# problems with servo and calculations

Hello

I have several questions:

1. I bought a servo (MG99R) which stated that the torque is 10kgcm .Yesterday, I used the travel beg weight tester in which the string was attached to the servo horn, so that the horn pushed the string to measure the pushing weight by the servo.
However, the servo can only managed to pull the weighter string by 2kg(measured from the weighter).

2)I tested a DC motor that I bought which has stall current of 2A with torque of 10kgcm, I watched youtube on how to measure the stall current(by making the shaft stop forcely with our hands). But the current shown was only 800mA. Why is that? is it the stall current on the specifications is wrong?if yes, that means the torque is also wrong?

1. I have a project in which I need to design a robot and make a design consideration. The instructor asked me to measure the torque needed and the rpm needed for that robot. However, I have no machanical background nor expert in electronics. The only parameters that I know is the weight of the robot. What formula should I use to measure the torque and the rpm of the motor?I have searched on the internet but I think I havent see any suitable formula to use.

2. I would like to check my Nrf24l01 on the oscilloscope, there are 8 pins which is Gnd,Vcc,Ce,Cs,Sck,Mosi,Miso and IRQ. Which pins should I connect to the oscilloscope?

Really sorry for mixed topic questions, but since my questions are more onto the motors so I put this questions on this forum. And i don't want to flood Arduino's forum.

You will need a very expensive oscilloscope if you are trying to monitor 2.4GHz signals.

I got my nRF24s working with this Tutorial

I suggest you use the TMRh20 version of the RF24 library - it solves some problems from the ManiacBug version

When you were measuring the pull from your servo, how long was the servo arm. 10kgcm means it can hold 10kg at a radius of 1 cm. Post a diagram showing what you were doing. Was the servo power supply sufficient? By the way if you force it to provide full torque for any length of time it may overheat and be damaged.

What voltage were you applying to the motor when it stalled at 800mA? You need to measure the voltage at the same time as the amps (with 2 multimeters).

...R

I bought this type of Oscilloscope and I think it is sufficient to measure the signal. The only problem is that i'm not sure which pin should I connect it to. I am afraid to connect those probe as I am new to electronics and afraid it might damage either the board or oscilloscope.

-Thank you for the link! Really appreciate it!

• I am using variable voltage adapter which only have 3V,4.5V,6V,7.5V and 12V. The maximum voltage the servo can take is 7.2V, so I was using 6V instead of 7.5V. Then when I measured it, it shows torque of 2kg. Even though it is not running on full voltage, I expect it to run nearly 10kg. What do you mean by diagram?Schematic or the picture of the experiment?

-The voltage was 12V using the same adapter as the servo, I have 2 multimeters and measure both of the voltage and also the current. But I just look after the current and ignore the voltage. I'm going to test it again soon and gonna see both of the readings.

That 40MHz Oscilloscope would be completely incapable of detecting 2.4GHz signals - however I doubt if that is what you want to do.

The real problem is that you have not told us what you want to measure with the 'scope. Without knowing that how can we help.

As long as you ar careful not to cause a short circuit it is very unlikely that you can damage either the scope of the nRF24.

How many amps can your "variable voltage adapter" provide at the different voltages? I suspect (with maybe 40% certainty) that it cannot provide as much current as the servo or motor require.

...R

I would like to check the sucessful packet received and also the distortion.

It just tell that it has rating of 2.5A.

How about if I use this type of battery? Will it be able to sufficiently provide enough voltage and current to both of the servo's and motors? *For servos, im gonna step it down using buck converter to 7.2V

Akh_f:

1. I bought a servo (MG99R) which stated that the torque is 10kgcm .Yesterday, I used the travel beg weight tester in which the string was attached to the servo horn, so that the horn pushed the string to measure the pushing weight by the servo.
However, the servo can only managed to pull the weighter string by 2kg(measured from the weighter).

Torque is tangential force times off-axis distance - you have to do a multiplication. The torque given in
the specs for the servo is the peak torque, it is never a continuous rating BTW.

2)I tested a DC motor that I bought which has stall current of 2A with torque of 10kgcm, I watched youtube on how to measure the stall current(by making the shaft stop forcely with our hands). But the current shown was only 800mA. Why is that? is it the stall current on the specifications is wrong?if yes, that means the torque is also wrong?

You measure the resistance of the motor (unpowered). Stall current = supply voltage / winding resistance,
so it depends on the supply voltage.

1. I have a project in which I need to design a robot and make a design consideration. The instructor asked me to measure the torque needed and the rpm needed for that robot. However, I have no machanical background nor expert in electronics. The only parameters that I know is the weight of the robot. What formula should I use to measure the torque and the rpm of the motor?I have searched on the internet but I think I havent see any suitable formula to use.

Go with an energy calculation - easy for climbing a slope, compare gravitation energy gained by the torque x radians
the motor shaft turned to get that far up the slope. Then bring time in to get power and shaft speed
to solve for torque. Power = torque x angular velocity.
Friction is harder to deal with, you really need some
measurements. Real drive trains are well below 100% efficient, note.

1. I would like to check my Nrf24l01 on the oscilloscope, there are 8 pins which is Gnd,Vcc,Ce,Cs,Sck,Mosi,Miso and IRQ. Which pins should I connect to the oscilloscope?

The only useful thing is sticking a current-probe on the power supply to monitor if its actually transmitting,
but current probes are rare to find. Basically the thing either works or it doesn't and the problem is 95%
of the time a software one.
[/quote]

What do you mean by energy calculation?In my case, im just assuming on a straight surface and not involving any slope. I am not quite understand with your explaination for the formula part..Im sorry because im slow.

But yeah, i understand for the you other answers.Thank you MarkT!I wish I have skills and knowledge like you.

You can do mechanics calculations in two ways, either forces, torques, speeds, or by
energy conservation. Energy conservation is a powerful tool that can lose tedious detail
and directly relate input to output (assuming you know the efficiency).

If going on the flat all the energy goes to friction and loss so its not so good...

However if you know the slope that can sustain motion, you know your propulsion
system has to be at least that powerful, so you can rethink the problem in terms of
energy to go up that slope.

Measuring the slope that sustains rolling motion is basically a measurement of system
friction.

Akh_f:
I would like to check the sucessful packet received and also the distortion.

It just tell that it has rating of 2.5A.

How about if I use this type of battery? Will it be able to sufficiently provide enough voltage and current to both of the servo's and motors? *For servos, im gonna step it down using buck converter to 7.2V

The 2.5A power supply should be OK as should that battery - but make sure the buck-converter can handle enough current.

An Arduino program is the easiest way to check if a packet is received.
I have no idea what you mean by distortion. If you mean errors in the data then that is very unlikely - the nRF24s have extensive error checking.

...R