grease gun using dc motor - current switch

[mqt]

Hi All,
I'm in the process of designing a grease gun which builds up pressure to 500psi using a 12V DC motor and a cam to move a piston which builds up the pressure. The system will be battery powered.

I'm having trouble coming up with a circuit which will turn the motor off once the current draw reaches a certain level (corresponding to the load on the motor once 500psi is reached). I then would like the motor to turn back on after say 5 seconds, and either
A) start running again if current draw is less than that required to build up 500 psi
or
B) turn straight off because the system is still at 500psi and therefore current draw is at the limit.

Are there any suggestions which you guys can give me or point me in the right direction?
Many thanks!

 

[Harald Kapp]

You can use a transistor and a resistor as a current sensor.
A 555 timer used as a monoflop can control the time.
The setup could look like this:

To the right of the voltage source V1 is the monoflop. The time constant can be changed by varying R1 and/or C3. Currently it is approx. 5s.
To the left of V1 is the current sensor Q1/R3. As soon as the current through R3 reaches ~0.7V, Q1 starts conducting and generates a low signal at C4. This signal triggers U1 and the output (Out) of the 555 goes high (~5V). The current limit can be set by chosing R3 appropriately. You may need some filtering (not shown) to prevent U1 from accidentally triggering due to noise.

The motor, the motor's energy source and a switch (relay) are not shown. They connect to the two wires leading off to the left of the schematic. The switch would be controlled by the output of the 555.

This circuit assumes a common connection of the control circuit (555) and the motor circuit. It also assumes Dc operation of the motor. If the motor operates from AC or from high voltage, this circuit will not work or will not be safe.
You can introduce safety by replacing the transistor by a photocoupler.
You can use the circuit with AC only after some modification of the current sensing circuit.

Have you considered using a commercial pressure limit switch? This might be easier and a lot safer than building this circuit yourself. If anything goes wrong with limiting the pressure, 500 psi is not a pressure to toy with!

Harald

You work at your own risk. I expressly refuse any responsibility for damages to persons or goods that may result from improper use of any advice I give.

 

[CDRIVE]

In order to give you accurate answers we need accurate information. This would include..

Motor draw under no load conditions.
Motor draw under load (as it's moving grease)
Motor draw @ 500PSI

As far as danger is concerned, it's quite low. This is because grease gun cylinders are extremely small (very low volume) and grease doesn't compress, so there's no danger that's usually associated with expanding gas or air caused by a rupture.

 

[mqt]

Thanks Harald and CDrive,

You're right, there is relatively low danger associated with this project.

The current draw on the motor at no load is <250mA
As the mechanical components are not fully finalised yet, I can't give an exact figure on current at 500 psi. From testing of prototypes, i've seen up to 6A.
I'm not sure what you mean by motor draw under load (as it's moving grease), as the draw will increase as more pressure is built up. So 0.25 < I < 6A ?

Many thanks for the input.

 

[CDRIVE]

Wow! That's quite a spread. I would think a motor that pulls only 250mA (no load) and pulls 6A @ full load is being severely overloaded.

Anyone else have an opinion on this?

 

[mqt]

You're right that is quite a spread...and that's not even at stall torque which would draw just almost 10A!

That's the reason why I require the current sensing circuit. There is the option of a higher reduction gearbox which would give me more with less current, but then my flow rate is also affected (there's also a peristaltic pump being driven off the same motor - not just the cam)

 

[Harald Kapp]

How ist he motor controlled? What voltage does it operate on?

 

[mqt]

it will operate at 12vdc via a relay.

 

[Harald Kapp]

at 12 V you could use my circuit. The current sense resistor would have to be 0.7V/6A -> ~0.12 Ohm.The "ot" pin of the 555 could control the relay.

There are two potential issues with my (quick and dirty) circuit that may need some more eaboration:
1) The 555 relies on a falling edge at the trigger input. The slope of the edge is created by the curent rise through the sense resistor. If the current rises to slowly (depending on the characteristics of your motor setup) it will not trigger the 555. In that case a more elaborate current sensing circuit using e.g. a comparator may be necessary. You may also consider a fail safe circuit that disables the motor if the current is too high regardless of the operation of the trigger circuit.
2) The current will probably very noisy. Some filtering may (or probably will) be required so the motor won't turn off at lower currents. This may take some experimentation and is a bit in contrast to the requirement for a comparatively steep slope to trigger the 555.

