Limiting stepper motor torque - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: January 03, 2013, 01:12:56 pm
content: I'm converting a Proxxon MF70 milling machine to CNC using a generic kit from 'Act Motor'. This lead screw nut on each axis is made from Delrin which is a nylon like engineering plastic. I'm worried that the stepper motors I'm using could strip the threads if I hit the end stops while I'm setting up and debugging the end-of travel limit switches. I've already set the current limit switch on the motor driver to the lowest setting. I haven't been able to find a suitable mechanical torque limiter.The specs are:Motors: 270oz (inch) (23HS8430).Stepper driver: DM54236V Power supplyIs there some kind of circuit that can detect an abnormal current/voltage profile that would indicate hitting an end stop and trip before the damage is done?I know nothing about how stepper motor drivers work or what feedback monitoring algorithms are used inside (if any).Mike

author: Kremmen
author_rank: Super Contributor
timestamp: January 03, 2013, 01:36:07 pm
content: Normally the switches are the only means. I suggest debugging them first with the motors unconnected so that you can verify that activation the end limits will cause the motors to stop. Only then connect them to the actual machine joints. Also, you maybe don't want to position the switches at the extreme last point of travel, but slightly ahead so that the motor has time to stop before hitting the physical end of travel. That is how it is usually done.

author: Gall
author_rank: Frequent Contributor
timestamp: January 03, 2013, 01:47:19 pm
content: Most stepper motor drivers use current feedback only. The motor is basically CC driven using PWM. Some really good drivers may use lower currents for hold and higher currents for higher speeds. The cheap TB6560 one doesn't.If you want to protect against tool collision, not only against the end limit hitting, then you could monitor the total consumption current of the motor. Or just add direct position feedback by adding a position gauge. Regardless of the methos there will be still danger of collision at higher speed since the rotational inertia of the motor is quite high.Just a hint: the GeckoDrive developer used to hang around on cnczone.com

author: Kremmen
author_rank: Super Contributor
timestamp: January 03, 2013, 04:19:38 pm
content: I am afraid the whole idea of current limiting is basically flawed. If you need to limit the current to avoid breaking your machine, by all means select smaller motors. A tool collision usually means the workpiece and generally the tool as well are goners, no limiting scheme is going to help. As to the end travel switches, a good control system has both soft and hard limits. if no soft limits are possible, then you will want to provide a suficient margin for an emergency stop before damage is done.Just limiting the torque is unproductive - why have the torque in the first place if you then turn it down?

author: JuKu
author_rank: Frequent Contributor
timestamp: January 03, 2013, 04:39:06 pm
content: Quote from: e100 on January 03, 2013, 01:12:56 pmCan't you set the screw that connects to the stepper motor axis looser?

author: Mikey
author_rank: Regular Contributor
timestamp: January 04, 2013, 05:39:10 pm
content: I have only been messing around with lower torque stepper motors that was not strong enough to actually destroy something. But what I did was to put the endstop switches in series on the enable wire to the stepper driver, so if something went wrong it would hit the switch, and the driver would lose the enable signal so it would shut down. Not the most advanced approach, but I find it a easy way to add a cheap and simple safety redundancy

author: Thor-Arne
author_rank: Unknown
timestamp: January 04, 2013, 09:29:05 pm
content: On commercial wood milling CNC machines the end-stop switches is connected to the emergency stop circuit and it cuts all the power to the driver circuit, only the PLS is left powered. The stepper/servo motors has a brake that needs power to disengage, so this stops almost immediately.You could test the end-stop switches (in a safe range) and drive steppers at max speed in the direction of the stop switch. Have at least as much weight as the axis is going to move so you can see how far the axis travels after end-stop.When it comes to tool and workpiece crashes... that's often considered a operator error.

author: Kremmen
author_rank: Super Contributor
timestamp: January 05, 2013, 10:07:09 am
content: While eevblog forum is a real good place for electronics, i still recomment that the OP should seek advice also from forums oriented more to cnc and machine tools. Forums such as cnczonehttp://www.cnczone.com/forums/and Practical Machinisthttp://www.practicalmachinist.com/come to mind. The members of those forums have real life experience on these things (not saying that eevbloggers don't) and have probably accumulated more experience between them than folks here.Having said that, i still stand by my earlier recommendation because i know for a fact that is how it is done. Why i know it, is i used to get paid for designing machine control systems not unlike the one now under discussion, though for far bigger machines.

author: GeoffS
author_rank: Unknown
timestamp: January 05, 2013, 11:11:53 am
content: The Gecko drives allow you to set the motor current hence reducing motor torque. As has already been pointed out, you don't want to do this, just select motors with the required torque to start with.A proper E-Stop that cuts all power to the CNC device is a must. This is usually separate from the limit/home switches which, in my case, only stop the stepper driver i.e movement.You will have a crash which will almost certainly destroy your tool and ruin your work piece. The best you can do is to have an effective way of quickly stopping things when it happens. Reducing torque is not the way to do this.