# Linear Actuator Control

• posted

Hi,

I am trying to control the following motor using PWM

I am trying to see the power requirements for this linear actuator. Its 12V and 12A motor at full load. How much Amp Hour lead acid battery is require d to move it like 20 degrees after every hour? The motor will not be runnin g constantly. It will increment like 20 degrees after every hour.

I just want to know how can I calculate the Amp Hour size of the battery.

Plus I am also unable to find out the frequency or time period of the PWM s ignal needed to control this motor. The data sheet does not say anything. I t only says duty cycle 20%.

I apologize if I am asking a stupid question.

jess

• posted

20 degrees? Is that on the data sheet somewhere? If not, you need to describe the mechanical arrangement connected to the actuator.

Gotta work on the basics first. As in my first question above.

Of course not. The PWM frequency and duty cycle is something you determine with your knowledge and math. The duty cycle to which the data sheet is referring is how often you can use the device at full load. In other words, you may apply full power at full load only 20% of the time.

Okay. But, have you been subjected to some training?

• posted

On Monday, November 16, 2015 at 10:02:22 AM UTC-5, snipped-for-privacy@gmail.com wrote :

2V and 12A motor at full load. How much Amp Hour lead acid battery is requi red to move it like 20 degrees after every hour? The motor will not be runn ing constantly. It will increment like 20 degrees after every hour.

signal needed to control this motor. The data sheet does not say anything. It only says duty cycle 20%.

It will depend on what the load is. I guess I'd just measure how much cur rent it takes.

George H.

• posted

Let's say load is 500 pounds.

How can I calculate mathematically PWM frequency? I controlled a servo motor a while ago and its data sheet says that operating frequency is 50Hz. Then I program the micro to do PWM around 50Hz.

jess

• posted

500 lb's is above the rated load! It looks like a DC motor, (not a stepper). I think a call to the manufacturer is is order.

George H.

• posted

Can you at least provide a clue as to what you want to control with PWM? Speed, Voltage, Current, WHAT?

• posted

I want to control current with PWM. Let's say that the load is 200pounds and motor moves 20 degrees every hour and PWM frequency is 5KHz then the motor requires 12A of current for the period of 200usec. Lets say if the motor moves 12 hours a day. Then 12 x 200usec = 2.4 msec a day.

So, if I use 12V 2AH lead acid battery then it will last for a very long time. Am I right?

jess

• posted

and motor moves 20 degrees every hour and PWM frequency is 5KHz then the m otor requires 12A of current for the period of 200usec. Lets say if the mot or moves 12 hours a day.

time. Am I right?

Wrong, as others pointed out. 20 degree is meaningless. To make it simple , let say you want to swing 180 degree per day, so 10 times of 18 degrees p er day. You have to first make your mechanism to swing 18 degrees for 6 in ches of movements, for a maximum of 60 inches. For 200 pounds, it would mo ve around 1.5 inches per second at 12A. So, you need to drive it for 4 sec onds per hour or 40 seconds per day.

Unless you have a deep cycle battery, don't drain it too much either.

• posted

The duty cycle on the PDF indicates run time. In otherwords, you can't run it any longer than 20% of the time.

It does not state how the time frame is measured, so I'll guess as to say the time it takes to do a full stroke you should wait 4 times that to allow it to rest before the next movement? That also could be fully loaded. As for current draw, assume maximum, add up the time in each hour of use * the current that you used. The amp hour rating of the battery will tell you how many hours of use you will get.

Depending on the battery technology the amp hour rating does not always mean it will kill the battery, because that's not good for most of them. It also does not always mean you can slam it for that rating constantly for an hour. There are other properties for batteries that need to be looked at for hammering them. The last time I looked into this, it was 10 Min use for lead acid at the full rated amps, then you let it rest for a cool down. But that was years ago.

Jamie

• posted

Degrees is a measure of angular displacement. That's a linear actuator. There's something missing in your question.

Even if it were a rotary actuator, or if we knew the stroke to rotation relationship, you still aren't giving enough information.

Moreover, it appears that there are at least two versions of the actuator, a 100lb version and a 400lb version. I gather that the difference is in the gearing, or perhaps the screw pitch.

They aren't explicit that the 12A rating is rated current, but their "speed vs. load" charts certainly indicate that fact -- this means that limiting current is up to you.

And just to make your life particularly jolly, there's no indication that the thing will stop dead with no motor drive. There's a good chance that it will, because it's a jack-screw design, but there's a small chance that it won't -- finding these things out late in the game can be kind of disappointing.

There's PWM and PWM. And both of those are different from the 20% duty cycle that they cite.

The first sort of PWM is "fast" PWM, where you drive the motor with an H- bridge at a high enough speed that the motor's inductance becomes part of a class D amplifier. With the right circuit you can use this to control the average current (and hence torque) to the motor.

