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Superimposing Low Frequency Dither over High Frequency PWM

ShewShew79

Nov 19, 2015
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Hello all, I'm sure this has been asked many times as I have read about dithering a PWM signal to overcome stick slip in hydraulic valves but nobody has ever said exactly how it is done, why? I use off the shelf controllers that use a high frequency PWM to control the spool valves in hydraulic systems for cranes to give them a fine control due to the high resolution PWM, for example Scanreco / Fabercom controllers from Italy. I like to tinker a lot and research in depth just to appease my brain with knowledge. I'm not looking for a straight answer to my problem but there are a couple of questions that I would like answering if I may ask them.
How do you superimpose a low frequency signal over a high frequency signal? And when you have, are you modulating the amplitude, frequency, duty cycle of the dither signal? can this be done in hardware or software or both? Has anyone successfully produced a working system of their own? I would like to see and image of what a dithered signal looks like on a scope, If I could I would figure it out myself anyway.
I have tinkered about with different techniques already and get all sorts of wonderful signals out but I'm not sure which one is right.
If anyone knows of a great book I could read then please let me know.

Sorry for the rambling on but its a question that unanswered all over the internet.

Thanks

Shewshew79
 

kellys_eye

Jun 25, 2010
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My understanding of it....

Apply the PWM to one input of a (N)AND gate along with a random, low frequency gate signal to the other input and you get your 'dithered' signal.

The amplitude is fixed, the frequency 'may' be fixed but the pulse widths can, and do, vary.

It can also be done in software of course....
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I haven't tried dithering a PWM signal, but amplitude dithering was common with analog servo loops and simply consisted of adding a small (usually sinusoidal) signal to the servo valve control signal. The effectiveness depended on how the servo valve was constructed because that affected the response time of the valve to the dithering signal. The end result was a small oscillation of the servo valve flow, adjustable in amplitude to a level just sufficient to overcome stiction in the valve or stiction in the load cylinder (or whatever the hydraulics were moving) without an overall change in load position.

The approach that @kellys_eye suggested, logically ANDing the PWM pulse train with a pseudo-random low-frequency pulse train (non-coherent with the PWM frequency) might work okay, but I would want to try it out on a breadboard with an oscilloscope to make some measurements before making a large-scale commitment. I do like working with hydraulics, but throwing around several tons of metal requires experience and patience if disaster is to be avoided.

On a side note: my first experience with hydraulic position control was the alt-az gimbaled turret mount for the M61A1 20mm Gatling Gun located in the tail of the B-52H heavy bomber in the 1960s. The gun was controlled by the AN/ASG-21 Defensive Fire Control System, which has since been removed, although both the G and H models of the B-52 are still operational and currently deployed.

This radar-controlled weapon system was a fascinating piece of electrical and mechanical power engineering by Emerson Electric in St. Louis, MO. The gun-turret gimbals were rotated with swash-plate hydraulic motors geared down to move the gimbals, each motor being roughly the size of a large human fist. The hydraulic power supply, mounted behind and below the gun, used a variable-angle swash-plate pump, driven by a 400 Hz three-phase electric motor, to provide a steady 3000 psig hydraulic pressure to the gimbal motors through Moog analog servo valves controlled by magnetic amplifiers. I had never heard of magnetic amplifiers until I encountered these. Essentially saturable-core transformers, with the saturation controlled by current through one control winding, said current being supplied by a pair of sub-miniature vacuum tubes, they were very reliable and virtually indestructible. Mag-amps were also used to control the 400 Hz servo motors driving the alt-az gimbals of the dual Ku-band radar antennas, as well as the gimbals of the control-line platform (see below).

