You might want to think about this some more . . .
all you will ever get from the wiper of this potentiometer is GND
Yes. GND is what i want in this case.
He get resistance to GND setting the first LM393 (-) input voltage. At (+) input he has sawtooth waveform from 555 so he can set output pulse duty factor. In both directions from center point pulse duty is the same.
From:
I understand that may be pulse polarity is some way reversed or may be actuator supply is reversed. May be when potentiometer is in center the actuator has no supply at all and that makes it returning to its center position.
I wonder how similar the effect can be achieved using just the 555 without the LM393.
OK, good point . . . . .
You are right. Pulse polarity (duty cycle) is reversed, but it is reversed again in the end, by a P-channel MOSFET (controlling a servo valve).
Likely possible alternatives:
If the actuator in question is an electric one with an electric DC motor, then the circuit here controls the speed of the motor and the polarity of the current to the motor (= direction of rotation) is decided with external switches or relays.
If it is a hydraulic actuator, this circuit either controls the hydraulic pump speed and the direction of movement is decided by switches controlling either of two electromagnetic on/off type directional valves (or coils on a common spool), or possibly, if the hydraulic circuit has proportional valves and not on/off type, this circuit controls the amount of current to the valves and switches control which one of two electromagnetic valves the current is routed to. Either way, the PWM of the current is the same regardless of direction, and direction is decided outside of the shown circuit.
The chosen connection of the potentiometer also gives a nice progressive curve for the control, which makes smooth control of the movement easier (good control of fine movements near the middle position as well as a nice acceleration and deceleration characteristic when the lever is moved from neutral to maximum position and opposite.
This is a good example of electronics adapted to a practical usage.
(Of course, safety aspects with possible danger resulting from various failure modes of the electronics must be considered.)
You are correct. I can add that the frequency of the PWM-signal is very low. Only 66Hz. That frequency makes the smoothest response from the propprtional valves.
I agree with your description but if it is just joystick rotating the standard potentiometer (without any gears) it will newer reach positions near potentiometer maximum/minimum.
Interesting! Thanks for the clarification. Where did you find this circuit? It might be interesting to see the original.
Interesting. I have seen specifications of 120Hz and 180Hz for proportional valves from a couple manufacturers, but perhaps the moving mass / inductance in the valves your circuit controls, is higher, necessitating the lower frequency.
Do you mind telling what brand / type the valves are?
While this is not likely the way I would design a circuit, the RV2 resistance to ground is:
Full CCW = ground
Full CW = ground
Mid point = 2.5k to ground.
For the last 20 years or so, using HALL sensors has become more common for such industrial applications. Hall sensors have âinifiniteâ life, donât need gears that can wear out or get misaligned) If safety is a real issue, such are for example with gas pedals in cars, big cranes, hydraulic stuff, etc, then even two hall sensors are used. I think there are special hall sensor chips for cars (gas pedals) which have two independent sensor circuits in the same IC.
And also of course, hall sensors are cheap 
Yes, weâve discussed that, and why itâs like that. Did you read the thread?
This idea, with the potentiometer in center at zero speed and the trim potentiometers aside, comes from my own mind. I had to figure it out when i buld my own all electric mini loader, that i accelerate and steer by a joystick. The loader must stand still as i release the joystick. And produce 5 to 0 kohm (for the motor controller) in both directions (forvard and back). Plus that i had to build in a microswitch for sensing forward/back.
For the PWM-part of the joystick in this thread i used the main parts from a standard PWM-regulator, and modified it with my ideas. Only paperwork, that not looks so great (tried to upload it, but i am only allowed to post one image). Therefore i want to explore Kicad.
I understand that people question my mid pot solution, with both âendsâ to ground, as my electronic-guru-friend shouted the first second he saw it âAre you really stupid? That wont work!â. Silence five seconds, then, with a nicer voice âOh yes, it doesâŚâŚâ
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I dont know the brand right now, but i will look for it when i visit he customer next time.
You are right, but this skylift (e.g. boom lift) is from about 1980 so i choosed to work this way. I guess i need a microcontroller to work with Hall:s?
You are right about safety. Especially on a skylift. My PWM-unit generates no output if the lever is released. If that fails, the output is stopped by the microswitches, that are open when the lever is released. If those both fails, at the same time, there is a âDead mans gripâ (a foot pedal that the operator must press) that stops the engine if released. Maybe there should be a fourth safety system, if all three fails simultainiusly, but i dont know what that could be.
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Heart rate sensor + bluetooth receiver? No pulse, and the machine stops.
Nice - itâs a good design which I probably wouldnât have thought of (although Iâm only a hobbyist). Just one point: the resistance goes from 0 to 2.5k, not 5k. (I made the same mistake myself.)
Where do you stop? A fifth safety system? A sixth?
Itâs a rhetorical question - there is no right answer for everyone. At this point in your design you would normally employ a Reliability Engineer (I was one) who will model the various failure modes and calculate the probability of all three protection devices failing simultaneously. You would then have to decide whether that is an acceptable risk or not. If you want to reduce the risk further, the Reliability Engineer would advise you on the best way to reduce that risk. It might involve another layer of protection, or changes to the existing layers.
Reliability Engineering is a fascinating job. One lesson you should take is that it is not possible to reduce the risk to zero. Another lesson is that Reliability Engineers are brilliant at spotting âgotchasâ that might undermine your efforts to make something safe (or reliable).
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