Dynamic Breaking in a PLC program

Ok...back to the ferris wheel...I have the seats moving and a reed switch to stop the seats but they will either drift too far or not far enough...So, I was thinking I could use dynamic braking for the lil' 3 V motor that I have running it.

I know I need a bridge rectifier but no clue after that...any ideas?

What I am trying to say is that during my loading and unload session, how can I have the reed switch activation completely stop the motor without any drift?

You can stop an ac motor by applying dc to it. In practice I suspect a mechanical brake will be a simple solution. Even applying a constant DC current would not completely lock the rotor although it would provide some resistance to movement.
I programmed a conveyor system MANY years ago that used a rectifier to charge capacitors will the motor was running and discharged the through the motor windings when the contactor dropped out. It did provide some braking but nothing really exciting.
Have you considered using an inverter drive??

In a normal application there would be either a Brake Motor or independant brake.
I have not read the other thread, sorry.
the only other way is to stop the wheel earlier and rely on inertia.
In the old days of 'PLUG BREAKING' the motor use to be thrown into reverse.
that may not be an option


EDIT
You did say 3 volt motor - I am asuming it is DC

mendonsy said:

You can stop an ac motor by applying dc to it. In practice I suspect a mechanical brake will be a simple solution. Even applying a constant DC current would not completely lock the rotor although it would provide some resistance to movement.
I programmed a conveyor system MANY years ago that used a rectifier to charge capacitors will the motor was running and discharged the through the motor windings when the contactor dropped out. It did provide some braking but nothing really exciting.
Have you considered using an inverter drive??

Click to expand...

Ummm...dont know what an inverter drive is

iant said:

In a normal application there would be either a Brake Motor or independant brake.
I have not read the other thread, sorry.
the only other way is to stop the wheel earlier and rely on inertia.
In the old days of 'PLUG BREAKING' the motor use to be thrown into reverse.
that may not be an option


EDIT
You did say 3 volt motor - I am asuming it is DC

Click to expand...

Yes...it is DC...the problem with the stoppage of the wheel being stopped early is that the 'drift' is almost always different

Ok - the Inverter mentioned is for an AC drive - so no good for this app.
Making an asumption this is a Model Ferris Wheel.
you will need to fit a small felt pad which can be removed/released with a small electromagnet.
You might be able to fit a permanent felt pad to give you a constant brake as the DC motor should have enough torque to still drive the Ferris Wheel.
try this with finger pressure

What I am trying to say is that during my loading and unload session, how can I have the reed switch activation completely stop the motor without any drift?

Click to expand...

Here is the previous history for those who want to help:

http://www.plctalk.net/qanda/showthread.php?t=77243

If the drift or coast time is always the same, you could use the reed switch trigger to control a timer that stops the motor at the exact time needed to coast to the next reed switch (seat position).

However you said the drift time is always different (bearings not good, wheel not evenly balanced, small DC motor very crude). In that case, you have to have addtional equipment, a brake of some type that can lock in the wheel in zero (almost) time.

Even if you immdeiately reverse the DC motor at the reed-switch position, it will not stop the wheel in 0 time, and could eventually damage the motor.

You are going to need another plan. Think outside the box (or the wheel). As I remember, you have control over only TWO ouputs, Motor Forward and Motor Reverse (no Brake). You were running the wheel 30 seconds forward and 30 seconds reverse, followed by the unload/load cycle. What if in the reverse cycle, you run the wheel full-speed for 25 seconds, but then for the last 5 seconds, you reduced the speed to a crawl by not keeping the motor output on constantly, but instead pulsing the Motor Reverse slowly until it stops exactly on a seat switch? (At that time, when the reed switch is on, you stop the pulse timer). You could experiment by setting up two timers, one to control the reduced-speed time, and the second to control the length of each Motor Reverse pulse.

Here is a way to determine the stopping characteristic time of each of the 4 seat positions (assuming that the wheel is always unbalanced by the same amount for each test). If the stopping RTO recorded times do not average the same for 3 or 4 tests, then you cannot use time to control the wheel to stop at the exact seat positions.

I know I need a bridge rectifier but no clue after that...any ideas?

Click to expand...

The quickest way to stop a DC motor (without a brake) is to give it a reverse-voltage pulse of a time duration that exactly brings the motor from 100% speed to 0 speed. For a real motor that would be destructive, but for your small model motor, it might work okay. In fact, if the motor is traveling forward, and you energize a reverse-direction pulse at any given seat position, the wheel might travel forward to a stop past the seat switch, then reverse until the previous seat switch is again ON.

Still, the reverse pulse will have some non-zero time duration. If it is longer than the average time that your reed seat switches are ON, then it probably will only work (without moving the the wheel in reverse) if you can energize the reverse pulse BEFORE the reed switch goes on. To do that you still need the average time between seats for each of the 4 positions.

Where are your seat position switches located? I hope that they are on the inside circumference of the wheel, as close to the center as possible. That will give you the longest ON time for each switch. If you locate them far from the center of the wheel, then your switch will only be on for a short blip, making it much harder to control the stopping of the wheel on any seat position.

you need a tiny solenoid actuator to stick out and stop the wheel. mount it in the frame, to be fail safe you need 100% duty rated solenoid and hold it energized to retract and allow the wheel to turn, when it slows to position, release the solenoid, the spring returns it to interfere with a spoke on the wheel. For a toy, I would do the reverse, so it's less of a pain in the butt when turned off. Align it with the loading positions.

Perhaps apply a time limited or reduced forward motor current to push against it during loading...I know nothing about your motor...

How big is this Ferris Wheel?

Can we get a nicely resized jpg of it?

That solenoid actuator is a good idea. That will provide some dynamic braking, as opposed to "Dynamic Breaking" as this post is titled! Dynamic Breaking is what may happen if the motor is reversed-pulsed to stop it.

In the previous thread, I proposed using a simple spring-loaded non-electrical flapper brake. If a flapper has the right springiness, and there are the same number of wheel spokes as seats, then the spring brake could be positioned to ride against wheel spokes so that it always slows and stops the wheel at a seat-loading position, after the motor is denergized.

A picture of the wheel would really help to weed out ideas that have no chance of working. Until then, we will keep beating our heads against the wall.

Dynamic braking is really just putting a load across the motor leads. Is there an easy way to either short the motor leads or put a resistor across them(after removing power)?

Is there an easy way to either short the motor leads or put a resistor across them(after removing power)?

Click to expand...

In this case of a 3 volt DC model-sized motor, the easiest, quickest way to "short" the motor is to apply reverse power. That is the equivalent of a Mega-Short+. Still the wheel probably will not stop with the same reed-switch-stop-trigger in the closed position.

What I am trying to say is that during my loading and unload session, how can I have the reed switch activation completely stop the motor without any drift?

Click to expand...

Oldskewl, Think of it this way: You are driving your car 50 MPH when a light turns red. You wait until you get under the light, then push the brake pedal to the floor, kill the engine, and throw the gearshift into reverse. Question: will your car stop under the light? The physical properties of inertia and momentum (mass x velocity) says that you can not stop a moving object (mass > 0) in 0 time. If you can, the Pentagon wants to talk to you about their next generation of tanks and rockets.

To successfully stop a vehicle at a traffic light, you have to anticipate the stop some xx seconds before the stop is needed. Your model ferris wheel has the same stopping characteristice, just a smaller mass and velocity, but not 0 momentum.

On the other hand, maybe your model wheel is a paper model only. In which case you can violate the laws of physics, but that will do you no good in the real world.