Controlling a Brushed Motor with DC Voltage to Avoid EMI (no speed control needed)

Question

I have a fairly simple project coming up. My task is to design a system which powers and controls a Pan and Tilt positioner for a camera. It has some weird and unavoidable requirements/constraints:

• No telemetry allowed in the Pan and Tilt positioner. Power must go straight from a control box, through a wire, directly to motors.
• PWM or AC should be avoided. The Pan and Tilt conductors will have to be in the same Minikevlar cable assembly as some low-voltage analog signals, and PWM would likely be too noisy.
• As the title suggests, speed control is not a requirement.

The Pan and Tilt will likely contain 24V brushed motors for both axes. My initial approach would be to use a switch joystick to open/close a circuit for each motor in forward and reverse, and supply it via a 24V+ DC power supply (I say 24V+ because I will be accounting for Vdrop). I would also need a current limiting device of some sort to protect the motor when it reaches end-stops.

My second idea would be to use PWM anyway, and if it causes interference in the sensitive low voltage signals in the cable, I could keep changing the PWM frequency until I find something that works (while driving the motor properly, of course). However, if I keep the PWM duty cycle at 100%, I feel like it defeats the purpose.

What are your thoughts, and how would you approach this? I'm assuming my solutions are missing something important because my experience with electronics is not quite as extensive as I want it to be.

Answer 1

[edit: assume electric-field coupling between the 24 volt PWM wires, onto the low_level analog. Assume a HIGH_PASS FILTER: a series Cap into a shunting resistance (the R_sensor, or R_source of the analog signal) to Ground.

Edit:We compute the edge_speed of the PWM, to reduce the 24 volts down to 0.001 volts, using the High_Pass Filter to do attenuation.]

Let us model the trash injection from fast-edge PWM signals, into the sensitive low_level analog.

Assume 10pF per foot between two wires (coax is about 100pF/meter).

At 150 feet, there is 1,500pF of electric_field energy storage.

Assume the low_level analog has 100 ohm resistance.

What slewrate (rise and fall time is permissible, in 24 volt PWM lines?)

For 1 millivolt induced noise into the analog, with 100 ohms, the limit is I = V/R = 0.001/100 = 10 microAmps.

Now we just compute the edge_speed (you might use L+C filter to slow down the edge from a motor driver, to have a SLOW edge out on the cable).

dV/dT = I/C (which comes from Q = C * V, and differentiate to get I = C * dV/dT

• dV/dT = 10uA/1.5nanoFarad = 1e-5/1.5e-9 = 0.7e-4 or 7,000 volts per second.

And the actual rise or fall time needs the 24 volts in the math

• Trise or Tfall = 24 volts / 7,000 volts/second = 24/7,000 seconds

And Trise or Tfall, to induce only 1 millivolt on 100 OHM_resistive line

• Trise, Tfall = 0.01 * 24/70 or about 3 milliSeconds

To have a PWM produce pulses that slow will be VERY INEFFICIENT (causing lots of heat).

But you can impose an L+C filter (series L, then shunting C, then the cable to the motor). You will need a Damping resistor in parallel with the inductor.

{edit: the suggested solution is

• efficient PWM driver with perhaps 100 Hertz pulses of variable duty cycle

• a L+C filter, to reduce the edge rate to 3 milliSeconds, located between the Driver and the 150 foot wiring

• the unavoidable large capacitance between the PWM wires and the low_level analog wires

• the sensors producing the low_level analog signals, with *ASSUMED output resistance of 100 ohms or less