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Edwin Dirnbeck
04-21-2016, 10:31 AM
I have 3 different machines that I built. They use cheap 1500 watt high power drill motors from 3/4 electric drills. These are brush type universal motors. I vary the speeds using a variable transformer or a router speed controller. These don't work well. They slow the motor down but when you load the motor they loose more speed.Any controller that has closed loop feedback needs to have a separate senser on the motor. I am looking for a universal plug and play controller that REALLY MAINTAINS THE SPEED, no matter what the add says they don't exist. SOOO I noticed when I have my AM RADIO tuned between stations and use my machines,I CAN HEAR THE RPM OF THE MOTOR.I think what I am hearing is the arcing of the brushes. I am quite an accomplished electro-,mechanical wizard,but all old school . I would like to build a controller using this radio interference for feedback.I have almost no experience with solid state electronics.I am 74 years old and it is time to learn. So maybe some of the wizards on here could point me in the right direction or shoot this idea down. Thank you ,Edwin Dirnbeck

mars-red
04-21-2016, 11:20 AM
I think there are some closed loop (or I guess it would be pseudo-closed loop) systems that monitor the load (current draw?) itself to compensate and give a more consistent speed. In fact, I think that the closed loop systems in some of the VFDs work that way. Speaking of VFDs, it's overkill, but you could use an inexpensive one for this purpose. You can get them for about $100 and I can tell you firsthand they work great for 3 phase motors but I haven't tried them on small universal motors. If you don't need to run more than one machine at a time, you could switch the VFD output to use a single VFD unit for all 3.

Anyway, getting back to compensating for motor load without needing a separate sensor, I came across this that might be of use: http://electronicdesign.com/electromechanical/load-measurement-dc-motor-needs-no-sensing-transformer

I have to imagine some of the readily available speed controllers out there make use of that idea, but I don't know how you'd identify one that had load sensing compensation versus one that didn't.

I'm curious about this topic as well, because I'd really like variable speed control on my little watchmakers lathe. I have a nice old universal motor that I could use, or the little DC motor I'm currently using.

EDIT: If accurate speed control is required, compensation based on load probably isn't what you want... in that case a true closed loop system using an external sensor is probably the only way to go.

boslab
04-21-2016, 11:44 AM
So you plan to replace a hall sensor with a rf receiver, sounds complicated with the added possibility of the receiver picking up other signals, if that makes sense?
However my electricery is limited in respect, mind I just watched an Eric dollard vidio, I don't think I'll ever recover!
In answer, try!, PWM occurs to me
Mark

MaxHeadRoom
04-21-2016, 11:48 AM
There is SuperPID for closed feedback control of a Universal motor.
http://www.vhipe.com/product-private/SuperPID-Home.htm
Max.

Forrest Addy
04-21-2016, 12:53 PM
Using commutator hash as a feedback reference signal. Hm. It probably has a frequency component based on the commutator bars count times RPM but how to separate a usable signal from the random electrical noise? Some combination of filters and freq to voltage ... Hell, I'm not smart enough to work out the details. The people who design shop tools are well aware of the competitive advantage of series wound motors with good speed regulation. You can bet in the last 40 years of solid state electronic motor controls they looked at all the possibilities. I can't imagine they would miss comm noise if there was any potential.

Couldn't hurt, try it.

There is an existing solution and that is to use a Dart or KB SRC motor control. They sense the back EMF of the motor to compensate for speed regulation and are adjustable over a wide RPM range. It's not perfect. In fact the speed reg is better in some parts of the range than others but it works quite well for most purposes. Some SCR controls have an input for governor generator feed back. This governor generator can be quite simple such as coupling a small Mibuchi style DC motor to the load motor shaft but that violates your initial requirements.

