Hi Jason, I understand your application better now and I do agree that if you have no speed feedback (other than back-EMF) then the open-loop effective voltage method does make sense in your case. You can derive the relationship from the standard buck converter equations. Sean On Thu, Jul 5, 2018 at 8:03 AM, Jason White < whitewaterssoftwareinfo@gmail.com> wrote: > Hello Sean, > > This is a small brushed DC motor going into a ~180:1 gearbox to actuate a > On/Off hydraulic valve. 15W 20V. There are cams and limit switches that g= et > adjusted for each type of valve. Part-wise I am limited to discretes, > op-amps, comparators, and logic gates. > > This is a fixed speed application, very size constrained. I need to limit > the 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 given 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 > motor. This controller is going to be used with multiple (very similar) > types of valve-gearbox-motor assemblies that do not exist yet. > > I am pretty rusty on my control theory, I have the following worries abou= t > a closed loop current feedback system: (1) it might oscillate or do > something unexpected due to my inexperience (2) it might consume more spa= ce > 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 > theory, I will gladly study it. But for now a simple fixed voltage-variab= le > current 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 o= f > > 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 moto= r > > with a sinusoidal current and vary the phase of this current relative t= o > > the back-EMF then you can also make it work like a boost converter (i.e= .., > > produce torque even when the back-EMF is higher than the supply voltage= ). > > This is often called either phase advance or field weakening (this latt= er > > 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 motor. > > > > 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-rever= se > > 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 effectiv= e > > voltage is desired). on-shorted means that you short the motor during t= he > > 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 curren= t > > 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 regenerativ= e > > braking. > > > > 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 hydraulic= s > in > > > aircraft. Size is rather constrained so PWM on the bridge is preferre= d. > > > > > > On Wednesday, July 4, 2018, Harold Hallikainen < > > harold@mai.hallikainen.org > > > > > > > wrote: > > > > > > > What is the advantage of using the H-bridge in this application ove= r > > 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 voltag= e > > > > 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/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 .