Part Number: LMR33630
Hi,
I need to control a gear pump (DC motor) with a flowmeter: reading the flow and change the voltage supply of the pump with a PID controller to keep a stable flow.
I'm using the LMR33630 to control the motor power supply and I digitally changed the buck output voltage with a DAC (range from 0 to 3,3V), in order to maintain the desired flow, like the circuit in the figure.
I need to control two same gear pump: one needs to have an high flow (500 - 1400 mL/min) and other one a very low flow (from 50 to 100 mL/min). Experimentally I found that to reach such low flow, the pump supply voltage need to be in the range 1/1,3 V (0,3A output current).
Between the DAC and the buck FB pin, I put a buffer to decoupling the stage and to supply the current. At the output of the buffer I use a diode the supply current only in one direction, in such a way that the output voltage can only decrease. I sized the resistors to reach the desired output voltage range: for DAC voltage from 0 to 1 the diode is open and the voltage is always 15V, for 1V DAC the buck voltage is at the maximum value 15V, for 3,3V DAC the Vout is nearly 1 V. R4 is used to have a fixed voltage (15V) even when the DAC is not active.
All the system works correctly with the high flow pump but I have problem to control the other low flow pump. When I reach a very low output voltage the DC-DC seems to turn off (voltage decrease to 0V), so I was wondering if there could be issues regarding output current or PID control loop.
I was thinking that maybe little oscillations in the PID controller, reaching the lowest output voltage set point, could switch off the DC-DC. For example, due the resistances tolerances, Vout could reach a value lower than the minimum allowable (1V), switching off the pump and the PID is no more able to control it.
Thank you