Anyway, there are no really cheap solutions that I know of. But there are several low cost ones:

http://www.ebay.com/sch/i.html?_odkw=SCR+AC+motor+control&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR2.TRC0.A0.H0.XKB+mot or+control.TRS0&_nkw=KB+motor+control&_sacat=0
'
Don't forget to look at Dart controls. There are others (Dayton and many automation controls hoses have private lable controls) but these two offer a wide range of reliable products and have a large installed base and good word of mouth.

J Tiers
04-21-2016, 01:07 PM
A series AC/DC motor, such as the "universal" motor in a drill, is INHERENTLY very poor at speed regulation. It's baked-into the design, and there is no way to avoid it*.

So, you HAVE TO have an external speed regulator to keep it on-speed. Otherwise it may vary speed over a 2:1 (or more) ratio.

Now, I do not know what sort of accuracy you think you want, nor why you think you want it. But do not be fooled, a standard induction motor does not maintain speed either. About the best an induction motor does is 5% loss of speed from no load to full power, and 10% from no load to the maximum overload point. And nobody has a problem with that in normal operation.

So, you really don't need absolute accuracy of speed control.

BUT, that said, the RF method is not particularly good. It's capable of being accurate in the sense of it ultimately being based on counting the number of commutator segments per unit time, which would give speed, but its really the long way around the barn when you come to actually DOING it.

You would be far better off to count fan blades, etc, optically, or with a hall effect sensor, etc.

You can then use a frequency to voltage converter, such as LM2907

http://www.ti.com.cn/cn/lit/ds/symlink/lm2907-n.pdf

http://www.digikey.com/product-search/en?keywords=lm2907

With that you get a voltage output related to speed, which you can compare against your speed pot voltage, and control the PWM or phase controller with the difference (error signal) between actual and setting.


* The basic issue is that the "normal" or "design intended" speed (generally called the "base speed") of a series DC or AC/DC motor changes with motor current. The more current, the lower the base speed. This is PERFECT for a drill, or a locomotive, but a bit disturbing or even disruptive for a lathe.

The problem with sensing current is that you cannot be sure how current relate to speed with a series motor. Current relates to TORQUE, but not to speed, and the same speed may require a different torque under dfferent conditions.

Even sensing POWER does not relate to speed, since power is related to speed x torque, and you only know one of those. If you have to sense speed, you may as well use it directly.

MaxHeadRoom
04-21-2016, 01:27 PM
The advantage of something like the SuperPID it uses AC switching (Triac) if using PWM then a suitable large DC power supply is needed.
The unit above uses a simple retro reflective sensor with very accurate rpm control at all loads
BTW, I do not have any vested interest in the site, but have used one of these controllers in the past.
This is the site of the guy that developed it, http://www.romanblack.com/index.htm#elec
Max..

ikdor
04-21-2016, 01:35 PM
The simple method of speed control for DC or universal motors is to compensate for the copper losses (which is basically emf control but then from the other end). A circuit measures the current and then adds voltage at a fixed ratio to compensate for the losses. This always leaves a fixed amount of voltage for the back emf at the required speed.
All walkmans used to work this way for their speed control and were pretty good at it.
It's simple for tiny motors with an opamp, but you could probably hack something with a class D audio amplifier chip for larger motors when so inclined.

garyhlucas
04-21-2016, 01:38 PM
Universal motors will run on DC. Just get a DC motor controller and adjust the IR compensation for reasonably good speed control. If you want better get one that will take a digital encoder. Dart sells the drive and a little blue hall effect encoder that you can install on the end of the motor shaft by tapping it for the 10-32 screw it comes with. It has a built in bearing and the cord restrains it from rotating. I've used them several times and they work great.

Paul Alciatore
04-21-2016, 02:31 PM
1. Using AM radio to receive the hash/noise from the brushes of a motor is a very poor way of monitoring the motor's speed. First, the radio receiver will pick up everything in the area and many things that are more distant. This unwanted "noise" will vary from one place to another so it will be almost impossible to separate the wanted noise from your motor from the unwanted noise of the environment.

