Hi guys!
I need some help with the power electronics that will control- and drive the solenoids in my solenoid engine:
There are 4 switches with bearings on the end of them, which get activated in four different intervals of the axle angle when an excentric part of the axle pushes them in. In short, there are 4 SPST switches that control when each pair of the solenoids should pull on their pistons. The axle is in 'flat-plane'-configuration which means that pairs of solenoids are always in sync (there will be 4 pairs, one switch corresponds to one pair).
The solenoids in the picture are obviously not spun with copper yet, but I have a couple that I've wound to use for testing. They will all be spun with ~78 g of 0.25 mm copper wire (tested once but the number of wounds is unknown) and each have a resistance of ~55 ohms. The maximum voltage (at the moment) that I've tried with them is 37 V when using a small test circuit powering two solenoids in parallell (circuit shown below). The current through each solenoid then becomes ~670 mA (this decreases a bit as the temperature of the resistor increases), but this makes the solenoid become very very hot after a while. This has however been in a test when I only turn the motor manually at very low RPM.
This leads me to
Q1: Do solenoids generate less heat when "pulling" on e.g. a mild steel piston, compared to just sitting and letting current pass through as if it were a resistor? My thoughts go "if there is nothing nearby that can be pulled/pushed, all the power (37 * 0.670 W) goes to heat, but if it's excerting a force on an object, all that power can't go to heat, some of it has to be 'spent' on pulling the object".
Regardless, I have not decided on how to supply the thing with power so for now I will use a PSU that goes up to 37 V, but this might very well be too much for the solenoids.
The testing circuit is a proof of concept for my thoughts on how to activate one pair of solenoids depending on the correct switch, and here's a representation:
There is an N-channel MOSFET (Q1, data sheet attached) that controls when the solenoid pair should be activated. This gets activated by the switch (SW1) which connects the output of a voltage regulator (U1). This voltage will only need to be above 4 V (the threshold voltage for the MOSFETS used in this circuit) and is decided on the ratio between R1_1 and R_2. The 'coils' L1_1 and L1_2 represent the two solenoids connected to the circuit, with D1 being the flyback diode.
The test circuit was made on a perf-board and simply applies the voltage VCC over the solenoids when the axle rotates to the position when the piston is "allowed" to pull, and disconnects VCC when the piston no longer is allowed to pull. All working as expected.
However: I will need to be able to control the forces that the solenoids excert, and therefore the current through them (and therefore the voltage them) so that I can implement some sort of throttle. I have some ideas but I'd like to hear your thoughts:
1. PWM the gate voltage and add some big capacitor near the drain of the MOSFET
3. Use a variable step-up circuit that takes e.g 12 V or 24 V and outputs anywhere from 0 V to 37 V (or somewhere in the vicinity) depending on a potentiometer?
#2 and #3 would be nice since the gate of the MOSFET would not be affected by the big changes, enabling me to get away with the simple activation method. What do you think? Any other ideas?
The final solution will be two PCBs attached to the front- and back side of the motor. I am obviously not finished but here's a preview at least:
The plan is to have each PCB control two pairs of solenoids each, but the above PCB only has one side filled up at the moment.
Q2: What would a good decoupling capacitance between VCC and GND be? Is it needed?
Q3: What would a good value for C1 be in this case? I only added it so that if I needed it on my PCB
I hope that I could sufficiently explain the problem. Grateful for all answers!
// Ephex
I need some help with the power electronics that will control- and drive the solenoids in my solenoid engine:
There are 4 switches with bearings on the end of them, which get activated in four different intervals of the axle angle when an excentric part of the axle pushes them in. In short, there are 4 SPST switches that control when each pair of the solenoids should pull on their pistons. The axle is in 'flat-plane'-configuration which means that pairs of solenoids are always in sync (there will be 4 pairs, one switch corresponds to one pair).
The solenoids in the picture are obviously not spun with copper yet, but I have a couple that I've wound to use for testing. They will all be spun with ~78 g of 0.25 mm copper wire (tested once but the number of wounds is unknown) and each have a resistance of ~55 ohms. The maximum voltage (at the moment) that I've tried with them is 37 V when using a small test circuit powering two solenoids in parallell (circuit shown below). The current through each solenoid then becomes ~670 mA (this decreases a bit as the temperature of the resistor increases), but this makes the solenoid become very very hot after a while. This has however been in a test when I only turn the motor manually at very low RPM.
This leads me to
Q1: Do solenoids generate less heat when "pulling" on e.g. a mild steel piston, compared to just sitting and letting current pass through as if it were a resistor? My thoughts go "if there is nothing nearby that can be pulled/pushed, all the power (37 * 0.670 W) goes to heat, but if it's excerting a force on an object, all that power can't go to heat, some of it has to be 'spent' on pulling the object".
Regardless, I have not decided on how to supply the thing with power so for now I will use a PSU that goes up to 37 V, but this might very well be too much for the solenoids.
The testing circuit is a proof of concept for my thoughts on how to activate one pair of solenoids depending on the correct switch, and here's a representation:
There is an N-channel MOSFET (Q1, data sheet attached) that controls when the solenoid pair should be activated. This gets activated by the switch (SW1) which connects the output of a voltage regulator (U1). This voltage will only need to be above 4 V (the threshold voltage for the MOSFETS used in this circuit) and is decided on the ratio between R1_1 and R_2. The 'coils' L1_1 and L1_2 represent the two solenoids connected to the circuit, with D1 being the flyback diode.
The test circuit was made on a perf-board and simply applies the voltage VCC over the solenoids when the axle rotates to the position when the piston is "allowed" to pull, and disconnects VCC when the piston no longer is allowed to pull. All working as expected.
However: I will need to be able to control the forces that the solenoids excert, and therefore the current through them (and therefore the voltage them) so that I can implement some sort of throttle. I have some ideas but I'd like to hear your thoughts:
1. PWM the gate voltage and add some big capacitor near the drain of the MOSFET
- Sort of how you'd PWM an H-bridge DC-motor controller, but with the 'PWM-switch' being in series with SW1 so that the axle-switch still decides when current is allowed to flow through the solenoids. Perhaps with a 555 timer that changes duty cycle from a potentiometer? This might however cause issues with the driving of the MOSFETS if the frequency gets too high(?). I might not get away with my super-simple way of activating a MOSFET as I did with my testing circuit.
3. Use a variable step-up circuit that takes e.g 12 V or 24 V and outputs anywhere from 0 V to 37 V (or somewhere in the vicinity) depending on a potentiometer?
- This also enables me to use a lower voltage powersupply (although higher current) so that it turns more into a desktop toy. I will also be able to more easily use it with batteries on future RC-cars or similar projects which I hope to be able to do.
#2 and #3 would be nice since the gate of the MOSFET would not be affected by the big changes, enabling me to get away with the simple activation method. What do you think? Any other ideas?
The final solution will be two PCBs attached to the front- and back side of the motor. I am obviously not finished but here's a preview at least:
The plan is to have each PCB control two pairs of solenoids each, but the above PCB only has one side filled up at the moment.
Q2: What would a good decoupling capacitance between VCC and GND be? Is it needed?
Q3: What would a good value for C1 be in this case? I only added it so that if I needed it on my PCB
I hope that I could sufficiently explain the problem. Grateful for all answers!
// Ephex
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