Hi,
I was just wondering what are the ways to detect foreign objects in the moving mechanism like human hand or any other object?
I need to stop the stepper once such object is detected.
Thank you.
That would depend extremely strongly on the mechanism, which you forgot to describe.
jremington:
That would depend extremely strongly on the mechanism, which you forgot to describe.
Hi Jremington,
Thank you for your reply.
The mechanism is a linear belt driven system similar to elevator. Unlike the elevator I have the belt attached to the load - driven by pulley at the top and the stepper at the bottom.
It moves in Z axis.
I need to stop the system if it hits or comes in contact with an object as it moves up or down.
The load consists of metal and plastic and painted(so I cannot use capacitive touch sensing).
I looked at the force sensors however, the load is really an odd shape which I cannot cover the whole surface of the load with a force sensor. Maybe I can get about 60 percent coverage with a force sensor.
I need a pretty sensitive sensing, if the load is forced even just a little bit than usual it needs to stop immediately.
Can you give us any hints about the size of this mechanism, the amount of power and size of typical load involved?
Post a photo.
jremington:
Can you give us any hints about the size of this mechanism, the amount of power and size of typical load involved?Post a photo.
The travel length of the load is around 2 meters. The load size is around 30cm.
About 40 kgs but I have counterweight attached to the load as well.
Not a lot of power, I have nema 23 sized stepper.
We await the photo or a diagram of the machine.
...R
Thank you for your replies.
Here is the load that has rails and my linear motion attachment at the back of the load. The travel is around 1.5 to 2meters in z axis.
In the attachments, you will find a human under the load risk of head injury if the system is energised.
And you will also find a hand getting caught between the load/linear arrangement.
Hope this is helpful.
Images from Reply #6 See Image Guide
And please reduce the size of images before posting them. 640x480 is probably sufficient.
...R
It's taking too long to download the images. I will try to look at them tomorrow morning.
...R
Our Occupational Health & Safety people would require us to built a cage around the machine.
Sorry, will reduce them next time.
Robin2:
It's taking too long to download the images. I will try to look at them tomorrow morning....R
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I cannot build a case for it sorry. Wish it was that easy 
jremington:
Our Occupational Health & Safety people would require us to built a cage around the machine.
Now that I can see the pictures I can say that I had no idea it was like that from your Original Post.
(The last picture suggests there is a girl in the cabinet )
I agree with @jremington. The real risk is not that the motor won't stop, but that the counterbalance will fail and the whole weight of the cabinet will come crashing down (girl inside it, won't like that either )
You could put some sort of U shaped hinged bar under the long edge of the cabinet connected to a micro-switch. Think of the push-bars on emergency exit doors.
...R
Robin2:
Now that I can see the pictures I can say that I had no idea it was like that from your Original Post.(The last picture suggests there is a girl in the cabinet
I agree with @jremington. The real risk is not that the motor won't stop, but that the counterbalance will fail and the whole weight of the cabinet will come crashing down (girl inside it, won't like that either
You could put some sort of U shaped hinged bar under the long edge of the cabinet connected to a micro-switch. Think of the push-bars on emergency exit doors.
...R
Not a bad solution but I am looking to avoid micro switches if possible.
All i need is, while that load is going up or down, if it is forced , stop the motor.
And make it so sensitive, if something is forcing it, just stop the motor
gkhn:
All i need is, while that load is going up or down, if it is forced , stop the motor.
And make it so sensitive, if something is forcing it, just stop the motor
Let us know when you figure it out.
What about putting a load cell where it measures the weight of the cabinet. If there is an object impeding movement the weight will appear reduced. But personally I would much prefer the micro-switches.
...R
Standard safety precaution is to prevent access to the area around the machine with a barrier.
You haven't indicated how fast it moves.
A low power servomotor will stall if the physical load gets too large, steppers tend to push
harder when stationary than when moving. Servo motor drivers can implement load
detection through current sensing and trip-out on over-torque. Such an approach can be
inherently safer as the maximum force the mechanism can exert is set by the motor size,
not determined by software - its is fail-safe. Assuming the normally operating forces are
low enough that the machine is not going to crush people. 40kg with a 40kg counterweight
might only need 100N force or so.