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Short answer
Stall current is the maximum current drawn1, when the motor is applying its maximum torque, either because it is being prevented from moving entirely or because it can no longer accelerate given the load it is under.
Free current is the current drawn when the motor is rotating freely at maximum speed, under no load2 other than friction and ...
17
Stall current is how much the motor will draw when it is stuck, i.e. stalled. Free current is how much current it draws when the motor has no load, i.e. free to spin. As you'd expect, the more strain on the motor, the more current it will draw in order to move; the stall current and free current are the maximum and minimum, respectively.
From a standing ...
15
@Ian and @Mark offer awesome (and correct) answers. I'll add one extra point for completeness...
There seems to be trend among less experienced designers to assume that stall current and free current equate to the maximum and minimum currents the motor might encounter.
They don't.
They are the effective nominal values. You can exceed these limits under ...
8
The 40mA current limit is the Absolute Maximum that an I/O pin on the ATmega328P can supply. Vcc on the ATmega can draw up to 200mA.
From page 313 of the datasheet:
The 5V that connects to Vcc and powers your chip comes from one of two places. Either the USB connection, which in most cases is limited to supplying 500mA. Or an external power supply (wall ...
5
Model Based:
Low cost solution is always software:
Develop a dynamic model which computes the load on the motors based on the motions (e.g. with Recursive Newton Euler Algorithm or Lagrange-Euler Method)
Make as simple model to handle losses in driver/motor
Make the difference between real torque and computed torque
It will be not accurate. At least 10% ...
5
All very good answers, but as a physics teacher I am concerned about some incorrect equivalences here that can only lead to confusion.
One form of [energy][1], eg [chemical potential energy][2], can be converted into other forms of energy (eg [electric potential energy][3], [kinetic energy][4], [sound energy][5], [thermal energy][6]). In the [SI system][7],...
4
When there is miscommunications between the microcontroller and a chip, assuming the electronics are not damaged, there could be a couple of things that can go wrong. Of course, further diagnosis is required. Typical things that could go wrong are (generally, not just in your particular case):
Connections: Is the chip connected to the correct power supply? ...
4
You are approaching the problem from wrong side. Current capability of a battery (25C) has nothing to do with that how much current will it actually source. It is the load (motors in that case) what defines the current, not the battery. You could use 10C, 25C, 50C battery, and the current flowing through the motors would be (approximately) the same, as long ...
4
The ONLY difference between a stepper and a servo is that a Servo monitors its position with an encoder, and may increase power if it gets behind, or reduce power if it gets ahead, or generates a 'fault' condition if it is unable to move to the proper position in a predetermined time frame.
There is no difference in power requirements. Any stepper can be a ...
3
I've experienced a similar problem before. For me, the root cause was insufficient battery power. When my motor tried to draw peak power it caused the battery supply bus to brown out. This led to supply voltage dropping below the minimum rated voltage for the motor controller. The master board was fine as the brownout voltage was still above that minimum.
...
3
As explained in my answer to What is stall current and free current of motors? when switching direction on a motor, you can end up drawing more than the maximum normal current rating. This can cause trips to be thrown, amplifier shutdown or voltage drops and loss of control.
One option is to try to reduce the stress on the motor and amplifier. For instance ...
3
First, regarding why “the same switch can be rated for different amperages”, the current-rating of a switch depends on switched voltage and current because switching a high voltage or current typically produces a more energetic arc than does switching a low voltage or current.
The switch you referred to, with its “Legion EPS21 10A 250VAC TV-5 8A/128A 250V ...
2
I know this thread is old but it's high on Google results and I would like to clarify for anyone who stumbles upon it in the future an answer not yet listed: this chip is not designed to read current accurately below 0.3A and only gives 20% accuracy above that. This is not listed in the board spec sheet but rather the chip spec sheet. If you're running ...
2
Firstly consider the servo torque limitations, if loading increase from specified limit the it start stalling and even can gear get fail and burn servo itself, possibly.
And another concern is with power supply. You are using metalgear servo with power supply of just 1 A.
As per the specification provided by greenonline (in previous answer), the servo needs ...
2
I assume that all the servos are identical, so it is enough to measure only one, and multiply the result by 9.
