Question on grounding (SDC2130)

6 years 9 months ago #29528719 by jwatte
I'm considering changing out existing controllers from another vendor, and the SDC2130 seems to be suitable for my application (a little bit overkill, but hey, that's a good thing, right? :-)

The one thing I wonder about is the "DO NOT CONNECT" note that separates logic signals from motor signals. Let me describe my system:


I am building a wheeled rover with four individually steering, individually powered wheels, using brushed DC gearmotors with motor-mounted encoders. The weight of the entire thing is perhaps 15 kilos; the motors are 12V/5A rated 37D gearmotors from Pololu.
This rover will not go over particularly steep terrain, nor will it ever fight any other robot, so I'm not so worried about the overheating or physical abuse aspect of the system.

The rover is powered by a single 10 Ah, 4S (12.8V nominal) LiFePO4 battery pack. This feeds into a M3-ATX power supply mounted in a mini-ITX PC, which is the main controller. Additionally, it feeds into a 5V auxiliary power converter, which powers some microelectronics/sensors. There are also four switching converters (UBECs) which each feed one hobby-style RC servo motor for steering. Finally, it feeds into two dual H-bridge controllers with serial communications and encoder support from "another vendor." (There are two such controllers, for a total of four motors.)

The main communications with sensors is through USB to a small microcontroller board, and from there using I2C or digital/analog I/O. The control of the motor controllers is through TTL-level non-inverted serial, out from a USB-based serial adapter from the computer. Each signal is a pair of wires -- ground, and signal -- which makes for a lot of ground loops; this is pretty much unavoidable given the pieces involved. I am not having signal integrity problems so far, so I think it's under control.

There is a single, common, chassis ground, that is tied to the negative of the battery; this includes the existing motor controllers.


The suggestions in another post in this forum is to "power all logic off the 15-pin D-sub connector," which clearly isn't tenable in this system.
Is it possible to use a pair of SDC2130 in this system, even though motor power and signal ground will be effectively connected? Or does the SDC2130 generate an actual voltage potential between logic ground and battery ground?

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6 years 9 months ago #29528720 by roboteq
Replied by roboteq on topic Question on grounding (SDC2130)
There are two reasons why we advise to avoid brining the ground that is on the 15 pin connector to the main battery ground wire:

1- It can happen that users forget to connect the main ground tab to the battery, or that this connection is loose. In such a situation, the entire current consumed by the motor flows through the ground on the small pin of the Dsub connector, burning the pin or more damage.

2- The ground reference is not as accurate, or it can be noisy. This can be a problem for analog sensors. There can also be other ground loop issues.

It is not always possible to avoid such connection. The controller will work with such a wire in place.

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