A motor control (PWM) question ...

pulse width, mostly.

Thanks, that's what I figured, but found that a lot of the documentation is kinda confusing on that subject ...

Without too much detail, the pulse width is the percentage of power being sent to the motor. If it is on 50% of the time (square wave) half the effective power is being sent.

Pulse width -- thus the acronym "PWM" (Pulse Width Modulation).

Actually, what matters is the duty cycle - the percentage of the time power is on.

Most drives use a constant frequency and vary the pulse width. But that's not universal. There are other approaches. Some of the newer drive amps generate a psuedorandom bit stream with the desired duty cycle, like the one-bit D/A scheme used in newer audio gear. This spreads out the noise spectrum, and tends to eliminate that annoying whine some servomotors produce.

John Nagle Team Overbot

Oooh -- that's an interesting idea. Do you have any web references on exactly what they're doing? One of my robots is using dual h-bridges with (unfortunately) major limitations on usable PWM frequency -- thus I get some noticable noise (especially at low speeds). Rather than replace the h-bridges -- which otherwise work well -- changing the programming on my motion control board would be an easy option.

TIA -- tAfkaks

The "pseudorandom bit stream with a desired duty cycle" seems almost self contradictory. I'm not sure I see any meaningful purpose in such a complication.

The servo whine you hear can be easily be eliminated by switching at higher than 20khz. You can virtually eliminate the whine of a lower frequncy PWM amplifier by adding an LC circuit between the PWM amplifier and the motor. The LC will smooth out the PWM spikes, and supply the motor with virtually DC current.

In general, however, a higher frequency means smaller inductors and compacitors. The only issue is transistor switching time. If the transistor has a slower saturation time, it will tend to be less efficient at higher frequencies -- and that means gets hotter and uses more power. These days, MOSFETs switch fast enough, and with enough current, that you can easily switch above audio range.

See

A similar technology, but at a constant duty cycle, has been used in DC/AC inverters for years, to approximate a sine wave.

John Nagle Team Overbot

John Nagle wrote: : Actually, what matters is the duty cycle - the percentage of the : time power is on. --With a Stamp would this be equivalent to varying the "pause" time?

There are some applications where that high a pulse frequency is not desirable e.g. r/c cars where the lower the frequency the greater the punch available from the motor, within limits. Usually we use anything between

1.5Khz and 8Khz depending on the motor and track condition. This helps in tuning the motor power to match the available grip levels on the race track

The PWM frequency should have NO affect on the amount of power or "punch" to motors. Most sealed DC to DC converters, which can supply great current, often run much higher than audio range, often as high as 100KHZ. If you are seeing frequency affecting the result of your output, it is likely you need to put an LC circuit between the amplifier and the motor. A DC electric motor works as an inductor, lowering the frequecy simply means you are saturating the inductor and basically driving the motor directly, you are losing a lot of the advantage of a PWM circit.

Idealy, a PWM circuit acts like a pure current source. Using a resistive load, the voltage output is directly related to the PWM. Increase the duration of the PWM, the voltage goes up along with current. Reduce the PWM, the voltage goes down.

Motors, unfortantely are inductive loads which are harder to characterize, but the theory is the same, you want the PWM amplifier to be pumping power into the system at a very controled rate based on the PWM. You have to either tune the PWM to the motor inductance or put an LC filter between the amp and the motor.

You could do PWM with a pause on a "stamp":

x= 10 (rate) start port1 = on pause X port1= off pause x goto start

I don't think the Stamp is fast enough to do PWM at a reasonable frequency though, and you would certainly need to amplify ttl levels with a transistor.

You would probably get the motor turning ON and OFF with a stamp+stampcode, or jitters, because the frequency isn't fast enough.

Rich

I know what you're saying and I do understand how PWM varies the power applied to the motor. You really should try it out for yourself before dismissing it offhand and please realise that we use permanent magnet DC motors NOT brushless motors with 5 or more poles and rare earth magnets. Yes brushless motors are starting to be used but at present they are comparable with brushed motor/speedo technology of about 8 years ago they are a let down mainly in the transient responses i.e. mid range responses.

There are reasons for the way the speed controllers are constructed and work the way they do, there are quite a few constraints:

Size and weight, about 40 x 30 x 20 mm weight as light as it can be usually about 20-25gm.

Motor specifications: 3 pole (540 size cans, 35mm dia x 50mm long including endbell) 25-45K rpm producing between 90 and 150W depending on the armature winding, running from 6 x 3300mAh sub C cells for a 5 minute race.

Currents experienced: average 30-45A and peaking at well over 100A at startup if full throttle is applied.

