Methods for sensing pressure needed

[originator]

I am using a belt driven actuator to move an object clamped to the belt. The motor used is a servo with a controller and no means of outputting torque being applied. Here are my options so far to sense pressure and output a pulse to trigger a stop, reverse, or whatever else. The sensor need only work in one direction, not both. This is a safety application to stop the movement on a set amount of pressure exerted on the actuator.

1. A spring loaded idling tensioner on the belt with limit switch mounted so that the tensioner will hit the switch with X amount of torque being applied in one direction only.

**I don't have room for this method

2. A laser fixed near the object being moved. A reflector being attached to the mechanism used for the attachment or the belt to the object, so that on X pressure, the reflector will tilt and cause the laser to not direct the beam back to a receiver, causing the receiver to output a pulse.

**A lot of hassle to implement, calibrate etc

3. A physical switch mounted on the mechanism used for mounting the belt to the object. On X pressure, the springloaded mechanism moves towards the switch.

**A two conductor cable is require to travel back and forth mny times following the object. High probabilty of failure after many movements.

4. baterry operated switch, no cables required

**Access is very limited, if at all, once installed, so changing batteries is not a good option. Assuming there is a battery that will retain it's voltage for 20 years+ when not being used, this may be be a last resort. The switch could turn on the circuit and would very rarely be used if at all. If using a baterry operated "remote", what then is the easiest and most reliable method for trasmission: This is my most likely option, assumoing I can place a battery compartment where it can be replaced every year. A 9 Volt style case being the easiest to implement.

1. radio
2. infrared light(Infrared LED aimed at a receiver) which turns on a receiver
3. ?



Note : Using the object for a conductor for part of the siganl is not an option



Thanks for any tips

 

[ScottyUK]

Presumably when the pressure exerted by the actuator increases, there is a corresponding increase in current drawn by the actuator motor? Can you sense motor current and use a threshold detector to determine when the motor current exceeds the trip value? This would appear to meet most of your requirements.

You say this is a safety application: personnel safety or plant & process safety? The two have different requirements from a legislative perspective, although there is frequently common ground between them. The severity of the hazard tends to determine what is required from the safety system: Europe is using the 'Safety Integrity Level' or 'SIL' assessment more and more widely, and this may be a good place to start as it defines what can and can not be used to implement a safety system for a given hazard level. Some hazard levels may preclude use of electronic controllers, require redundancy, or require multiple levels of redundancy using dissimilar principles, for example.

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I don't suffer from insanity. I enjoy it...

 

[originator]

Thanks for the idea for putting in a current sensor on the motor. I don't know mcuh about servo motors, or where you would start to get a current measurement. My assumption is that to do what you suggested would require tapping into the motors power cable, taking a current measument off it and sending that to a comparator? I a short on knowledge for that circuit,b ut I guess it would not be too complicated. The current exceeds a level, and trips a circuit to output a pulse.

The application is a door that will be in residential and business settings. I already have touch sensitive glass for the door for sensing a live object and retracting, infrared beams for sensing motion across a path, but for the rare case where an object got placed in th door path and could not be detected by the photo stuff, then I want to make sure the motor doesn't sit there grinding trying to get past the object. I am incorporating heat sensors(105F) and smoke sensors. These will automatically open the door as an option in case of heat of smoke. There is a battery backup in case of power outage that can be programmed to open the door on power out as well. I am building in every saftey mechanish possible to avoid law suits later on when I realease to the public for sale.

I will do some research on how to build the circuit you described.

 

[ScottyUK]

I think you've started another thread in the motors forum asking a related question, so it's probably best to continue the discussion there rather than fragment ideas by running two concurrent dicsussions in different fora.

As I'm writing a post anyway, the easiest place to sense current might well be in the power supply to the servo motor. If it is a COTS servo amp, they often have a current sensing feature built in which you could make use of. Failing that there are numerous manufacturers of current sensing relays from the likes of Omron, SAIA Burgess, Telemechanique, Siemens, ABB, etc which do all the work of giving you a volt-free contact operated by a current threshold. They usually accept up to about 10A - 15A before an external transducer or CT is required.

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I don't suffer from insanity. I enjoy it...

 

[dbaird]

Would "safety mats" to detect people/objects work?

http://www.sti.com/products/intro3.htm

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.