During search mode, the antenna gimbals drove the radiated antenna beam in a raster pattern, controlled by micro-switches that actuated at left/right and up/down limit-stops from Teflon-coated cams. Even with a Teflon coating, wear was extreme from the constant and rapid raster-scan, so a stamped, thin, sheet-metal actuator was mounted on each micro-switch, interposed between the cam surface and the micro-switch button actuator. This sheet metal part sacrificially wore out, sooner rather than later, as the Teflon coating on the cams deteriorated. When it wore completely through, the antenna quit scanning. Replacing it was difficult in the field as it was nearly inaccessible and secured by two long screws through the body of the micro-switch. Getting the micro-switches re-aligned after replacing the actuator was even more difficult, but the alternative was to remove the entire antenna assembly on the flight-line and replace it, usually a considerably longer job. And we didn't have a lot of spare antenna assemblies tested and waiting on the shelf, so us "bag draggers" were "encouraged" to replace the actuators without removing the antenna assemblies... while working ten feet above ground in the open air on a B-5 stand with below-zero wind chill conditions. And three years later they asked me why I didn't want to re-enlist!

The same pump responded quickly to pressure changes caused by changes in hydraulic fluid-flow demand because it also powered a much larger hydraulic motor that turned the gun barrels when the gun was fired. You wouldn't want to break lock on a target you were tracking just because the gun barrels started spewing out several hundred rounds per second. (Some who have been there and done that have said it sounds like God farting when GAU8 "Vengence" 30mm Gatling Gun, the M61A1's big brother, is fired from an A-10 "Warthog" fighter/bomber. The A-10 was literally designed around this gun.

Anyway, back to the B-52H... the gun turret motion was slaved to the motion of a three-axis device called a control-line platform or CLP. The purpose of the CLP was to remove the effects of roll, pitch, and yaw motions of the B-52H aircraft, which presumably would be undergoing some pretty fantastic pilot maneuvers if it were necessary to fire the gun during a combat mission. These maneuvers mainly consisted of rapid roller coaster-like changes in altitude and lateral position, called jinxing, and are rarely seen because they put humongous stress on the aircraft. However, because of the size of the B-52, there is little alternative for survival against an attack by fighter-launched missiles or cannon fire from the rear.

The CLP consisted of two nested gimbals for pitch and roll and an outer gimbal for yaw motions. Mounted to the roll gimbal were three mutually perpendicular precision rate-gyroscopes that measured the rate of change of gimbal motion with respect to the rotation axis of each gyro. During normal flight, the gimbals were oriented with the yaw, pitch, and roll axes of the airplane and the gyros were caged so as not to respond to aircraft yaw, pitch, and roll motions. Once radar target tracking started, the CLP was initially aligned with the tracking radar antenna beam and the gyros were un-caged to allow the fire control problem to be solved by the ballistic computer or BC.

The BC was an electro-mechanical marvel consisting of servo motors, resolvers, and multi-gang potentiometers, all connected with precision anti-backlash gears and placed inside a pressurized cylindrical metal case with external forced-air refrigerated cooling of the heatsinks inside the case. The BC took analog inputs from the radar system representing range, azimuth, and altitude and calculated where to point the Gatling Gun based on a myriad of environmental inputs such as pressure altitude, pitch attitude of the B-52, indicated air speed, the type of ammunition loaded, and (for all I knew about it) maybe the phase of the moon. The end result, all calculated continuously and in real time, was an x-y or azimuth-elevation positioning command to the CLP.

This was also my first encounter with discrete circuits that I later identified as operational amplifiers, although the Air Force and Emerson used a different descriptor. By any name you choose to call them, they were op-amps and they did amazing things inside the BC... well amazing to a twenty year old airman still wet behind the ears and fresh out of tech school. The experience later came in handy after my enlistment was up and I had talked my way into a technician job at UDRI, one month after separating from the Air Force.