2. The user, your customer will have to tune the radio receiver to an unused frequency in whatever area the tool is being used in. Total PITA from a consumer viewpoint.

3. The hash you receive will be very broad band and will have to be filtered with a low frequency filter. Designing this filter will require assumptions about the frequency of the desired signal and this will limit the speed range attainable.

4. You will have wires between the motor and the controller anyway: the power wires. So adding one or two more for feedback is no real problem.

5. A speed sensor, perhaps an optical one, can be easily added for little cost: LED for light and a photo-transistor. These components can be purchased in a single package that just mounts near the shaft. It will give you a nice CLEAN signal that will directly translate from frequency to revolutions per second. This will be a lot less expensive than adding an AM radio receiver. And a lot easier to design. And the user does not have to tune anything.

There is a reason why things are done the way that they are. You will drive yourself nuts trying to make that AM receiver work. Look into tach circuits and figure out where it can be mounted.

CalM
04-21-2016, 02:41 PM
You could reinvent the wheel

Or get one of these
https://www.google.com/imgres?imgurl=http%3A%2F%2Fi.ebayimg.com%2Fimages% 2Fa%2F(KGrHqF%2C!oEF!QIybNRLBQV37Gs6u!~~%2Fs-l300.jpg&imgrefurl=http%3A%2F%2Febay.com%2Fitm%2Fbaldor-dc-motor-adjustable-speed-drive-motor-bc-1-dc-baldor-%2F261099808686&docid=CgpwGwrkC2J6lM&tbnid=3POafKf13FI8KM%3A&w=300&h=200&itg=1&client=ubuntu&bih=796&biw=1356&ved=0ahUKEwiokIXur6DMAhXBPz4KHdnQD68QMwgeKAEwAQ&iact=mrc&uact=8

Baldor DC Motor Adjustable speed drive. The actual electronic device is rather ubiquitous under the maker's name.

MaxHeadRoom
04-21-2016, 02:54 PM
Baldor DC Motor Adjustable speed drive. The actual electronic device is rather ubiquitous under the maker's name.

Baldor relabel KB drives for their line.
Max.

Mike Amick
04-21-2016, 03:39 PM
If you really want to do exactly what you say. Its kind of like an electronic steam punk thing.

There are many Frequency to voltage converter chips out there. You could build a circuit that
"hears" the wine from the radio, converting that to a voltage that helps control the speed of
your motor.

If the wine starts lowering in Freq .. it would just pump up the juice to the motor to keep it
at that freq.

All the other suggestions here are great and how it "should" be done .. but .. your drummer
may want a different beat.

The Artful Bodger
04-21-2016, 04:08 PM
Edwin, I can see some advantages in your proposal the most obvious being that one controller unit could be used on different machines without alteration to the machine at all, just plug it through your magic box!

I have a device in my car which uses alternator hash to determine engine speed and it seems to work quite well but I have no idea of how accurate it may be.

In your situation I would consider a filter on the mains input to your box leaving a pure 60/50Hz followed by a take off for your detector circuit. Your detector would filter out the mains frequency leaving whatever noise is coming from the motor. Half wave rectify the noise and you have a pulse train representing motor speed.

So no need for the AM radio link but still using your idea of using brush noise(?) as the feedback signal to your controller.

Brgds
John
ZL2AYQ

Jim Williams
04-21-2016, 05:44 PM
The variable speed electric drills I have seem to have good torque at low speeds. Possibly you could use the electronics from a variable speed electric drill to control the speed of your devices.

Jim

Mark Rand
04-21-2016, 06:32 PM
The motors will be shunt wound (no load/speed stability on series wound motors!!).


If you want speed control with good torque, separate the armature and field connections then feed the armature from the speed controller and the field from the controller's supply lines. That's what happens inside the variable speed drills, Dremels etc.

J Tiers
04-21-2016, 07:35 PM
What the OP states he has is "universal" series AC/DC motors. He says they have brushes, which generally confirms his other statement.

