[SOLVED] Agile actuator challenge

[mename]

Dear all,

I have faced a challenging problem that may be interesting for those who are in agile/precise control.

Realizing an actuation system for the following application, I appreciate any help/feedback:

• A 0.5 kg load needs to be moved up and down between positions A and B. The (vertical) distance between A and B is 1 meter. The trajectory followed for the up and down motion is free.
• The load needs to be standing still in point A for 0.2 seconds, has 0.2 seconds to move up to point B, should be standing still in point B for 0.2 seconds and has 0.2 seconds to move down again to point A.
• Specific requirements: Energy efficiency is of utmost importance and the system has to be powered via a standard net switch (230V, 16A, 50-60Hz, single phase). This motion has to be continuously repeated.

My initial approach is:

1- Regardless of the motor type, I should setup a continuously running system (no start and stop several times a second!).
2- The actuator should move the load preferably on a circular path A-->B-->A-->B... in 0.2 sec and the load should be detached from the mover at point B by a detacher (electromagnetic detach/attach mechanism?) while the mover continues to run ensuring the constant running of the motor. 0.2 sec later, the attacher (electromagnetic detach/attach mechanism?) attaches the load to the mover while the mover takes the load to point A back again, and so on.
3- The rotational moving mechanism will render unnecessary the motor to change its rotation 2-3 times a second as it does if you go with a linear actuation style (back and forth).
4- The detachment and attachment mechanism, if it works, will render unnecessary the motor to stop for 0.2 sec while the load is at point A or B. An electronically controlled timer will energize the attach/detach mechanisms at A and B.
5- For the sake of energy efficiency, a single-phase ac motor may be chosen since it is required to power the system via single-phase 230V 16A switch.
6- Or, should I design an actuation system where I control the start-stop cycle of the motor for every 0.2 sec? If so, what type of control can I employ? A PID feedback control with position/velocity sensors? I suspect any ac motor, regardless of number of phase or use of VSD, will sustain such an agile movement for long time.

What do you think?

Regards,
Maxime

 

[barry]

It sounds a LOT more complicated to add a second mechanical device (your 'detacher') than to simply have your controller turn the motor/actuator off. Further, turning the motor off is a lot more efficient than turning a detached on.

I don't think you need a PID control. Since your system is pretty well defined I think you could get away with just having a fixed trajectory profile. You didn't state any requirements for overshoot, etc.

 

[mename]

Thanks Barry,

Using an AC motor, how can I manage so quick start-stops-wait cycles, 0.2 sec each? Can the motor withstand? I want precise control with an error margin of 1 mm.

 

[barry]

Well, that 1mm requirement makes a big difference. I'm not sure how you can get that with just an ac motor. Can you use a servo motor or step motor? Regardless, youre going to have to have some kind of profile; you can't just run at top speed and then suddenly stop or disconnect something. The AVERAGE speed of your mass is 5 m/sec.

 

[mename]

Can you use a servo motor or step motor?

Yes, I can go with a servo motor. It is my understanding that a servo motor is a just modified ac/dc motor for precise control purposes.

But one of the main question stays there: Is it possible even for a servo motor to accelerate from zero speed to high speed and then decelerate and finally stop, all in 0.2 sec? And keep on doing this with the load?

 

[BradtheRad]

To move the mass that rapidly, will require a large robust servo. Its own motion needs to be amplified with a lever or pantograph or gearing. You may or may not find one which can stand up to continually duty.

 

[barry]

Not being an expert doesn't stop me from offering an opinion, so: I think, as you suggested, you need some kind of closed-loop system. In other words, some kind of feedback. A step motor might be an easier solution, since you can actually run it without feedback, but that 1mm accuracy is tough.

 

[KlausST]

Hi,

if you want linear acceleration and linear deceleration to move the object form A to B within 0.1s then you need 157m/s^2
This is 16 G.
62 J of energy.
1.3kW peak power

Only for the object. But for sure you need some mechancal construction where the object is mounted. It has additional inertia.

Calculate with 5 times of may calculated energy and power.

Klaus

 

[mename]

To move the mass that rapidly, will require a large robust servo. Its own motion needs to be amplified with a lever or pantograph or gearing. You may or may not find one which can stand up to continually duty.

Indeed. High frequency motion will eventually cause the gearing/screw etc. to fail structurally. To avoid that I wonder if a direct drive ac motor (where the load is directly attached to the motor ) would be wiser option.

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Hi,

if you want linear acceleration and linear deceleration to move the object form A to B ...

Klaus

Klaus, the load will go from A to B in 0.2 sec and this high speed will cause stress related problem sooner or later. That is why, I think it is better to setup a rotational actuator where the load A will follow the circular path to B, similarly from B to A again. No need to change the turning direction. And if a direct drive connection is feasible, no need to use any transmission gear/pulley.

Then I am left with the question of whether I can reach 300 rpm (this is roughly equivalent to 10 m/s vertical speed ) speed with the direct drive motor. The max allowable power is 3680 Watts (230V 16A switch).

 

[KlausST]

Hi,

with the rotational approach you need at least 2.47 times the energy and power than with linear movement.

Klaus

 

[FvM]

BLDC or induction motor as direct drive is the best you can do for the problem. A vector inverter is probably necessary for precise positioning and high torque. You need to calculate the maximum torque and respective motor peak current.

Dynamic power can be pulled and restored from/to an oversized DC bus capacitor, so 230V/16A should be feasible.

Consider that the huge imbalance requires a massive foundation for the machine.

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Classical machine engineering would have solved the problem with continuously rotating flywheel, crank arm and conrod, I think.