How to drive a brushed DC motor with a DC signal, or filtered PWM signal?

Question

I need to drive a brushed DC motor (inside a linear actuator) with an electronics circuit. The cable feeding the actuator has, in addition to supply_A and supply_B wires, two wires for feedback signals (hall effect), and supply and ground for Hall-effect sensors.

I have so far used an off-the-shelf DC motor driver from DFRobot (DFR0601), which takes a logic PWM signal as input and spits out a high-power PWM signal which is fed directly to the motor.

This drives the motor fine, but the square-wave PWM signal creates severe crosstalk with the Hall feedback signals and completely messes the position monitoring, which I need to rely on (I am using two actuators and they need to operate without one getting ahead of the other).

I therefore need to address the crosstalk issue. As a temporary solution, I am adding an RLC filter between the driver module and the motor(s). This means filtering a high-power signal, which is not ideal. Here is a screenshot of the filter circuit.

This is only a temporary solution, as it is too bulky and too expensive for my actual application. Simulation yielded currents as high as 7 A through L and C. Parts I ended selecting are:

• 0.47 Ω, 5 W resistor
• 6.8 μH, 10 A shielded inductor
• 100 μF, 25 V ceramic capacitor

I will receive the parts in about a week, so I have not tested it yet, but as mentioned, this is only a short term patch.

I am looking at a more space efficient and power efficient way to address the crosstalk issue. Options I can think of are:

1. Find a more efficient way to filter the PWM signal at the output of the DFR0601 module
2. Find another driver module which outputs a non-PWM signal (must fit within 50mm × 50mm x 12.5mm)
3. Ditch the DFR0601 and design my own driver, which would output a non-PWM signal (available real estate is approx 50 mm x 50 mm x 22 mm)
4. ??

I searched for alternate modules, but most are bigger than the real estate I have available.

Separating the motor power wires from the low-power Hall signals could have been an option, but it introduces other complications which I would prefer avoiding. Besides, the motor power signals do run through my main PCB, close to where the Hall signals enter my microcontroller, so there would still be a risk of crosstalk there even if I separated the wires.

Can anyone advise on ways to address this issue? If I filter the "logic" PWM signal, what could be used to drive the motor(s)? Would it be advisable to use op-amps?

Other information:
Main supply: 28 VDC

Actuator specs:
Max voltage: 24 VDC
Motor resistance in stall mode: 3.3 Ω
Motor resistance under typical load/speed: 15 - 30 Ω
Cable run is approx. 12 feet Hall pulse rate is in the vicinity of 110 Hz The Hall supply line is fed from the 3.3v out of the RPi Pico

Microcontroller: RPi Pico
Max PWM freq: 62.5 MHz (currently set to 60 kHz due to DFR0601 limitation)
Plenty of pins available on the Pico if necessary (GPIOs, PWM, UART, SPI, I2C)
Need to drive two channels, both in forward and reverse modes.
Motor speed accuracy is not important, as long as both actuator units operate in sync (think of sit-stand desk as the application).
Actuators are operated for 5 to 10 seconds continuously, then are not operated for a period which can span 1 to several minutes.

Here is a screenshot of the current PCB layout (region of interest only). You can see MOT_1_A and MOT_1_B being the high-power PWM signals going to the DC motor, and HALL_1_A and HALL_1_B being the Hall signals going to RPI Pico inputs.

Here is a snapshot of signal measured at one of the signal GPIOs (RPi Pico), with the hall signal being disconnected at the motor end, so whatever appears on the scope is only high-power PWM interference being picked up through the cable run and possibly to some extent by the PCB traces.

Answer 1

Can you put a hall sensor amplifier near the sensor itself? I presume so, since you say that there are independent sensor ground and power lines available at the sensor end.

My recommendation is to reduce the impedance to ground at both ends of the hall output data line, and buffer the signal (with an an emitter follower, for instance), to drive sufficient current along it:

^{simulate this circuit – Schematic created using CircuitLab}

It's much harder for capacitively coupled noise to influence such a low impedance line. Magnetically induced currents (due to, say, a motor right next door) will also have a hard time competing with the 20mA or so that must flow around that data/ground loop, to register a "high" at the receiver end.

Don't forget the supply decoupling capacitor at the sensor end; sharp signal transitions will cause serious dips in power supply voltage there, due to those long, inductive power lines.

I'm not sure if you need R3. If the hall sensor has a push-pull output then you can omit R3. If it's an open collector output type, then you'll need R3.

The scope trace shows noise with strong components at 50kHz and higher, but not much below that. I imagine your hall sensor output is below a kilohertz, so you could filter away a lot of that noise quite effectively, with a simple RC filter at the receiver end:

^{simulate this circuit}

R4 and C2 form a low pass filter with cut-off at around 10kHz.