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Controller for small proportional solenoid valves

Beat%20Ludin
Prodigy 70 points
Other Parts Discussed in Thread: INA253, DRV110, DRV120, DRV110EVM

I'm not sure this is the best forum for this question so feel free to re-locate my inquiry.

I want to control small proportional solenoid valves (<2W, max drive current <200mA)  as finely and precisely as possible preferably via I2C (or SPI or UART). I would like to be able to add some dither, too.
So I guess I will need a (>12bit) PWM source, a MOSFET, a compensated current measuring component (like INA251?) and (an ADC that creates) a feedback to the PWM source.

I could build this up and program a microcontroller to act as an I2C-controllable PWM source with an internal or external ADC and a current sense amplifier (e.g.) INA253. Is there a simpler, more integrated solution?

TIA,

Beat

  • 0
    TI__Expert 6330 points

    Hi,

    We will review this topic next week. Have a great weekend!

    Cheers,

    Aaron

  • 0
    TI__Expert 4940 points

    Hi,

    Based on your description a solenoid current controller will meet your requirements: DRV110 or DRV120.

    The solenoid current peak, hold and peak duration are set by external resistors, DRV120 has integrated FET with 250mA max current.

    The peak and hold current can be controlled by replacing Rpeak/Rhold with programable DAC's:

      

    Regards,

    Costin

  • 0 in reply to Costin Cazana
    Prodigy 70 points

    Thanks for your reply. This might be a possible solution. However, the controllable range of I(hold) is limited, if I interpret the datasheet (section 7.3) correctly, i.e. it might not be possible to control the whole span of the proportional valve. Or am I missing something?

  • 0 in reply to Beat%20Ludin
    TI__Expert 4940 points

    Hi,

    What the span you need?

    You can change also Ipeak/Ihold with Rsense

    Regards,

    Costin

  • 0 in reply to Costin Cazana
    Prodigy 70 points

    The current at which the valve opens is dependent on the spring which is adjusted by the manufacturer individually and varies noticeably. The get a good signal to noise ratio, I want to have a high duty cycle of the 90-95% at the maximum opening of the valve (incl. some headroom to compensate for the temperature-dependent resistance down to less than 10% to be sure to include the closing point.

    If I want to regulate the valve continuously, this would happen via I(hold), which is limited to 1/6 of I(peak) and I presume the maximum duty cycle is limited accordingly. But by replacing the 'keep' capacitor with a resistor, I can keep V(keep) < 100mV, so I(hold) will be active indefinitely.

    All of this is theoretical, however. I now have a DRV110EVM and a DAC at hand. What I find is that PWM is only regulated continuously at low duty cycles. Above 20%, the duty cycle starts to become very irregular, with interleaved >100% duty cycles This is to be expected according to the manual (7.3.2.). The average current is still regulated but the lower-frequencies generated make the valve hum and rumble at large regions of the DAC settings. Maybe this can be overcome by adjusting R(sense) optimally, but I haven't found that sweet spot so far

    Another big worry is that the built-in reference will not be very accurate/stable. I want to control the valve as precisely as possible, i.e. the same current should always bring it to the same position. 

    Of course, I could replicate the parts of the DRV110 that I need. Using a low temp coeff sense resistor and a stable reference at the upper rail of the DAC as inputs for a hysteresis controller. Would you say that is the best way for an analog PWMed current controller? Or can you see a more elegant way?


    Or - a different thought - would it be possible to use the DAC directly as the reference for the hysteresis controller instead of controlling the internal reference of the DRV110 with it? (I'm thinking about the 4 n.c. pins of the DRV110...)

    Thanks for your help,
    Beat

  • 0 in reply to Beat%20Ludin
    TI__Expert 4940 points

    Hi Beat,

    The main reason for variable duty cycle is the slope of the current. At slow di/dt noise can trigger randomly the comparator. A very slow di/dt will force >100% duty cycle. The solution is to increase di/dt. Since the inductance can not be change, increasing the headroom across the valve with the valve supply voltage will increase di/dt and duty cycle will be stable and <100%. After the duty cycle is stable you can adjust the duty cycle with the supply voltage.

    Please send me a simple schematic of your external reference idea at:

    costin.cazana@ti.com

    I'll close this thread sine we'lll continue thru e-mal

    Regards,

    Costin