The best would be if you could use a lab bench variable power supply, like this. With one of these you do not even need a multimeter as these power supplies are able to measure the current they are sourcing. You simply just need to connect the "+" ...
1
Running a motor with a higher voltage might reduces its lifetime. However there are some cases where we want to run motors above the rated voltage. According to the equation V = IR, current increases with the voltage since the resistance is contant in a motor. But when it is getting heated, the resistance gets low and the current increases. This process will ...
1
It's maybe not this at all, but when a motor or any inductive system suddenly stop it create a short opposite surtense that can affect your microcontroleur. The solution is to ad a flyback diode within your motor driver. Most of the motor's driver (including the H-bridge) are provide with flyback diode built-in but you can check if yours got one.
Here the ...
1
You can try to put a capacitor in the motor terminal. Very close to the motor.
1
50k4 did a good job of enumerating the different ways to measure torque in the joints. So I won't go into that. But if you want to detect unexpected impacts, another option is to use accelerometers. Example here
1
From the TowerPro website from the MG995:
Specification:
Weight: 55 g
Dimension: 40.7×19.7×42.9 mm
Stall torque: 9.4 kg/cm (4.8v); 11 kg/cm (6 V)
Operating speed: 0.20 sec/60° (4.8 V); 0.16 sec/60° (6.0 V)
Operating voltage: 4.8~ 6.6 V
Gear Type: Metal gear
Temperature range: 0- 55 °C
Dead band width: 1 µs
servo wire length: 32 cm
...
1
Nice detail in your question.
The somanet site makes a big deal about their firmware being open source, so you should be able to check the timing in the code (or otherwise get the help of the somanet engineers).
A 15kHz PWM is the slowest I've seen in a long time and could indicate that that they had trouble fitting the commutation algorithm to their ...
1
You are correct in that they are related, via Ohm's law:
$$
V = IR \\
$$
The difference is in what gets regulated. A voltage source will keep supply voltage at a constant level by altering the current that is supplied. If the effective (Thevenin) resistance goes down, then the applied current must go up to keep the supplied voltage constant.
Similarly, if ...
1
If I might add to the previous answers: I noticed in the referenced datasheet that the 20A maximum current rating for this driver is limited to less than 10 seconds and the maximum current is further limited to 10A for less than 60 seconds. Maximum continuous current ( >60 sec.) is then given as 5A without heatsinks but with, I'm assuming, decent cooling ...
1
First of all the link you posted states that your motor driver carrier board has built-in overcurrent protection...so...where is the problem?
If it does not have:
Put a fuse in series with the motor that will protect the controller (you can find slow fuses that allow higher current for short time). Then implment a current control loop with output ...
1
Heat is an important problem with a series resistor used to limit stall current. Wattage of I amperes through R Ω is I²R, so 7.35 watts of heat are produced at the motor's nominal 7 A operating current, and 93.75 W at a 25 A stall current.
You can avoid some loss at the 7 A level by using an inexpensive light bulb as a resistor. The filament's resistance ...
1
Of the choices you presented, PWM + BJT/FET is most efficient. An H-Bridge is a prime example of that.
-BJT's dissipate heat in proportion to their "on-ness"
-FET's dissipate heat based on their (very low) on-state resistance, plus a small loss during the actual switching.
By PWM'ing, you are approximating an analog voltage in proportion to the BJT/FET ...
1
Yes, that looks like it will work. Keep in mind that the arduino io pins aren't powering any logic. Most Arduinos can provide more than 500 mA from their 5v pin assuming that they are connected to a power supply other than usb (not that you want to draw much current from it). The io pins on atmega328 based boards can provide 40 mA, so powering anything more ...
1
The Adafruit Motor Shield for Arduino should work, if two conditions are satisfied:
the Adafruit Motor Shield for Arduino can limit its own current, or there's enough data available so that you can.
the motor you want to use can generate sufficient torque on 3 amps
The answer to (1) should come from the data for the Adafruit Motor Shield for Arduino. The ...
1
According to this discussion on the Arduino forums, you can draw about 1 amp through the Vin pin before frying the "polarity protection" diode that sits between the power supply and the pin.
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