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Unread 04-04-2018, 11:58 AM
Lesafian Lesafian is offline
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Possible Issues with Feeding Voltage to a 775 Constantly?

Last weekend we competed at the Shepherd District Event. We quickly realized that we wanted to reduce the gear ratio on our lift motors to increase the speed of our lift.

After changing from a 100:1 to a 16:1 on our center carriage, there is significantly more slip (duh). In order to prevent this, we are going to feed enough voltage to the motor to help reduce the slipping, but not drive it upward when not necessary.

Theoretically, this should work, but are there any issues with this method that we should be aware of?
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Unread 04-04-2018, 11:59 AM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Lesafian View Post
Theoretically, this should work
Please show your calculations.


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Unread 04-04-2018, 12:18 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Lesafian View Post
After changing from a 100:1 to a 16:1 on our center carriage, there is significantly more slip (duh). In order to prevent this, we are going to feed enough voltage to the motor to help reduce the slipping, but not drive it upward when not necessary.

Theoretically, this should work, but are there any issues with this method that we should be aware of?
By slip I think you mean the elevator drops (back drives) on its own when power is removed right?

If so, yes you can hold position by applying voltage to the motor. There are issues you should be aware of doing this. Without careful control, a "stalled" motor has a very low resistance, and zero air cooling, so it will pass a lot of current and heat up very quickly to internal-wire-melting temperature. Look carefully at the last graph on this page "Locked Rotor Stall": http://motors.vex.com/vexpro-motors/775pro You'll see that applying more than 3 or 4 volts to a stalled 775 pro will kill it pretty quickly.

That's fine though - if you program it to never go above 4 volts (or current limit to maybe 20A). Just be aware that will also reduce the motor torque to less than 33% of it's rated value (and the power would be only ~10% of its rated value), so double check your calculations to make sure the elevator will still hold position if you limit the voltage or current this way.

Last edited by nuclearnerd : 04-04-2018 at 12:57 PM.
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Unread 04-04-2018, 12:34 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by nuclearnerd View Post
By slip I think you mean the elevator drops (back drives) on its own when power is removed right?

If so, yes you can hold position by applying voltage to the motor. There are issues you should be aware of doing this. Without careful control, a "stalled" motor has a very low resistance, and zero air cooling, so it will pass a lot of current and heat up very quickly to internal-wire-melting temperature. Look carefully at the last graph on this page "Locked Rotor Stall": http://motors.vex.com/vexpro-motors/775pro You'll see that applying more than 3 or 4 volts to a stalled 775 pro will kill it pretty quickly.

That's fine though - if you program it to never go above 4 volts (or current limit to maybe 20A). Just be aware that will also reduce the motor torque to less than 33% of it's rated value (and the power would be only ~10% of its rated value), so double check your calculations to make sure the elevator will still move if you limit the voltage or current this way.
Perhaps you could verify my method.

The motor controller used on both the "inner and outer" lifts is the TalonSRX.
Both of the TalonSRX's are controlled using the PercentOutput method.

I'm not quite sure how to calculate the output voltage relative to the percent output, but I do know that there is a built in function talon.getMotorOutputVoltage(), so I assume I could use that to drop the output percentage if the returned value of that function exceeds 4.

Also, when you say it can never exceed 4 volts, do you mean it should never exceed 4 when it is stalled (a.k.a holding its position), or it should never exceed 4 even when I'm trying to lift it higher or lower?
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Unread 04-04-2018, 12:36 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Ether View Post
Please show your calculations.


Lol, I suppose by "work" I really meant, "It will hold its position... it might ruin the motor, it might not..."
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Unread 04-04-2018, 12:40 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Lesafian View Post
Perhaps you could verify my method.

The motor controller used on both the "inner and outer" lifts is the TalonSRX.
Both of the TalonSRX's are controlled using the PercentOutput method.

I'm not quite sure how to calculate the output voltage relative to the percent output,
You can scale PercentOutput to 13 volts at 100% to get a pretty close idea of how much voltage you are sending from the Talon to the motor

Quote:
Originally Posted by Lesafian View Post
Also, when you say it can never exceed 4 volts, do you mean it should never exceed 4 when it is stalled (a.k.a holding its position), or it should never exceed 4 even when I'm trying to lift it higher or lower?
You only have to worry about when it is stalled

Last edited by nuclearnerd : 04-04-2018 at 12:56 PM.
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Unread 04-04-2018, 12:43 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Lesafian View Post
Also, when you say it can never exceed 4 volts, do you mean it should never exceed 4 when it is stalled (a.k.a holding its position), or it should never exceed 4 even when I'm trying to lift it higher or lower?
4V @ locked-rotor (aka Stall, aka max current for the given input voltage).

