http://www.microchip.com/design-centers/motor-control-and-drive/motor-types= /brushed-dc-motor Go down the bottom to 'documentation' tab and work through the app notes fo= r brushed DC motors. -----Original Message----- From: piclist-bounces@mit.edu On Behalf Of Jason = White Sent: 05 July 2018 13:04 To: Microcontroller discussion list - Public. Subject: Re: [EE] DC Motor PWM, Average Voltage? Hello Sean, This is a small brushed DC motor going into a ~180:1 gearbox to actuate a O= n/Off hydraulic valve. 15W 20V. There are cams and limit switches that get = adjusted for each type of valve. Part-wise I am limited to discretes, op-am= ps, comparators, and logic gates. This is a fixed speed application, very size constrained. I need to limit t= he max speed over 20-40V supply voltage from -50C to 100C. Too fast is bad,= too slow is acceptable. It would be "nice" to have a constant speed but gi= ven the design constraints I'm not sure that I can actually do that. Am I correct in thinking that a fixed (PWM output) voltage would produce a = relatively fixed max speed? The load and motor characteristics almost certainly very with temperature -= but I don't have control over or data on that. I don't know the exact moto= r. This controller is going to be used with multiple (very similar) types o= f valve-gearbox-motor assemblies that do not exist yet. I am pretty rusty on my control theory, I have the following worries about = a closed loop current feedback system: (1) it might oscillate or do somethi= ng unexpected due to my inexperience (2) it might consume more space on the= small PCB (3) different valves might require different currents. If you could point me to an example/app note that goes into the control the= ory, I will gladly study it. But for now a simple fixed voltage-variable cu= rrent scheme seems like a better fit -Jason White On Wed, Jul 4, 2018 at 11:43 PM, Sean Breheny wrote: > I would definitely recommend considering PWM drive of motors in terms > of the motor current as a function of motor speed, PWM duty, and > supply voltage, rather than an effective voltage. This is because the > transfer function from duty to effective voltage is moderately > nonlinear overall and very nonlinear near zero duty. Taking this > nonlinear voltage expression and using it in a simplified > voltage-driven motor model will result in complex equations which are not= very accurate in predicting motor behavior. > > Is this a brushed or brushless motor? > > A brushless motor driven by PWM is, as some have pointed out, > effectively a buck converter because the inductance of the motor acts > to create a voltage drop between the back EMF and the supply voltage. > If you drive the motor with a sinusoidal current and vary the phase of > this current relative to the back-EMF then you can also make it work > like a boost converter (i.e., produce torque even when the back-EMF is hi= gher than the supply voltage). > This is often called either phase advance or field weakening (this > latter term comes from the fact that the out-of-phase current in the > coils produces a field which opposes or weakens the permanent magnet > field as seen by the coils themselves). > > Brushed motors have the additional property that the commutation > transients dump the stored magnetic energy in the motor coils (instead > of 6-step commutated brushless motors where most of the stored energy > is fed back into the supply via the body diodes of the driving MOSFETs > or the protection diodes added to the switching elements if they are > BJTs or IGBTs). This is slightly less efficient but it does reduce the > effect which the motor inductance has on the electrical dynamics of the m= otor. > > If PWM is operated at a high enough frequency that the current flow is > continuous through the motor coil from PWM cycle to PWM cycle (which > is almost certainly what you want because it is more efficient and > more linear in duty to torque relationship) then there really are only > two kinds of PWM for a two-terminal (brushed) motor: on-reverse and > on-shorted. on-reverse means that you alternate between applying > positive and negative voltage to the motor during each portion of the > PWM cycle (positive during the ON portion and negative during the OFF > portion, assuming positive effective voltage is desired). on-shorted > means that you short the motor during the off portion of each PWM > cycle. There is no true OFF because even if you turn off all switching > elements the protection diodes will cause current flow from the motor > inductance back into the power supply, which is the same thing as > on-reverse operation (but less efficient because of the diode voltage > drop). On-shorted prevents this backflow into the power supply but it > produces less maximum braking torque and prevents useful regenerative bra= king. > > Sean > > > > On Wed, Jul 4, 2018 at 10:01 PM, Jason White < > whitewaterssoftwareinfo@gmail.com> wrote: > > > I need reversal, it is a solenoid/valve controller used for > > hydraulics in aircraft. Size is rather constrained so PWM on the bridge= is preferred. > > > > On Wednesday, July 4, 2018, Harold Hallikainen < > harold@mai.hallikainen.org > > > > > wrote: > > > > > What is the advantage of using the H-bridge in this application > > > over > just > > > using a buck regulator? The buck regulator would use just one > > > switching transistor, a "catch diode," an inductor, and an output > > > capacitor. This > > is > > > assuming the motor only needs to rotate in one direction (no > > > voltage polarity reversal). > > > > > > Harold > > > > > > -- > > > FCC Rules Updated Daily at http://www.hallikainen.com Not sent > > > from an iPhone. > > > -- > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > View/change your membership options at > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > > -- > > Jason White > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- Jason White -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/chang= e your membership options at http://mailman.mit.edu/mailman/listinfo/piclis= t -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .