Driving PMDC motor with default high - Page 1
Forum section: electronics / projects

author: robotix3
author_rank: Regular Contributor
timestamp: September 14, 2016, 03:35:41 pm
content: A co-worker and I have noticed that on a custom board using the DRV8837 motor driver that if you default to holding both motor terminals HIGH and then PWM one terminal low you will get the motor to move at much lesser potentials, thus increasing the range (thus speed precision), rather than defaulting LOW and then PWMing HIGH on the other terminal.Is this because there is less inductance on the path to ground than the path to V+? Is this bad practice?

author: H.O
author_rank: Frequent Contributor
timestamp: September 14, 2016, 04:00:42 pm
content: Do you mean the control IN-puts? I don't see a way to drive both motor terminals high.The difference is that when you do it one way the motor is basically disconnected during the "off phase" of the PWM, when both IN-puts are low the outputs are high imedance. When you do it the other way the motor winding is shorted during the "off phase" of the PWM, when both IN-puts are high the outputs are both driven low shorting the motor winding.

author: robotix3
author_rank: Regular Contributor
timestamp: September 14, 2016, 04:36:39 pm
content: Quoteyes I mean the INs, of course the motor driver basically just amplifies the input on the output. You drive them both high by giving both sides 100% PWM (DC @ VCC). The motor does not move when you do this because the potential difference is 0V.Basically I visualize the output in the default LOW scenario to be a sawtooth (due to a delayed rise time), and the output in the HIGH scenario to be a more squared waveform.

author: tatus1969
author_rank: Super Contributor
timestamp: September 14, 2016, 06:22:10 pm
content: your observation is correct. This table should explain what happens1) PWM on both channelsPhase1 Phase2 Result---------------------------------------0% PWM 100% PWM full forward50% PWM 50% PWM stop100% PWM 0% PWM full reverse2) PWM on one side onlyPhase1 Phase2 Result---------------------------------------GND 100% PWM full forwardGND 0% PWM stopVCC 100% PWM stopVCC 0% PWM full reverseYou see that in the first scheme, you need 0%<-->50% for the full speed range, and in the second you need 0<--->100%. This effectively halves the gain.Both schemes have their advantages:1) PWM on both channels+ you can fully control the motor in both directions with only one control signal2) PWM on one side only+ less radio interference+ higher efficiencyIf you do not need the capability to reverse motor direction, then always go for (2), and tie one motor lead directly to GND. This further improves efficiency (one transistor less in the path) and you only need 1/2 motor driver. If you need both directions, you can still go for (2) but you need two control signals then: PWM, and direction.

author: tatus1969
author_rank: Super Contributor
timestamp: September 14, 2016, 08:06:42 pm
content: just reread your initial post and realized that I give the right answer to the wrong question. What you describe should not be the case. You should observe exactly the same behavior from the driver if you invert both PWMs. BUT your motor will turn in the opposite direction. And this does actually matter for most brushed DC motors. They are mostly optimized for one rotation direction, this is done by rotating the commutator block slightly against the stator. Some are even adjustable. This phase adjustment gives better max RPM.