The second sort of PWM is slow enough to bring the motor to a substantial fraction of it's rated current (or all the way) for long enough to guarantee that it'll actually start turning against friction. This is used to attain fine control in the presence of friction, because just a PID controller generally won't do when friction is high.

The 20% duty cycle that they cite is basically saying that the motor as a whole can only dissipate about 1/5th of the power generated at the rated

12A current. Again there's missing information -- they're not telling you the thermal time constant of the motor. At a guess I'd say the thermal time constant is between one and five minutes, so that lets you give it 10 to 60 second bursts at full power, with waits in between.

Naive, perhaps, but not really stupid.

There's information missing, on several fronts. You need to know how much force you're going to exert with the thing, you need to know if you want continuous motion or if you do, indeed, want to just move a bit once an hour, and you need to know if the thing is going to turn under load with the motor disabled.

If you know these things, then you use them along with the current vs. load chart on the data sheet to estimate the current used by the motor. Then you use that current plus what you know about your power electronics to estimate the current draw on the battery. Then, with a time vs. current estimate in hand, you average the current over time (one cycle if you're so lucky), then multiply that average by how long you want the whole thing to run between charges, then derate that (because you don't want to suck a rechargeable battery dry on a regular basis) and then you have a number.

Moreover, it depends on the battery you use -- you don't want a car battery here, because they aren't designed for long slow discharges. You want to make sure to get a deep discharge battery ("golf cart" or "RV" battery).

20 degrees/hour plus a linear actuator sounds suspiciously like 15 degrees/hour with a linear actuator, which sounds like you're tracking the sun or other astronomical object. In that case, and if you're pushing a mirror, antenna, or other device around during the day and bringing it back to "sunrise" position over night, you want to include the total energy used for the return stroke as part of your whole energy/ cycle calculation.
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Tim Wescott
Wescott Design Services ```
• posted

Whatever battery you use, it will need to provide at least 12A when the load is turned on. A 2AH battery may not be able to provide 12A, even for 200usec, so you need to check that.

Aside from that, you will not get 2AH from a 2AH battery if you draw 12A, assuming the battery can provide that. Battery capacity decreases as the rate of discharge increases. For example, a

12AH battery will not last for a full hour at 12A current. Google "Peukert's law" to read about that.

Next, a 200usec (I did not check your math) pulse will not result in 200usec motor motion. The motor will not move instantaneously when a pulse is applied. You need to check with the manufacturer to find out what the effect of your PWM frequency & duty cycle will be.

You need to consider the mechanical operation you want to achieve. The output of a linear actuator is linear distance, not degrees, so "20 degrees" is meaningless. Are you looking for continuous motion over a 12 hour period, or can the motion be intermittent? (For example one inch every hour during the first minute of the hour followed by 59 minutes of no motion.) And along with all of that, you need to consider if you'll get the precision you need.

Ed

• posted

All the power in the worls won't help. Linear actuators don't rotate, they extend.

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• posted

12V and 12A motor at full load. How much Amp Hour lead acid battery is req uired to move it like 20 degrees after every hour? The motor will not be ru nning constantly. It will increment like 20 degrees after every hour.

Same for the power steering rack and pinion, they move left and right and t urn the wheel around 45 degrees. In fact, i think power steering R&P might be a better choice. They are stronger and would need less space.

To move solar panels (i assume that's what the OP is doing), she needs a fl at pyramid . Namely, moving along the base of the triangle, fixed at the t ip. The actuator she quoted need 60 inches for full movement (with full po wer), the pyramid would be 5 feet tall below the panels. Power steering E& P would be around 1 foot tail. Smaller movement also need stronger motor.

I am still trying to figure out how to wire up the 60A motor from a 2014 fu sion, but i am not doing solar panels.

• posted
40 secs a day! So, if I go with 20AH, 12V battery then the discharge rate would be 200mA per day. How long the battery will last for?
• posted

Don't you plan on recharging it? Although the discharge is small, it is literally tearing itself apart. It won't last very long without recharging. Are you planning on using it like one-time disposable battery?

• posted

How many hours in a day? Have you always struggled with basic math? (Sorry to be so snarky.)

0.2A*24Hrs = 12AH.
• posted

That's not the whole story; at FULL TORQUE the motor will take

12A, but we have not ascertained that the friction, mass, and acceleration will require full torque. At full speed, the motor will take 12V, but we don't know if that's ever going to occur, either.

The '12A' rating means that the motor probably saturates (and then burns up or trips its overload) if it is stalled and the current rises above 12A.

• posted

That sounds like you were controlling an RC-style servo, which is NOT just a bare motor -- it's a completely different animal from that linear actuator.

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Tim Wescott
Wescott Design Services ```
• posted

That would be at least 20/.2 or 100 days.

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