Electromechanical replacement for pneumatics 3

[imcjoek]

I have an existing system consists of a 5.5" long arm which pivots on bearings at one end, and loads against a product at the other. The load of the pivoting arm is provided by a pneumatic dashpot (a low friction air cylinder, essentially).
We have applications which require this pivoting arm, but they do not have a compressed air source.

I am looking for an electronically driven alternative to a small (1/2" bore) air cylinder. Some general requirements.
1) Must be easily back-driven / low stiction. So anything with a lead-screw or substantial gear reduction (worms, multi-stage) is out.
2) The arm needs to see ~1Nm (8.85 lbf-in) torque.
3) The arm swing is 90deg. For reference, I'm looking at a 1-2" stroke for a linear solution. (Of course this depends on the force capabilities of the actuator).
4) Adjustable and linear force. Spring generally don't have enough adjustment range and their force increases with increasing displacement.

Thoughts so far:
Simple Brushed DC Motor (Mabuchi or similar), gear driven ~10:1 or so to the shaft. It's simple, inexpensive, and gives me the torque, adjustability, and easy back-drive I'm looking for.
I'm worried about the motor/brushes survival since they are not actually designed to be nearly stationary 24/7. The motor could be oversizde such that a lower current can be used. So long as the motor can dissipate the heat, will it survive? Or is this just terrible practice...?

Solenoid actuator:
They seem to be more designed for engaging/disengaging things. I have not been able to find any that actually function like a pneumatic cylinder.

Motor Driving a clutch:
Avoids the motor eating-itself-alive problem, but the cost of an adjustable clutch (hysteresis, whatever) can be prohibitive.

Any ideas? Thanks.

 

[VEBill]

My immediate reaction is to install a small, inexpensive compressed air source.

 

[imcjoek]

For a variety of reasons some of our customers cannot or will not use compressed air in their equipment.

Or that's what I'm told. It's entirely possible this is one of "those" sales/marketing things.

 

[ScottyUK]

imcjoek,

Sounds like what you are looking for is a servomotor. I'm not sure that you will want servomotor prices, but it is perhaps a starting point for some creative Googling.

 

[imcjoek]

I'm certainly familiar with servomotors and their prices. Couple orders of magnitude above where I need to be.

 

[MikeHalloran]

The electrical device you're seeking is called a torque motor. ... because it exerts torque without necessarily rotating. They usually come without bearings of their own, or a housing, and are built into other devices. They are even available in the form of a sector. ... because they are custom made, and expensive. An oversized servomotor is probably a better choice.

Best, or least worst, choice is a small air compressor and an air cylinder.



Mike Halloran
Pembroke Pines, FL, USA

 

[imcjoek]

Thanks. I've heard of torque motors before, but wasn't aware of what they are used for. Looks like it's back to the old torsion spring compromise.

Out of shear morbid curiosity, what prevents a "solenoid air cylinder" from being made? I'm envisioning something a bit like a coil-gun, with a linear pot for position feedback to an inexpensive uC controlling the individual coils (I guess each coil needs its own mosfet h-bridge, which could get expensive). In effect, a linear brushless DC motor.

Off to the patent office...?

 

[catserveng]

Something like this?

http://www.ultramotion.com/products/

Or maybe an electric governor actuator, only thing might be hard to find 90 degree rotation direct from the actuator by adding a non-linkage may increase effective travel

http://www.governors-america.com/225_series.html

Or a governor linear actuator,

http://www.governors-america.com/alr_series.html

Not sure if these will work but there are other suppliers with similar devices and different ranges. voltages, and control inputs like Woodward, Heinzmann and Bosch.

Hope that helps, Mike L.

 

[ScottyUK]

How about one of the cheap & cheerful actuators used by the HVAC guys to control dampers? For example

http://www.johnsoncontrols.com/cont...ac_systems/hvac/actuator_product_family.html.

There are literally hundreds of manufacturers of these things. I'm not sure they go small enough for what you need, but I haven't really looked in detail.

 

[imcjoek]

Something like this?

http://www.ultramotion.com/products/

Looks like a pretty standard linear actuator. Intended more for "Go to X position" than "Push with X Force". A better analogy of what I'm looking for might be "electronic spring".

How about one of the cheap & cheerful actuators used by the HVAC guys to control dampers? For example

http://www.johnsoncontrols.com/content/us/en/produ....

Interesting idea, but it looks like they're mostly synchronous motors designed to run off 60hz. Not so sure that would work if the actuator is effectively stalled and repeatedly back-driven.

Looks like there isn't an obvious solution to this one that I've missed. I suspect I'll wind up (ahem) with a compromised solution involving a torsion spring with many coils.

Thanks everyone.

 

[ScottyUK]

Those actuators are designed to be permanently energised in a stalled state. Some variants have a spring return so one position requires power and the other requires none, others require power both ways. They are usually seen on dampers in ductwork where there is a torque due to airflow against the damper. Get one off ebay and have a play.

Similar devices are used to actuate zone valves on hydronic heating systems - they often contain a Slo-Syn motor and a little gearbox with spring return; these devices sit in an energised stalled state for long periods of time.

 

[VEBill]

Good suggestion by ScottyUK. My HRV (air handling) system has two of those 'damper motors' and they stay energized for many months straight.

 

[jraef]

Linear thruster perhaps? They are a linear motor, no rotary conversion.

http://ab.rockwellautomation.com/Motion-Control/Actuator/Integrated-Linear-Thrusters#/tab5

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[btrueblood]

Agree with Scotty and VE1BLL. Many of those will operate on 24 vDC as well as 60Hz/50Hz 24 vAC; the better ones rectify any AC input and feed a brushless DC motor. See also Siemens, Belimo, Neptronics, KMC, Invensys. Google (brand name) + "damper actuator"

 

[mikekilroy]

cost point including electronics?

sounds like $ 200 total? 1 pc or many?

i will guess ballpark prices of approaches:

- $500 servo motor + 500 servo drive
- $1000 torque motor + 500 servo drive
- $50 stepper +$100 stepper drive
- $150 cheap linear actuator with say 12vdc motor
- $500 linear motor + $300 driver (

http://nipponpulse.com/products/overview/linear-shaft-servomotors)

- $100? your cheap dc motor/gearbox

We sell/apply all but the last choice. I think our nippon linear motor sounds right but not sure of price you need. I also do not see why you think your cheap dc motor would not survive sitting in one spot pushing for long time - brushes wear out for 2 reasons: 1) friction of going around and around & 2) sparking - from going around and around at high current or voltage. Your app would do neither so therefore your little cheap motor should last nearly forever based on the data given so far.....

 

[imcjoek]

cost point including electronics?

It would be replacing a dashpot & regulator that are $50 and $25 respectively. Quantities in the ~100/year area.

I also do not see why you think your cheap dc motor would not survive sitting in one spot pushing for long time

Just concerned with overheating it. Although I guess that can be avoided with the right power supply method.

I am going to investigate the HVAC damper thing some more.