cylindrical coordinate robot

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cylindrical coordinate robot

Postby hasnid » Mon Apr 03, 2017 8:00 am

I am trying to make a e robot , with a payload of 200 gr load. The arm rotates 360 degrees. The arm moves horizontally, and up and down
I am confused about my choice of motors...the more i read the more it becomes difficult for me to decide between servo and stepper with encoders.
kindly give your expert opinion.
kindly also guide me in motor selection
Last edited by hasnid on Mon Apr 17, 2017 8:32 am, edited 1 time in total.

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mparadis
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Re: cylindrical coordinate robot

Postby mparadis » Mon Apr 03, 2017 10:08 am

The motor you should choose depends on the requirements of your project. Servo motors are cheaper and lighter, which is why you often see them for small remote-controlled robots. Stepper motors are usually heavier, but with an encoder they are much more precise and give you more options for control (controlling velocity and acceleration).

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Re: cylindrical coordinate robot

Postby hasnid » Mon Apr 03, 2017 9:46 pm

thanx alot for your reply

yes i do understand that servos are cheaper but precision is a very important parameter in my project ...so keeping in view your expert advice and my own research i have decided to go for stepper.
Now since i m not very confident about motors i have few more questions

i understand for selecting a motor i need to calculate holding torques...but do i also need to calculate torques at each actuator when the payload is accelerated from its static position?
what other parameters do i need to keep in mind while selecting a motor(i m thinking of using Arduino family controller)

thanks alot

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Re: cylindrical coordinate robot

Postby mparadis » Tue Apr 04, 2017 7:47 am

You only need to worry about holding torque if you think there's going to be a significant external force working against your motor when it's at rest. The holding torque needs to be higher than the strongest force you think it'll have to oppose in this way.

Generally speaking, you should calculate how much torque will be required to actuate the joint in the worst case scenario, and then add a bit of extra torque to give yourself some room for unforeseen factors.

Other parameters to pay attention to:

Step angle - Most bipolar stepper motors have a 1.8 degree step angle, which is then further reduced by the gearbox. So a 1.8 degree stepper motor with a 100:1 gearbox would have a 0.018 degree step angle. However, just by having a gearbox, you introduce backlash error (error caused due to the space between gears in the gearbox) of 1 to 3 degrees. Luckily this error is non-cumulative, so it doesn't get worse over time. If 1 to 3 degrees of error is too much for your application, you may want to consider getting a stepper without a gearbox. Some steppers also have a 0.9 degree step angle to improve precision.

Voltage - Stepper motors are controlled using constant current drivers, and that driver needs a certain size of power supply in order to run certain motors. Make sure that your system is capable of providing the appropriate amount of voltage to your controller. For example, our stepper controller requires a 12V DC power supply for most steppers, but 24V for some of the largest ones.

Weight - Obviously weight is an important factor in robotic applications.

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Re: cylindrical coordinate robot

Postby hasnid » Tue Apr 04, 2017 7:56 am

thanx alot once again
can u suggest some stepper motor as precision is important for me ;my payload is very small 100g...meaning holding torque is not much...i may not go for gear reduction but i do need small step

Maybe encoders can be used in feedback for better precision

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Re: cylindrical coordinate robot

Postby mparadis » Wed Apr 05, 2017 8:18 am

For precision I recommend the 3330. Using an encoder will ensure that you will still know the exact position of the encoder even if the stepper stalls or missteps due to external forces.

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Re: cylindrical coordinate robot

Postby hasnid » Wed Apr 05, 2017 8:50 am

thanx


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