Almost all mains-powered portable drills have had universal motors, because they have such high torque at low speeds. Often as high as 250% of full load torque, and generally limited only by iron saturation. A speed controller CAN make them behave much better as far as load vs speed performance. When they are used as drills, there is no need, since YOU are the speed controller.

If you separate the armature and field, you lose much of the advantage of the motor type, and you have to replace what is otherwise automatic, with some additional form of controller.

Edwin Dirnbeck
04-21-2016, 11:18 PM
There is SuperPID for closed feedback control of a Universal motor.
http://www.vhipe.com/product-private/SuperPID-Home.htm
Max.
I am looking for a plug and play unit .The one you refered to needs a reflecter installed on the motor shaft. Thanks anyway,Edwin

J Tiers
04-21-2016, 11:49 PM
I am looking for a plug and play unit .The one you refered to needs a reflecter installed on the motor shaft. Thanks anyway,Edwin

You will probably need SOME sort of speed feedback. The back EMF sensors are possible, but usually need to be tuned to the motor for the EMF AND then for the PID control as well (may have workable generic settings for the PID). Speed sensors need only the PID control tuned, and again may have generic settings that will work well enough, so the speed feedback types are probably more "plug and play".

If you want total plug and [lay, no adjustment, etc, you may be disappointed. There may be something out there, but....

Edwin Dirnbeck
04-21-2016, 11:54 PM
1. Using AM radio to receive the hash/noise from the brushes of a motor is a very poor way of monitoring the motor's speed. First, the radio receiver will pick up everything in the area and many things that are more distant. This unwanted "noise" will vary from one place to another so it will be almost impossible to separate the wanted noise from your motor from the unwanted noise of the environment.

2. The user, your customer will have to tune the radio receiver to an unused frequency in whatever area the tool is being used in. Total PITA from a consumer viewpoint.

3. The hash you receive will be very broad band and will have to be filtered with a low frequency filter. Designing this filter will require assumptions about the frequency of the desired signal and this will limit the speed range attainable.

4. You will have wires between the motor and the controller anyway: the power wires. So adding one or two more for feedback is no real problem.

5. A speed sensor, perhaps an optical one, can be easily added for little cost: LED for light and a photo-transistor. These components can be purchased in a single package that just mounts near the shaft. It will give you a nice CLEAN signal that will directly translate from frequency to revolutions per second. This will be a lot less expensive than adding an AM radio receiver. And a lot easier to design. And the user does not have to tune anything.

There is a reason why things are done the way that they are. You will drive yourself nuts trying to make that AM receiver work. Look into tach circuits and figure out where it can be mounted.
Could not the radio receiver be built right into the controller and therefore be closest too and very influenced by the motor that I am trying to control.Thank you for your reply ,it was very thoughtful ,but I will have to disagree with your statement that there is a reason things are done the way that they are. The only reason things are done the way that they are is,someone hasn't figured out a better way yet. Thanks again Edwin

PStechPaul
04-22-2016, 12:19 AM
I think Microchip has an application note on speed control for PM motors using the spikes of voltage or current due to the commutator action. I'll see if I can look it up.
http://ww1.microchip.com/downloads/en/AppNotes/00894a.pdf
https://www.microchip.com/design-centers/motor-control-and-drive/motor-types/brushed-dc
http://www.nxp.com/documents/application_note/AN10513.pdf (BEMF speed sensing)

Otherwise, you can get an inductive proximity sensor that will respond to a conductive (and not necessarily magnetic or iron) piece connected to the spindle of the lathe, or perhaps the chuck, or even the workpiece itself. It uses 6-36 VDC and is only about $3:
http://www.banggood.com/Wholesale-NPN-DC-6-36V-Inductive-Proximity-Sensor-Detection-Switch-p-41603.html

