handle limiting the current. At present I'm just > running the motor on its rated voltage. > > The second quesiton is, as I eventually use larger motors, how do I > properly choose the protection diodes I will use in parallel with the > windings? I have some 1N4148's but they seem small. I also have > several larger ones like 1N4004 and other, similar part numbers. > > Thanks in advance. > > --HC Since this is your first stepper motor project I would start by staying within the Voltage and current ratings of the motor. Once that is all working you can try pushing the boundaries for enhanced performance. But at least you won't 'let the smoke out' of the motor your first time. George H.
From: HC on 22 Feb 2010 17:50 On Feb 17, 7:19 am, N0S...(a)daqarta.com (Bob Masta) wrote: > On Tue, 16 Feb 2010 15:57:04 -0800 (PST), HC > > > > > > <hboo...(a)gte.net> wrote: > >Hello, all. I'm trying to build my first stepper motor controller and > >I have two questions. In trying to learn about stepper motors and > >controllers I read the document AN907 from Microchip among other > >documents and sites:http://ww1.microchip.com/downloads/en/AppNotes/00907a.pdf > > >In that document the author writes about current limiting a stepper > >motor that is purposefully driven at a voltage that is higher than its > >designed voltage in order to get the current up more quickly > >(overcoming the inductance of the coils) and then using current > >limiting to prevent the motor from being run at higher than maximum > >current. Several methods are discussed including PWM and using a > >current-sense resistor. > > >I think I understand the reasons why a current-sense resistor is not > >ideal (cost and heat dissipation). PWM sounds great and I've used a > >PWM circuit for controlling the speed of a DC motor in the past. > >However, let's say the motor is rated at 5V and I choose to drive it > >with 10V so I'd need to limit the duty cycle to 50%, what happens when > >the winding is first energized? > > I'm no stepper motor guru, but I think the above > is suspect. If you double the voltage (on a > purely resistive load, at least) the power > dissipation is *squared*. The equivalent duty > cycle to maintain the same dissipation would thus > be only 25%, not 50%. > > But that's what you'd want the long-term average > to be. Presumably, you would get much higher > instantaneous power when starting up, which would > then taper off to something like 25%. > > Or am I missing something here? > > Best regards, > > Bob Masta > > DAQARTA v5.00 > Data AcQuisition And Real-Time Analysis > www.daqarta.com > Scope, Spectrum, Spectrogram, Sound Level Meter > Frequency Counter, FREE Signal Generator > Pitch Track, Pitch-to-MIDI > DaqMusic - FREE MUSIC, Forever! > (Some assembly required) > Science (and fun!) with your sound card!- Hide quoted text - > > - Show quoted text - Hello, Bob, thank you for your reply. I'm sure that's not right on my part; it was just a loose (and inaccurate) recollection I had at the periphery of my thoughts at the time. :( --HC
From: HC on 22 Feb 2010 17:51 On Feb 17, 1:36 pm, George Herold <ggher...(a)gmail.com> wrote: > On Feb 16, 6:57 pm, HC <hboo...(a)gte.net> wrote: > > > > > > > Hello, all. I'm trying to build my first stepper motor controller and > > I have two questions. In trying to learn about stepper motors and > > controllers I read the document AN907 from Microchip among other > > documents and sites:http://ww1.microchip.com/downloads/en/AppNotes/00907a.pdf > > > In that document the author writes about current limiting a stepper > > motor that is purposefully driven at a voltage that is higher than its > > designed voltage in order to get the current up more quickly > > (overcoming the inductance of the coils) and then using current > > limiting to prevent the motor from being run at higher than maximum > > current. Several methods are discussed including PWM and using a > > current-sense resistor. > > > I think I understand the reasons why a current-sense resistor is not > > ideal (cost and heat dissipation). PWM sounds great and I've used a > > PWM circuit for controlling the speed of a DC motor in the past. > > However, let's say the motor is rated at 5V and I choose to drive it > > with 10V so I'd need to limit the duty cycle to 50%, what happens when > > the winding is first energized? Let's say that I'm using a frequency > > of 31kHz or so, it seems that, when the winding is first energized, > > the voltage may need to be applied longer than the 50% pulse to get > > the current to the maximum the motor is rated at, and then run it at > > the 50% duty cycle once the current is at the maximum. If you look at > > the document you will see that Figure 21 shows a longer-than target > > pulse at the beginning of the power cycle and then short pulses (lower > > duty cycle) to maintain the target maximum current in the winding. > > Therefore, I must conclude that the circuit is requiring some kind of > > current detection in the winding. And if I rely soley on calculated > > duty cycle for the PWM, obviously I would have to be careful of the > > duration of the pulses (50% duty cycle at 0.5 hertz would give 1 > > second of full power to the winding, so pulse duration can impact the > > current across the motor). That said, it would seem that a way to > > monitor the current in the winding would be important. > > > So the first question is: what method would be best to measure the > > current in the motor windings considering that I am using a PIC (I > > have several to choose from including the PIC16F630, PIC16F684, > > PIC16F54, and some PIC16F84's) to control the motor? My test motor is > > a 6-wire unipolar and I intend to power the common junctions with > > positive voltage and just switch the negative with a MOSFET (I have > > some TIP41C's I will try). I am good with the PWM control but I am > > not sure how to handle limiting the current. At present I'm just > > running the motor on its rated voltage. > > > The second quesiton is, as I eventually use larger motors, how do I > > properly choose the protection diodes I will use in parallel with the > > windings? I have some 1N4148's but they seem small. I also have > > several larger ones like 1N4004 and other, similar part numbers. > > > Thanks in advance. > > > --HC > > Since this is your first stepper motor project I would start by > staying within the Voltage and current ratings of the motor. Once > that is all working you can try pushing the boundaries for enhanced > performance. But at least you won't 'let the smoke out' of the motor > your first time. > > George H.- Hide quoted text - > > - Show quoted text - Hello, George, thank you for your reply. That makes the most sense to me. I will do that. In the meantime I am still trying to learn as much as I can and am still reading, as time permits, the site suggested to me by the first responder (Jones on stepper motors). --HC
