RPM regulation

[dacflyer]

ok, i might have asked this before cannot rember<p>(memroy is like a flashbulb here)<p>i need a device to control the rpms of a gas engine,
the engine does not have a built in governor,,as small engines of the lawn mower vareity..<p>this engine i have i want to use on a large generator 12kw..belt driven..
runs at 1800 rpms but i need something to boost rpms to compensate for load demand,,,<p>most home generators have this but its mechanical/internal to the engine..<p>i need something that will work on a engine such as a geo-metro engine,,,something maybe servo controlled if possible...<p>it would be great if circuit could be getting its info from pulses off the coils input...much like the rpm tach gets its info
in my mind this should be a easy thing to make,,,but i cannot think of how to make it...<p>i immagine a diagram block...
1 tach input
2 processor?
3 servo controller
4 servo<p>am i thinking right? if so,,how do i make this?
thanx for any info....i bet EDD WATLEY knows...heehee hes the guro of all here

[cato]

I think automotive community calls the circuit you describe a cruise control.

[dyarker]

The generator controller will control the output voltage via the current it feeds the field winding. You need to maintain constant engine RPM for about 60Hz out.<p>A cruise control isn't exactly right because they measure RPM after the tranmission, and try to keep the wheel RPM the same as when the button was pushed. Should be useable though if you put the sensor at the 1800RPM shaft. The tricky part is modifying the cruise control to be permanently set at 1800RPM. Also useless for automatic switch back to normal power.<p>If you don't care about auto switch-back and want to do it yourself:
Transformer isolate the geneator output (120V to 6V), half wave rectify (no filter), clamp to 5V, scmitt trigger, then feed to one input of a Phase locked loop (PLL) IC. The other PLL input comes from a crystal oscilator divided down to 60Hz. The analog output of the PLL goes to a Pulse Width Modulation (PWM) circuit that drives a selinoid on the engine's carburator. Add a bought mechanical governor to the engine, set to like 1900RPM, in case the circuit fails. (No sense over-revving the engine for a bad transistor!)<p>Make sure a qualified electrician does any AC mains work.

[Bernius1]

I think Cato's idea is very good, excepting that instead of vehicle speed sense, you'd tie that input to a voltage sense in the output, so that as soon as the voltage falls below a set point ( say 100V ), the system activates. If you're good, the throttle response will be proportional to added load, as a car's cruise ctrl is proportional to MPH drop. Good luck !

[dacflyer]

Hmmm making it sound more complex than i am thinking...
i am not a circuit designer...but can follow a made schematic...also a solonoid for throttle control is no good,,rather it be a servo type,,
the unit has to be able to maintain the 1800rpms
during load demands,,<p>other small engines have mechanical governors but i need to have one to use on a engine that does not have such a thing,,,so i though a electronic one would be best...<p>something that can take the readings from coil input..and feed the info to a large r/c servo...<p>do i make it sound too easy or is it much more complex... i want to keep it simple as possible...thanx...

[dribach]

my suggestion: get a small risc microcontroller with PWM output and at least one external interrupt. should be pretty easy.

[haklesup]

I assume that your generator must maintain input revs at 1800RPM so that the output will be 60Hz. A change in speed means a shift in output frequency (depending on the load this may be tolerable within a narrow range).<p>When the load increases, the torque required to spin the generator increases and transfers that load to the gas engine via the belt. What you need is a mechanism that will increse power to the engine so that RPM will be constant as the torque is changing. (we probably all knew this but I wanted to clarify)<p>Generator RPM has ltttle to do with output power (voltage and current), that is determined by how much copper there is in the windings. and how much force (torque) you apply to the shaft.<p>Consider this scheme. Put a slotted disk (optical encoder) on the motor shaft. Use a photomicrosensor to sense the slots as they go by. This will give you a square wave tachometer signal (may need to buffer with an op amp). Run that into a Frequency to voltage converter IC and put that output into a comparitor. Place a reference voltage on the other input of the comparitor to set the speed. The comparitor output voltage will be an error signal (you will need a bipolar supply +/-). This voltage can be used an input to a linear actuator (see your search engine) either directly via your own driver ckt or via a driver provided with the actuator. If the throttle is a knob then a rotary servo might be easier to integrate (mechanically) than a linear one (I assume the motor has a throttle you can control, newer lawn mowers make it very hard to change the speed). <p>It's straight forward but you will need to control the damping (response time) of the actuator so that it does not oscillate. Finally the spacing of the slots in the encoder will determine the resolution of the speed control. They need to be spaced enough so that at 1800 rpm they don't blur into a continuous signal but close enough so that a small change in RPM will be sensed. There are optical encoder kits designed for specific speeds out there.<p>Whats your budget?