One of my first tasks there was to help revive a huge hydraulically-operated dynamic fatigue-testing machine, whose California contractor went bankrupt trying to get it to work. It took us almost a year of effort, but eventually it did work and it met the original design specs. We had to replace the original motor-oil hydraulic system with a real mil-spec 3000 psig hydraulic system, and replace the home-made voice-coil-driven servo valve with a real Moog proportional-flow servo valve. It took quite a bit of hubris to believe a small machine shop could duplicate the performance of a Moog, or that ordinary motor oil was a suitable substitute for high-performance hydraulic fluid. The test stand was capable of exerting over 200,000 Lbf static load, and ±100,000 Lbf dynamic load at 60 Hz. Total ram displacement was limited by the hydraulic cylinder to about twenty-four inches, but the test coupons were quite stiff and only required a few inches of displacement for slow fatigue tests, less for high-frequency (up to 60 Hz) fatigue tests.

The gun turret was freed from its stowed position (usually pointed dead aft) and moved to align its azimuth and elevation axes parallel with the azimuth and elevation axes of the CLP. The CLP roll axis maintained its fixed orientation in space, moving with roll motions of the B-52, but it's actual position was just another input to the ballistic computer that continued to calculate the predicted projectile trajectory. Presumably the guns could accurately fire even if the B-52 "stood on its wing" in flight. AFAIK this was never tested on a real aircraft, and probably not tested in the field either. The system was tested with towed targets and one (probably apocryphal) story alleged that the fire control radar "locked on" to the turnbuckle connecting the target tow cable with a similar cable affixed to the aircraft. According to the story, the gun shot the turnbuckle off of the tow cable.

From a practical point of view, dithering the amplitude of a DC control signal should be similar to dithering the duty cycle of a PWM signal. In both cases, the effective amplitude of the control signal is modulated or dithered.
 

kellys_eye

Jun 25, 2010
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^ fascinating insight - thanks Hop.

It's amazing how the violence and destruction of war means the creation of systems that would otherwise never come into existence or be onward-adapted to peaceful tasks. Of course, in war situations, cost never seems to enter into the equation.
 

ShewShew79

Nov 19, 2015
6
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Nov 19, 2015
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Thanks kellys_eye for your input, I have a NAND gate IC and a function generator I'm going to mess about with and see what signals out I get, I will be sure to post some images. On the other hand I am totally blown away excusing the pun with hevans1944 response. I love interesting historical facts about aviation technology and from someone who has worked on the kit even better. I am a big aviation buff, so I find it very interesting. I live very close to the MOOG factory in Tewkesbury England and various other aviation design and build firms,
Messier-Bugatti-Dowty etc.
Thanks guys, I will be back soon!

 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Of course, in war situations, cost never seems to enter into the equation.
Cost always enters the equation! It just gets "kicked down the road" by issuing fiat currency and selling "war bonds," but it is always a major factor in the decision of what gets funded and what doesn't.

After WWII, the United States made a conscious "policy decision" to never again be caught unprepared for war, whether it be "conventional" or thermonuclear war. In the 1950s everyone was afraid of the Soviet Union because they had developed the atomic bomb after stealing secrets of how America did it. No multi-billion dollar Manhattan Project necessary in the USSR! And with their avowed goal of spreading Communism world-wide, aided and abetted by China and eventually Cuba, it appeared that nuclear war was not only possible but even imminent.

The strategic bomber was not a decisive factor in the defeat of Germany and its Axis allies (Italy, mainly). It took "boots on the ground" to do that, soldiers, armor, and artillery, as it does in any traditional war. My deceased father, who entered the war late as a B-17 bombardier, said they were reduced to bombing civilians near the end of the war because all the military targets had been destroyed. But still Hitler refused to surrender. All wars create a motivation to discover and build better ways to destroy things. However, adding weapons of mass destruction changes the game forever.

A mentor once told me that the reason we have avoided nuclear war to date is because the politicians on both sides would be vulnerable. The USSR built thermonuclear weapons of humongous size for the express purpose of targeting underground shelters built to protect United States government leaders. We built a fleet of strategic bombers and ballistic missiles to be launched from underground silos and submarines for the specific purpose of making sure the enemy politicians, emerging from their bomb-proof bunkers, would not have a world they wished to return to or could survive in. Dad flew as a navigator/bombardier on B-47 strategic bombers, and for a short time prior to that on B-36 bombers. He said the crews never expected to return from an actual combat mission.