From: HC on 3 Mar 2010 18:02
On Feb 16, 5:57 pm, HC <hboo...(a)gte.net> wrote: > Hello, all. I'm trying to build my firststeppermotorcontroller and > I have two questions. In trying to learn aboutsteppermotors and > controllers I read the document AN907 from Microchip among other > documents and sites:http://ww1.microchip.com/downloads/en/AppNotes/00907a..pdf > > In that document the author writes about current limiting asteppermotorthat is purposefully driven at a voltage that is higher than its > designed voltage in order to get the current up more quickly > (overcoming the inductance of the coils) and then using current > limiting to prevent themotorfrom being run at higher than maximum > current. Several methods are discussed including PWM and using a > current-sense resistor. > > I think I understand the reasons why a current-sense resistor is not > ideal (cost and heat dissipation). PWM sounds great and I've used a > PWM circuit for controlling the speed of a DCmotorin the past. > However, let's say themotoris rated at 5V and I choose to drive it > with 10V so I'd need to limit the duty cycle to 50%, what happens when > the winding is first energized? Let's say that I'm using a frequency > of 31kHz or so, it seems that, when the winding is first energized, > the voltage may need to be applied longer than the 50% pulse to get > the current to the maximum themotoris rated at, and then run it at > the 50% duty cycle once the current is at the maximum. If you look at > the document you will see that Figure 21 shows a longer-than target > pulse at the beginning of the power cycle and then short pulses (lower > duty cycle) to maintain the target maximum current in the winding. > Therefore, I must conclude that the circuit is requiring some kind of > current detection in the winding. And if I rely soley on calculated > duty cycle for the PWM, obviously I would have to be careful of the > duration of the pulses (50% duty cycle at 0.5 hertz would give 1 > second of full power to the winding, so pulse duration can impact the > current across themotor). That said, it would seem that a way to > monitor the current in the winding would be important. > > So the first question is: what method would be best to measure the > current in themotorwindings considering that I am using a PIC (I > have several to choose from including the PIC16F630, PIC16F684, > PIC16F54, and some PIC16F84's) to control themotor? My testmotoris > a 6-wire unipolar and I intend to power the common junctions with > positive voltage and just switch the negative with a MOSFET (I have > some TIP41C's I will try). I am good with the PWM control but I am > not sure how to handle limiting the current. At present I'm just > running themotoron its rated voltage. > > The second quesiton is, as I eventually use larger motors, how do I > properly choose the protection diodes I will use in parallel with the > windings? I have some 1N4148's but they seem small. I also have > several larger ones like 1N4004 and other, similar part numbers. > > Thanks in advance. > > --HC Hello, all. Where I am now is I have learned a fair bit about programming Micrchip PICs and think I've got that well enough to finally implement a test design. I'm taking the suggestion that I start with rated voltages initially, so no need for current sensing yet. I started looking for an H Bridge circuit for my test motor because running it bipolar yields higher holding torque than unipolar operation. I have a number of BUZ11 N-Channel MOSFETs that I wanted to use and, I've read, they offer less resistance to the load so less heat is generated when they are operating, particularly switching. Plus, I don't think I have a P-Channel MOSFET in captivity and I live in the sticks so I can't just run to the store and grab one. So, after reading tons of stuff on H Bridges I kind of cobbled together a piece that works and I wanted to fly it out there and see if I've done any good or if I'm screwing up. I have the schematic done up and saved on Photo Bucket: (http://s938.photobucket.com/home/ nunyabusiness11). I hope that link works. If not you can hit the site and search for my username listed at the end of that link. I've wired it up and it works. It is simply 1/4 of an H Bridge, one of the upper legs supplying positive voltage to the motor. Currently I did not put in any clamping diodes around the motor because the MOSFET has one and it'll work for just seeing if the circuit works. Basically, I'm using 5Vdc from the PIC (simulating it now by just touching the input to +5Vdc or Gnd) to activate a NPN phototransistor. That switches +12Vdc to ground via a 3k3 pull-up resistor (it is located before the phototransistor). That line (before the phototransistor) is connected via a 10k resistor to the base of a PNP 2N3906. The collector is tied to +12Vdc via a 1k resistor. The emitter is connected to ground via a 2k2 pull-down resistor. The N Channel MOSFET gate is tied to the line after the 2N3906 but before the 2k2 pull-down resistor. The N Channel MOSFET is tied to +12Vdc on the drain and the source ties to the motor which itself is tied to ground. I used the phototransistor (SFH615A) to isolate the 5Vdc circuit from the 12Vdc circuit. When the input to it gets +5Vdc it switches on the phototransistor which allows the base of the 2N3906 to go low switching it on. That switches voltage to the leg with the 2k2 pull- down resistor and to the gate on the MOSFET. I've tested it a few times, just powering the system up and touching the input directly to +5Vdc or Gnd. The motor holds securely when the input is energized and seems to freewheel correctly when the input is grounded (after I changed the pulldown resistor on the MOSFET gate from a 3k3 to a 2k2). I'd appreciate input as to whether or not this is a good idea or if I simply got lucky that nothing smoked. I'm not an EE by any means and this circuit could be the silicon equivalent to a crack-baby. Thank you. --HC |