Percent output will adjust the SRX output duty cycle so that some percentage of the present battery voltage is delivered to the motor. Presuming your battery outputs 12V, 33% of 12 gets you 4V. This assumption may not be valid in all cases though .

There are other threads to discuss this already, but beware that setting a SRX current limit will limit current draw from the battery (and into the input) of the SRX. It does not, however, directly limit current draw through the motor. The motor current draw can exceed the set limit by a decent margin under the right conditions - there is additional work required to hard-limit the motor current draw.
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Unread 04-04-2018, 12:50 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by nuclearnerd View Post
You can scale PerentOutput to 13 volts at 100% to get a pretty close idea of how much voltage you are sending to the Talon


You only have to worry about when it is stalled
Thank you very much!!!
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Unread 04-04-2018, 12:52 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Lesafian View Post
Also, when you say it can never exceed 4 volts, do you mean it should never exceed 4 when it is stalled (a.k.a holding its position), or it should never exceed 4 even when I'm trying to lift it higher or lower?
When stalled (or running slowly). If you give a 775 pro a full 12V, it only draws ~700mA, which is about 8.4 watts. Because the 775 pro has an internal fan, if running in free or near free conditions you can give it 4 or even 12 volts indefinitely. The fan blows the heat out, keeping the temperature safe. Temperature is what kills motors.
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Unread 04-04-2018, 12:52 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by gerthworm View Post
4V @ locked-rotor (aka Stall, aka max current for the given input voltage).

Percent output will adjust the SRX output duty cycle so that some percentage of the present battery voltage is delivered to the motor. Presuming your battery outputs 12V, 33% of 12 gets you 4V. This assumption may not be valid in all cases though .

There are other threads to discuss this already, but beware that setting a SRX current limit will limit current draw from the battery (and into the input) of the SRX. It does not, however, directly limit current draw through the motor. The motor current draw can exceed the set limit by a decent margin under the right conditions - there is additional work required to hard-limit the motor current draw.
What if I were to setup a simple equation that would return the battery voltage from the DriverStation, and calculate the needed output percentage to achieve 4v. Would this solve that issue?
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Unread 04-04-2018, 12:56 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by nuclearnerd View Post
You can scale PerentOutput to 13 volts at 100% to get a pretty close idea of how much voltage you are sending to the Talon


You only have to worry about when it is stalled
Just thought about it further, giving the TalonSRX 33% will most definitely raise the lift, and at a decent speed too. A more realistic value to hold it in place would probably be like 5-10%. In this case, it would be completely safe, right?
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Unread 04-04-2018, 01:11 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Just thought about it further, giving the TalonSRX 33% will most definitely raise the lift, and at a decent speed too.

How do you know that? Please show your calculations.


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Unread 04-04-2018, 01:24 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Ether View Post
How do you know that? Please show your calculations.


This this this.

@OP

See: http://motors.vex.com/vexpro-motors/775pro

Learn it. Know it. Love it.

Figure out the holding torque required for your lift. Look at the performance chart for the 775. Figure out what voltage you need to apply to get your holding torque. Look at the locked rotor stall data. What's your time margin? No need to guess when all the relevant data is at your fingertips!

Edit:

Perhaps more to the point - get PID control on that sucker. If you're using a 775 Pro you might be using a Versaplanetary gearbox. If you're using a VP gearbox you can get a VP encoder and enter the world of good robot control.
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Last edited by JamesCH95 : 04-04-2018 at 01:28 PM.
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Unread 04-04-2018, 03:29 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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Originally Posted by Ether View Post
How do you know that? Please show your calculations.


I don't have any calculations or numbers, I know from testing.
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Unread 04-04-2018, 03:30 PM
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Re: Possible Issues with Feeding Voltage to a 775 Constantly?

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I don't have any calculations or numbers, I know from testing.
Would you please describe the tests you ran.


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