I used one for my Tig-Tach module that I connected to my Air Motor to read RPM:


https://www.youtube.com/watch?v=8naEh12f4IE

I used a PIC to sense the switch transitions and measured the time between them and inverted that for speed, and I also measured the time the switch was closed so it could be used as a Tachometer and Dwell meter. I also incorporated a driver for a transistor ignition, but that wouldn't be useful on a lathe unless you wanted to send a spark into the work at a specific point determined by the position of the sensor. It should be an easy Arduino project as well. I have just interfaced a 1.8" TFT 128x160 graphics display to an Arduino Pro Mini. You can get both, plus a solderless breadboard, and a set of leads, for under $10 total, from Banggood or eBay. You can also add a $3 Bluetooth module and pair it with a tablet computer that can show the RPMs.

The motor speed is somewhat proportional to applied voltage, although not so much for a series wound universal type. However, as mentioned, using a DC PWM drive may be more stable than the phase-fired TRIAC control on the AC line, and you can read the filtered DC voltage and current to regulate it. As for using a radio, you might be able to shield it from external sources and position it close to the motor, and if you hear a whine proportional to speed, you should use a scope on the speaker or headphone jack to see the waveform and exact frequency and amplitude. Then you can design a filter to get rid of RF hash and accept only the frequencies within the desired band, and then process it so that it can be interpreted by a microcontroller. An analog approach may also be possible using an F/V converter and a comparator or PID loop circuit to keep the motor running at the desired speed.

Forrest Addy
04-22-2016, 12:56 AM
I've often wondered about speed regulation when running a series wound motor as a DC shunt motor. Separate the armature and field leads. Run the field at something less than full load amps with low DC voltage and feed the armature variable voltage DC to the desired RPM. No feedback, just the DC straight from a variac with a bridge rectifier or full-wave SCR drive. Run the field from one of those cool little 5 volt switching power supplies on eBay.

It worked years ago when I tried it using an emergency light battery and a big resistor for a field supply and the variac/bridge for armature power on a vacuum cleaner motor. It ran smoothly and promptly up and down a wide range But I couldn't apply a mechanical load to the motor. It didn't have enough shaft sticking out to apply a quick and dirty load. I don't think the field was intended for a 100% duty cycle; it run a little worm. Shunt connected RPM's went up to about 2/3 the free-running speed when connected as a series wound motor. However I never pursued it. The field battery pooped out and I never resumed the experiment.

PStechPaul
04-22-2016, 01:41 AM
As I understand it, a series wound motor has about the same size wires for the field and the armature, but the field runs on DC all the time so its voltage remains very low to produce the magnetic field proportional to current. The armature, however, runs on AC due to commutator action, so its voltage will go up with speed. If you separate the windings, you will need a very low voltage or current regulated supply for the field, while the armature will take a much higher voltage as the RPM increases and the rapidity of the alternating flux in the rotor's inductance cause current to drop as RPM increases, but dependent on the torque required for the load.

Paul Alciatore
04-22-2016, 02:10 AM
Well, far be it for me to rain on your parade. Go for it. And keep us posted on your results. You might re-read Artful Blogger's post above. I agree that an actual radio receiver is probably not needed, just pick up the static with some kind of antenna that is placed close to the brushes.

I await the reports of your results.




Could not the radio receiver be built right into the controller and therefore be closest too and very influenced by the motor that I am trying to control.Thank you for your reply ,it was very thoughtful ,but I will have to disagree with your statement that there is a reason things are done the way that they are. The only reason things are done the way that they are is,someone hasn't figured out a better way yet. Thanks again Edwin

Mark Rand
04-22-2016, 05:15 AM
I say again, the motors used in drills etc. are not series wound, they are shunt wound.

Series wound motors have poor or no speed control with varying load and can't be used for these applications.

Putting the controller on the armature while feeding a constant voltage to the field is the simplest way to get variable speed and sensible torque on a universal motor.