The variable audio range PWM drive frequency has been tested and used over many years, and is proven to work. There has never been any commercial r/c speed control equipment that uses supersonic PWM frequencies, that is used in the DC/DC converters you are on about because having that whine in your ear all day will get on your nerves.

Most of the PWM circuits used in the cars are a bank of MOSFETS (running as a switch not sine wave generator) inserted between the motors neg terminal and the battery negative terminal, the battery positive is wired directly to the motors positive terminal (giving forwards and brake control only, the brake FET is between the battery + ve and motor -ve terminals ), so no fancy H bridge circuits (only on reverse capable speed controllers). These devices are all about compactness and minimal losses in the power path giving max power to the motor at full throttle and minimal things to go wrong such as LC networks. The only other components near the power path are snubber capacitors and a shottkey diode across the motor terminals.

So in conclusion, yes it would be nice to have an electrically clean and efficient designed H bridge, LC filter and so on but at the end of the day the space constraints, cost and robustness nothing beats what's there at the moment. If you can design and build something superior for the same cost as a decent unit today but just as robust or more so then I'll be the first to buy you a pint and your first commercial unit.

Chris

I don't think you're correct about this, especially when you're talking about a voltage range where the losses in the flywheel diode are so significant. At very low resistance and voltage, the inductive kick will be discharged and lost - with no further loss - entirely within a single low frequency PWM off cycle. At a high frequency, the loss is basically continuous throughout the off cycle, and higher on average.

A higher switching frequency could yield better results than either way, if the braking mosfets were used to provide active commutation, reducing the losses in the flywheel action.

Make that an L - you don't want or need any C. But without active commutation, the L will increase the flywheel losses at low frequencies to match those at high ones.

Clifford Heath.

I've never head it called a "flywheel" diode, we've always called it a flyback. Technically speaking, I think "flywheel" is probably a bad name considering what it does.

Anyway, there should be little loss in the diode, the energy should be stored in the LC circuit.

This is the bahavior of an LC circuit poorly matched to the frequency.

The thing about LC circuits is that, if properly tuned to the frequency of the PWM, there is very little loss. The theoretical loss should be zero, but there is always core saturation, coil resistence, capacitor leak, etc. If you are losing power in an LC circuit, it has only one place to go, and that's heat and that's waste, and potentially smoke.

You should put a capacitor across the motor terminals, and feed the motors with two inductors, and diode protect your h-bridge. The LC should be tuned so that you see very little ripple at half load/duty cycle at the PWM frequency. Depending on your current, you may need a big core and capacitor, if you increase the PWM freqency you can make them smaller.

If tuned correctly, the flyback voltage from the LC circuit will forward bias the diodes, effectively shorting out one end of the LC circuit, forcing the current back to the motors. This is how a switch mode system gets its efficiency. The only loss in the system should be the voltage drop of the semiconductors, coil resistence, and capacitor leak. You should never be dumping power to a heat sink.

"Flywheel diode" is a fairly common term. If I had to guess, I'd say it originally comes from the action of the diode when the motor is freewheeling. During this time the motor acts like a mechanical flywheel. I've seen the term used for anything that uses a coil, not just motors. I note the term is used more in the UK and Austrilia -- just as they call an electron tube a "valve." Clifford hails from Australia.

I am curious: have you ever built a PWM controller or circuit like the one you describe? Not that I doubt you, but even manufacturers of motor control chips, like Analog or STMicro, often state an "ideal" PWM frequency of 16-22 kHz for the kind of small motors commonly used in robotics. It would be great if you had a URL to a circuit of your design that demonstrated the principles that you mention. That way more people could try it out.

-- Gordon

In a couple jobs I've had yes.

A PWM motor amplifier is very similar to a PWM switching power supply as well.

I don't know how they can say anything about frequency without knowing anything about the motors. For instance, (assuming no LC filter) you would use a MUCH higher frequency with a low inductance pancake motor than you would with a very high inductance wound coil motor.

I'm not sure I'd be qualified to teach electronics theory, but I would strongly suggest people study a bit of theory. Horowitz and Hill "The Art Of Electronics" is a great book.

Probably in your local library.

I don't think there's much on PWM motor control in Art of Electronics, and besides, I would think most people who are designing PWM controls for motor speed, in or out of the audio frequency range, have read the book.

I'm not a motor control designer, but I've purchased (for my work) several ready-made commercial servomotor driver/amplifiers, costing from $100 on up. I think one had a PWM frequency of over 25 kHz. All the others offered more in the range of 100 Hz to 25 kHz. This is fairly typical I found.

Again, if you think your design has the benefit of avoiding audio frequencies for PWM, and improves efficiency while not sacrificing torque, I'd like to see a design. I know a couple of guys at work who'd love it! Maybe you have a Web page for posting a quick schematic?

-- Gordon