1 and 2 combined lead to the conclusion that is is probably better to use a low side current sense amplifier plus a filter (low pass) and a schmitt-trigger (comparator) to trigger the 555.

Harald

 

[CDRIVE]

Harald, I think it would also be advisable to have a current limiting resistor in series with the base of your current sense transistor. Without one a spike could easily destroy the base emmitter junction.

 

[mqt]

With point 1, i'm reading through a 555 timer documentation and under monostable operation it says:
'the circuit triggers on a negative-going input signal when the level reaches 1/3 vcc'
Would the 555 timer not trigger even if the trigger signal ramped linearly to low, so long as it goes below Vcc/3 ?

 

[CDRIVE]

With point 1, i'm reading through a 555 timer documentation and under monostable operation it says:
'the circuit triggers on a negative-going input signal when the level reaches 1/3 vcc'
Would the 555 timer not trigger even if the trigger signal ramped linearly to low, so long as it goes below Vcc/3 ?

Yes, I recently posted a plot indicating this. I'll post the link if I can find it.

 

[CDRIVE]

Here's the plot.. As you can see a linear ramp has no trouble triggering it.

 

[mqt]

That's fantastic news!
thanks very much

 

[Harald Kapp]

If the slope is too slow, the capacitor in the trigger line in my diagram will effectively block it. You may have to experiment with this capacitor.

 

[Harald Kapp]

This is an improved circuit using a comparator:

Note: the comparator needs to be able to sense voltages near GND level.


Harald

 

[CDRIVE]

I'm only posting this for the educational value it may have for others reading this thread. I say this because Harold's use of a comparator is a better choice for the following reason.

Using BJT's for my current sense requires Rsens to be at least 0.1 Ohm. This value may be too high and cause loss in motor torque. A comparator can be made far more sensitive and does not have to overcome the Vbe knee of a BJT to turn it on. Ergo Rsens can be reduced with a comparator.


I'm posting this for its triggering uniqueness, as it's DC coupled. Anyone who's used 555's long enough knows that if the Trigger pin is held low the 555 will never time out. This Rube Goldberg overcomes that and allows DC coupling to the Trigger pin, so it will respond to a very slow inverse ramp voltage. When the 555 triggers the Out pin goes hi, turns Q3 on, which, in turn, turns Q2 off. This lifts Q1's emitter off Gnd potential and turns it off. This releases the Trigger pin.

As you can see, U1 times out.

 

[mqt]

Thanks for the input CDRIVE and Harald.
I will play around with these circuits and see how it performs.

Cheers

 

[CDRIVE]

Please clarify something for me. The object of this project is to interrupt power to the motor when back pressure becomes excessive and causes corresponding excessive motor current. How long the power is interrupted is determined by the RC time constant of the 555. The time constant on the attached plot was made short @ about 1 second. This was done to show what happens if the pressure is still high when timeout occurs. The circuit re-fires and repeatedly times out (see spikes on plot) until the pressure has fallen below trip current, which is also shown. This will occur with mine or Harold's circuit.

Note: In this circuit I'm simulating an increasing and decreasing motor current ramp. You don't have this luxury. It also includes the relay which my last one didn't. So, here's the question...

Since the motor is tripped (off) what clears the high pressure conditions before timeout occurs and repeatedly re-fires the circuit? Maybe I'm over thinking this. When the circuit trips do you simply release the trigger (switch) and move on to the next grease fitting? Doesn't the cam that you mentioned have to reposition itself at a start point?

 

[mqt]

I believe your understanding is that the grease gun is connected to a grease nipple and the motor is run until there is high pressure inside the equipment being greased, then the motor turns off.

What is actually happening is the grease is being primed to a high pressure inside a chamber. When this chamber pressure is high, the circuit switches the motor off. As a machine gets greased, the pressure inside this chamber will drop causing the motor to kick back on.

does that answer your question? Or have i overlooked something major..