MrSleepy
04-22-2016, 05:53 AM
I say again, the motors used in drills etc. are not series wound, they are shunt wound.

An again you are quite wrong.
.
Shunt is parallel ... Universal drill motors are series ...ie AC->1/2field->brush->Armature/commutator->brush->1/2field ->AC.

The only shunt wound drill motors i've ever seen were in the 400hz Bosch motors. Shunt wound motors (ie the standard 180vDC rated) with seperate field and armature connections are normally much higher quality motors , and not found in cheap disposable
crap like modern power tools.

Mark Rand
04-22-2016, 09:51 AM
An again you are quite wrong.


Beating myself over the head in order to extract both by boots from my mouth! :mad:

The one that I had taken apart a number of years back was variable speed and had a rectifier feeding the field and TRIAC + rectifier feeding the armature. DOH!

Edwin Dirnbeck
04-22-2016, 09:53 AM
Edwin, I can see some advantages in your proposal the most obvious being that one controller unit could be used on different machines without alteration to the machine at all, just plug it through your magic box!

I have a device in my car which uses alternator hash to determine engine speed and it seems to work quite well but I have no idea of how accurate it may be.

In your situation I would consider a filter on the mains input to your box leaving a pure 60/50Hz followed by a take off for your detector circuit. Your detector would filter out the mains frequency leaving whatever noise is coming from the motor. Half wave rectify the noise and you have a pulse train representing motor speed.

So no need for the AM radio link but still using your idea of using brush noise(?) as the feedback signal to your controller.

Brgds
John
ZL2AYQ
Yes ,yes this is just the information I am looking for. Could you please post the brand of tachometer that you have? I see all of the incredible technology that is routinely included in teenagers game controllers and quadcopters ,ect.but power tools are still mostly stuck in the 1950 era. YOU turn them on . they slowly come up to a single speed and most annoying take forever to slow to a stop. Edwin

Edwin Dirnbeck
04-22-2016, 09:57 AM
If you really want to do exactly what you say. Its kind of like an electronic steam punk thing.

There are many Frequency to voltage converter chips out there. You could build a circuit that
"hears" the wine from the radio, converting that to a voltage that helps control the speed of
your motor.

If the wine starts lowering in Freq .. it would just pump up the juice to the motor to keep it
at that freq.

All the other suggestions here are great and how it "should" be done .. but .. your drummer
may want a different beat.
Thanks for your reply. I think it is understood that almost everyone that reads this forum hears a different drummer. Thanks again ,Edwin

MaxHeadRoom
04-22-2016, 10:19 AM
I am looking for a plug and play unit .The one you refered to needs a reflecter installed on the motor shaft. Thanks anyway,Edwin

But is a simple dab of paint or a 3m reflective tape.
Here is the extensive Posts for those that have used it, you are going to have to fit some kind of feedback sensor to control a Universal motor accurately.
http://www.cnczone.com/forums/diy-cnc-router-table-machines/112658-cnc-manufacturing-forum.html
Max.

Forrest Addy
04-22-2016, 10:27 AM
I say again, the motors used in drills etc. are not series wound, they are shunt wound.

Series wound motors have poor or no speed control with varying load and can't be used for these applications.

Putting the controller on the armature while feeding a constant voltage to the field is the simplest way to get variable speed and sensible torque on a universal motor.

The motors used in battery drills are PM DC and their trigger speed control uses PWM (pulse width modulation) to vary the RPM of the motor. PM mean "permanent magnet" A PM motor behaves much like a shunt wound DC motor. PWM is essentially a rapidly cycled switch whose on and off time (duty cycle) varies according to trigger setting. The effect is variable voltage and the pulsed power is fly-wheeled by the inductance of the motor.

Most all corded hand portable drills are powered with a series wound motor. Their motors are carefully designed and matched to a compact SCR control embedded in the trigger. The control senses back EMF and adjusts SCR on-time to regulate motor RPM in response to load (and trigger setting.) Modern variable speed portable hand tools are so well designed and matched the tool's RPM regulation rivals that of a shunt wound motor.

