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 get 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 about a closed loop current feedback system: (1) it might oscillate or do something 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 theory, I will gladly study it. But for now a simple fixed voltage-variable 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 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 a= nd > very nonlinear near zero duty. Taking this nonlinear voltage expression a= nd > using it in a simplified voltage-driven motor model will result in comple= x > 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 volta= ge > 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 higher 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 transien= ts > 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 whi= ch > 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 line= ar > in duty to torque relationship) then there really are only two kinds of P= WM > for a two-terminal (brushed) motor: on-reverse and on-shorted. on-reverse > means that you alternate between applying positive and negative voltage t= o > 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 dio= de > voltage drop). On-shorted prevents this backflow into the power supply bu= t > it produces less maximum braking torque and prevents useful regenerative > 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 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 switchi= ng > > > transistor, a "catch diode," an inductor, and an output capacitor. Th= is > > 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/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .