Weak motor or a bad component somewhere in the circuit?

[ebmeneophite]

Hello. Been lurking this forum for quite some time and just wanna say it's a great forum.

I have a small problem, maybe someone could point me in the right direction. I'm a newbie at electronics but have a somewhat grasp of fundamentals. (I think  )

I have a Mammography unit that has up/down movement provided by a motor. When I press the UP button the motor pulls the c-arm up, even if I apply some downward pressure. When I press the DOWN button it will move for a quick sec then stop, if I help it by applying downward force then the motor can pull down.

At first I thought it was a bad motor, so I ordered a new one, but I get the same problem with the new motor. I thought it might be the rails where the c-arm travels but I can easily move it up/down when the motor is disengaged.

So I hooked up the motor without mounting it in the machine and when pressing the UP button the motor will turn even if I try holding the rotation with my hands, but when pressing the DOWN button the motor will stop when I apply medium force to stop the rotation. I also hooked up a multi-meter while pressing DOWN and while preventing it to rotate and what happens is that the voltage rises then it goes to zero. It seems like the circuit is sensing that the motor stalling but it's too sensitive.

Unfortunately these machines do not come with circuit schematics only block diagrams, so I'm wondering if someone could explain what component arrangement could be used to sense the motor stall.

Thanks

[Rasz]

step one of every repair is using your most powerful instruments - eyes and brain
post pictures of the boards

[ebmeneophite]

step one of every repair is using your most powerful instruments - eyes and brain
post pictures of the boards

Here's the pics.

There's a CPU board which I guess controls all the signals and there's also a Motor Control board which is where I think the fault is.

[Rasz]

are those relays connected directly to the motor? do they click when you change direction?

[ebmeneophite]

Yes they are clicking.

[Rasz]

doubt its that simple, but maybe arm was blocked at one point and power draw spike cooked relay contacts
check measuring voltage drop directly on the relay while motor is on, or measure voltage in front/after relay

[Gyro]

Worth looking for dry or cooked solder joints on the relays and connectors too.

[bookaboo]

I doubt its relay contacts, they tend to fail suddenly (cooked by thermal runaway) or chatter before fail (work when physically tapped).
Sounds more like the motor control board monitors the current to the motor to detect how much force is being applied, possibly to prevent limbs being crushed or even just the motor being overloaded. If that's the case then when moving the arm UP you should be able to make it cut out with some physical force.

Repair would be tricky and might involve figuring which component monitors the current then getting the datasheet for that and going from there.
Your best hope is that the "down" function is mechanically impeded. If you are competent at the voltage involved you could try swapping the UP/DOWN cables to the motors and observe.

[amyk]

What type of motor is it? DC? If so, most likely it will be driven in an H-bridge arrangement. Check the semiconductors in it. You'll need to trace out the schematic yourself.

[ebmeneophite]

Thanks to everyone in this thread. I found the culprit. It was an uncalibrated compression sensor.

Thanks

[Gyro]

Damn, I was just about to suggest that! 

Glad you found it.