I say most, but not all. Brushless DC motors are appearing on the scene both battery and corded. These are essentially a synchronus three phase motor controlled by a PWM VFD control embedded in the trigger switch - a very sophisticated, powerful, efficient, and compact arrangement but presently expensive.

A very few corded hand portable power tools were powered by a PM DC motor and controlled by a full-wave SCR DC control in the trigger switch. I have one example dating from the '60's. An odd mustard gray color tool made by Thor - a good maker of the day. It's a very light and compact tool but not too powerful. It readily overheats when driving screws Its virtue is its small size (8" long x 1 3/4" wide - IIRC) and very low weight (about a pound.)

Never seen a shunt motor powering a hand portable tool but there is no reason one cannot be made in these days of sophisticated electronics and motor design..

Maybe the OP's motor problem is better solved with a stepper motor or a servo motor, one whose RPM and shaft angle are under digital control. I have a slick little 300 watt brushless servo about the size of a Campbell's soup can. Turns 6000 RPM. It has a 2000 count rotary encoder on it and the control responds to step and direction control and quadrature. These aint cheap costing over $1.5K new. If you want such an animal for less than $200, used (read "usable", you REALLY have to be in the right place at the right time.

But the OP is looking for simple and cheap. I do ramble on.

J Tiers
04-22-2016, 01:30 PM
I have 3 different machines that I built. They use cheap 1500 watt high power drill motors from 3/4 electric drills. These are brush type universal motors. I vary the speeds using a variable transformer or a router speed controller. These don't work well. They slow the motor down but when you load the motor they loose more speed.Any controller that has closed loop feedback needs to have a separate senser on the motor. I am looking for a universal plug and play controller that REALLY MAINTAINS THE SPEED, no matter what the add says they don't exist. SOOO I noticed when I have my AM RADIO tuned between stations and use my machines,I CAN HEAR THE RPM OF THE MOTOR.I think what I am hearing is the arcing of the brushes. I am quite an accomplished electro-,mechanical wizard,but all old school . I would like to build a controller using this radio interference for feedback.I have almost no experience with solid state electronics.I am 74 years old and it is time to learn. So maybe some of the wizards on here could point me in the right direction or shoot this idea down. Thank you ,Edwin Dirnbeck


I am looking for a plug and play unit .The one you refered to needs a reflecter installed on the motor shaft. Thanks anyway,Edwin

The end result here seems to be more constant speed control, and I can certainly understand that, The series motor is inherently not constant speed, and varies over a huge range with loading.

With all due respect, the OP has demanded a certain type controller, and appears to be carefully EXCLUDING all other varieties. He wants what he wants, and will not even put a reflector on a shaft, he wants ZERO connections beyond what already exists. Now, I hate to call it unreasonable, but it does seem rather uncompromising to specify the performance AND the specific means of attaining that performance, without also explaining why no other solution is acceptable. Perhaps it is just because he wants to, and that's fine.

The radio feedback idea is one which can likely be done, given sufficient desire. It is an interesting idea, but I won't call it a "high school project". It involves several different areas of electronics, and has a number of possible ways for it to NOT work in a satisfactory manner.

Sparking of that type has a good deal of high frequency content, and a low-gain receiver in the 50 MHz range would pick it up. AM demodulation is certainly easy enough, and a frequency to voltage converter would extract a control voltage, to be used to adjust the firing angle of the SCR or triac. I'd suggest SCR plus rectifier, because triacs are a compromise and are not available with as nice characteristics, but a triac would work. Some idea of the frequency range from the commutator at the desired speeds is needed to set up the F-V converter.

It is likely that a back EMF sensor type control would work as well or better, and be considerably less complex, less prone to poor operation due to RF interference. Either type would satisfy the "no extra connections" / ""nothing extra added" requirement.