[dacflyer]

DRIBACH >>> simple you say?
how do i go about making this????

[dacflyer]

haklesup >>> an encoder wheel does not sound useable
dust dirt etc..<p>as for circuit design,,i cannot do this..unless i have a schematic....*sigh*

[josmith]

If you need to maintain voltage and frequency is unimportant than use voltage for feedback If you need to maintain 60hz than you can use the line frequency from the generator as feedback.<p>For voltage control you can go low tech amd use a solenoid acting against a spring. <p>If you go the microprocessor route you can use a radio control type servo for output.

[cato]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by dacflyer:
haklesup >>> an encoder wheel does not sound useable
dust dirt etc..<p>as for circuit design,,i cannot do this..unless i have a schematic....*sigh*<hr></blockquote><p>This is why I pointed out that you are asking for a cruise control. Just buy one.....

[haklesup]

There is another although less efficient way to regulate the output. That is to let the RPM of the generator vary and feed the AC output into a rectifier then back into an inverter (AC-DC-AC converter). This basic scheme is used by wind turbines. If you get a good inverter (off the shelf) then the voltage and frequency will be reasonably well regulated. What you will sacrafice is current. When you run out of current, the voltage will sag. You can stick a bank of lead-acid batteries in the DC stage and it will act as storage and a capacitor for peak loads. As for the rectifier, use a high current schottky type so that you waste less power on the diode junctions.<p>Hard part in this scheme is getting the DC voltage to be what the inverter wants it to be. I'm guessing the raw AC off the generator is 120V so a rectified DC of that would be about 84V. Automotive inverters are made to take in 12V. You can either make a whopper of a 12V regulator or find a way to get the inverter to accept a larger voltage (anybody know if this is possible)<p>Yes, cato, I do agree a cruise control would probably work, only difficulty is in sensing and signal conditioning of a tach signal to substitute for the one it expects. Digital counters could be used to divide the 1800 Hz square by 2, 4, 8 etc. <p>A curise control would have a rather slow reaction time, it may take second or more for the RPM to recover from a change in load demand. Response time and damping would be important, too fast or under damped and it (the speed) will oscillate, too slow and you might as well do it by hand.<p>A magnetic pickup (magnet on shaft, coil of wire nearby) would be simple and resistant to enviornmental contamination. There is probably an automotive component specifically for this use.<p>Even if I hashed out a schematic for my design it would be difficult to compute the gain for some of the stages and the delay/damping constants in the feedback loop. This is because the control part of the loop is electronic and the feedback is mechanical. deriving (never mind solving) equasions for this is beyond what I already forgot from college. This would be a trial and error design.<p>Chris

[cato]

Rectified DC of 120AC is a lot closer to 170 Vdc than it is to 84.<p>I would use a hall effect sensor for the magnetic pulse pickup.<p>If these aren't part of the cruise control kit, I'm pretty sure they ARE part of many anti-lock brake systems.<p>By the way, I just glanced back at the original post. As I recall, 1 horse power is about 700 watts. So, 12KW would be less than 20 horse power. I'm pretty sure even a Geo metro engine would barely notice a 20 horse load. You might be able to just set the speed with the idle screw and ignore the frequency and voltage drifts.

[dacflyer]

CATO >> all the cruise kits have to have a speedometer input,and some engines do not support vacuume..as with diesels....
doesn't have to be a metro motor just something in the area..but i mainly refer to any engine with out a mechanical governor...
micro processors out of question too...
was looking something more conventional...

[Will]

I built some 200 kW generator sets about twelve years ago. We used commercial modules for the speed/load controls so I didn't have a lot of electronic design to do. The speed feedback/measurement however was achieved by drilling to flywheel casings and inserting a commercial sensor in there (I can't remember whether it was Hall Effect or some other - they are relatively cheap - just proximity sensors fast enough for the job. They detected the teeth on the flywheel. We ran at 1800 rpm (Four pole generators) and there were about 200 teeth on the flywheels so that we could sense a time of 100th of a cycle i.e. about 166 microseconds. This was fast enough to detect the engine slowdown due to increased load and open put out a 4 - 20 mA signal which drove the throttle actuator. The system worked near perfactly i.e. we never detected any changes in the (60 Hz) frequency due to load changes. The real problem is that your control system needs to be tuneable so that you can finely adjust the control terms to match the engine/generator dynamics. We had PID controllers for this (All part of the relatively cheap engine controllers). Building an elelctronic controller with a 4 - 20 mA output is not a big deal (But I don't know how to do it - but I know exactly how they operate)) it just takes a few op amps and there is another thread on this board in which it is discussed - See posts of Bob? - Hope this helps