Nuclear winter is a real possibility, followed by disease and famine, after a global thermonuclear war. Mutual assured destruction works, but only if everyone involved remains sane. Lately, that seems to be a pretty damned big "if" when it comes to some world leaders.

During WWII the United States had to play "catch up" to compete. Fortunately we had the industrial infrastructure and a cadre of skilled workers that enabled us to do just that. Were it not for the "sneak" attack by the Japanese on Pearl Harbor on December 7, 1941, the USA might never have entered the war. And I might be speaking German as my native language. There was a strong isolationist sentiment in America that persisted, even after VE (Victory in Europe) Day on May 8, 1945. It was hard to raise money that spring and summer to continue fighting the war in Asia. Fortunately, there was a Plan B: The successful Manhattan Project. Japan surrendered on August 15, 1945, barely three months after VE Day, following the atom bomb dropped on Hiroshima on August 6, and another one three days later on Nagasaki, the same day the USSR declared war on Japan and invaded Manchuria.

I grew up learning about all the wonderful technical advances in radar and servo mechanisms (to name just two) that were made as direct result of trying to fight (and win) World War Two. This continued, albeit at a slower pace, during the Korean Conflict and, much later, during the Vietnam War where the process was accelerated again... not because the United States had to but because we wanted to. Color me cynical, but I was heavily involved in futuristic weapons development during the Vietnam War Era. Vietnam was our field testing ground. This turned out to be so successful that we tried it again by invading Iraq and Afghanistan, with similar results.

The next field trials may be conducted "real soon now" in North Korea, if some backdoor deal can be arranged with China to get them to agree not to interfere, perhaps for "the greater good of mankind" or to preserve their expanding economic and industrial base. Check out what is going on in and around Guam, and at bases in Japan, to see how this may play out in a war near you. AFAIK, there have been no definitive studies published on how a lopsided or one-sided nuclear war would work. I would be looking for an excuse to make a "first strike" and immediately go nuclear to decimate North Korea's ability to retaliate in kind. If you are gonna be a bully, you might as well go large.

Anyhoo, keep me posted on how that PWM dithering works out. I don't have a hydraulic test stand available at the moment to test the concept.

Hop
 

ShewShew79

Nov 19, 2015
6
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Nov 19, 2015
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hevans1994 , I will for sure keep you posted very soon, But thank you for your memories again, I feel like I should holiday in Florida and come visit, Listen to you for a week and leave complete. I'm not sure my wife would agree but hey a man has to sacrifice something once in his life. YOUR MY KINDA GUY!!! hahahahhahhahaha
Speak to you soon I hope!
 

ShewShew79

Nov 19, 2015
6
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Nov 19, 2015
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Hi guy's
Update on my research into superimposing signals. I recently tested a Scanreco crane controller and recorded my results with an oscilloscope. Knowing what a tried and tested systems PWM signal looks like would help me produce my own. In the image is a 2khz fluctuating PWM signal with what appears to be a 60hz signal affecting it. Sorry for the crude paint editing.

scanreco1.JPG

Having this has given me a target to aim for. So with this in mind my research started. I found this document from Texas Instruments http://www.ti.com/lit/ug/slau508/slau508.pdf showing the technique of mixing two signals to create a PWM signal that looks like what I'm trying to do, awesome.

Using a LM 2901N Comparator I have been able to reproduce the signal almost exactly.
My Test1.JPG

Again, sorry for the crude paint editing, hahahaha.
I will follow this up with some more detail soon, including the schematic and signals used to generate the PWM. I have attached the circuit to a mosfet being driven in a high side configuration and powered a solenoid with it. You can feel the valve gently vibrate which is the desired affect with higher resolution over the current control.

More technical updates soon, I promise.
Please note that I am not in anyway a professional electrical engineer, I just like to know how things are done.

Regards

M

P.S
The following image is a screen shot of the sig gen outputs, I forgot I took one.
My Test2.JPG
 
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