The application data books for SCRs and Triacs are full of suitable back EMF control schemes, many including values. While the best performance is probably going to come from tuning of the control to the motor, acceptable performance is likely possible by pretty much using a control for any similar motor within a 2:1 range of the same power directly, adjusting values only for current/voltage ratings as necessary.

When I get time, I'll look for a circuit i the apps data I have. I've got tons of it, the problem will be finding the type wanted in the "haystack". It will be back EMF, I have NEVER seen any control scheme using RF pickup of spark noise.

I found one quickly, here is is

http://img.photobucket.com/albums/0803/jstanley/wiring/series%20motor%20control%20scheme%20A1_zpszw55f2ek .jpg (http://smg.photobucket.com/user/jstanley/media/wiring/series%20motor%20control%20scheme%20A1_zpszw55f2ek .jpg.html)

http://img.photobucket.com/albums/0803/jstanley/wiring/series%20motor%20control%20scheme%20A2_zpsm6br6r9v .jpg (http://smg.photobucket.com/user/jstanley/media/wiring/series%20motor%20control%20scheme%20A2_zpsm6br6r9v .jpg.html)

Seastar
04-22-2016, 02:56 PM
To the OP
I have spent my entire life designing radio equipment, simple and sophisticated. Frequencies from 10 MHz to 10 GHz.
I view the design of your proposed "noise" receiver as an almost impossible task for you IF you want absolute reliability under all conditions.
I wonder why you would not accept a sensor on the shaft of the motor to drive a closed loop speed controller?
Dart Controls,
www.dartcontrols.com
has everything you need for low cost control if you will accept a sensor on the motor shaft.
Bill

The Artful Bodger
04-22-2016, 03:41 PM
Edwin, I am afraid the gadget I have is not a tachometer but it does demonstrate the principal..

https://www.youtube.com/watch?v=b9XAC-BvUyo


I support your idea of a deriving the feedback signal without modification at all to the machine being controlled as that allows controlling different machines by merely plugging them in to your controller.

PStechPaul
04-22-2016, 04:38 PM
That's a pretty cool device. You could probably direct the output of the speakers to a PA system mounted under the hood, so everyone else can "enjoy" the sound. It would also be a good idea to add something like this to EVs and hybrids, which are often silent, and pose a danger to blind people who rely on sound to detect approaching vehicles.

It would also be interesting to attach a microphone to the head of the lathe to detect bearing and gear noise, which also serve as RPM indicators. But all of these indirect means may be problematic because the sound changes depending on load and other factors. These may provide false information to the controller and cause instability, which, on a lathe, may range from annoying to dangerous.

RB211
04-22-2016, 05:30 PM
Call me crazy, but am I the only one that thinks the easiest thing to do is drive an H bridge consisting of appropriate mosfets, with a micro-controller, that also reads the quadrature output of a sensor counting every time the shaft makes one revolution? Oh wait, that is exactly what the SuperPid is...
Yes, you would need to program the micro-controller with a PID algorithm, all fun stuff really. Funny, if I was to do this the old fashioned analog way, I would find that immensely more difficult. The difference between generations.

Mike Amick
04-22-2016, 06:01 PM
That's why I thought Edwin .. might be one of those gentleman that thinks ...

"I wonder if that could be done ... THAAAT way."

I actually have a little bit if that, in myself.

I once built a robot platform, out of a fighter jet seat base. It looked real cool, all alum and
rivets all the way around it .. just cool.

But I controlled it with a phone type key pad ... transmitting through a Radio Shack walkie
talkie. On the base was the talkie recvr, feeding the DTMF tones to a demodulator. Not the
most efficient way to do it ... but .. I was just messin around and it worked really well.

And I do think it is kind of a steam punk way of thinking. (thats a good thing)

And Edwin DOES have something on his mind, hesitating to use the easiest methods to
accomplish what he wants done.