Hi, I want to measure the current and/or torque of small hobby-class DC motor. Are there relatively inexpensive good sensors that can get the job done with high accuracy? Torque sensors that I found cost hundreds or even thousands of dollars. They are also big. I also read that current sensors signals are noisy.
Did you study the specifications of the expensive torque sensors? Can you apply that to a small motor for your project?
Are you looking for a range of torque based on motor speed or are you looking for maximum stall torque?
Paul
Paul_KD7HB:
Did you study the specifications of the expensive torque sensors? Can you apply that to a small motor for your project?Are you looking for a range of torque based on motor speed or are you looking for maximum stall torque?
Paul
When I saw the prices, I searched for other products as even if they can get the job done, I cannot afford to pay.
I would like to measure the current and/or torque of the motor while it is turning.
bbqq:
When I saw the prices, I searched for other products as even if they can get the job done, I cannot afford to pay.I would like to measure the current and/or torque of the motor while it is turning.
But, do you understand how the expensive instruments measure the torque? If so, you can duplicate that method for your own project.
There are two ways to measure the torque of a turning motor. Both entail balancing that torque with an equal but opposite torque. The easy way to hinge the test motor mounting and add a coil spring the control the rotation of the test motor. A brake is applied to the test motor armature shaft and as more braking is applied, the motor will react by rotating and stretching the coil spring. Record the length of the stretched spring and then use a spring scale to see how much pull is required to stretch the spring. That is the motor torque at that speed.
The electrical torque measurement sort of duplicated that experiment by using a second motor to provide braking force to the test motor armature shaft. Current applied to the second motor attempts to make it rotate the opposite direction of the test motor and the current applied correlates to the test motor torque.
There are other methods that measure the heat produced by a brake applied to the test motor armature and relate that to the power produced by the motor at that given rotation speed.
Paul
You can save yourself a lot of trouble!
The Pololu company has and uses the expensive equipment needed to test and verify the torque, power and current performance of the motors they sell, and they support their products.
So, if you buy a motor from Pololu you get a quality product at a reasonable price that meets the stated specifications, and the engineers are always happy to respond to questions posted on their forum.
I have no connection with Pololu, just uniformly good experiences with their products.
bbqq:
Hi, I want to measure the current and/or torque of small hobby-class DC motor. Are there relatively inexpensive good sensors that can get the job done with high accuracy? Torque sensors that I found cost hundreds or even thousands of dollars. They are also big. I also read that current sensors signals are noisy.
Not quite, Hall sensors are noisy. Only some current sensors use Hall sensors. Some are shunt-based, and for
small currents shunt measurement is usually the best/simplest/cheapest approach.
Mechanical torque sensing is specialist and expensive when the torque to be measured is on a rotating
shaft.
MarkT:
Not quite, Hall sensors are noisy. Only some current sensors use Hall sensors. Some are shunt-based, and for
small currents shunt measurement is usually the best/simplest/cheapest approach.Mechanical torque sensing is specialist and expensive when the torque to be measured is on a rotating
shaft.
Thanks. For using shunt, do I have to cut the wire which corresponds to power supply to the motor and then connect the two pieces with a shunt in between?
how big of a motor ? there is the GY-169 / INA169 current sensor. should work upto 60V on small hobby motors . just have to figure what your load resistor would be .
Assuming its a DC motor you can easily measure the current with a simple resistive shunt. If you also measure the shaft speed, you can calculate the torque.
The mechanical power Pm supplied by the motor is given by
Pm = M*w, where M is the torque and w the rotational speed.
If we ignore frictional loses the mechanical power Pm is equal to the electrical power Pe supplied to the motor - which is
Pe = V*I Watts.
I have more detailed information and circuit calculations on my own site.
1 Like
bbqq:
Thanks. For using shunt, do I have to cut the wire which corresponds to power supply to the motor and then connect the two pieces with a shunt in between?
Yes, ironically a this kind of shunt is in series with the load (but parallel with the voltage sensor,
hence the name).
Current shunt resistors are 4-terminal devices, as they need to be used with a Kelvin connection.
All you need is a low value resistor that will produce a voltage across it that is a small percentage (say 1 - 5%) of the supply voltage to the motor. So if your motor runs from 12V and draws 5A on full load a suitable resistor would be 0.1 ohm theereby dropping only 0.5 V.
0.5V 5A = 2.5 Watts - so I'd choose a 5W resistor.
If you wish you can amplify the output, or measure it directly using a 1.1V internal reference if you use low side sensing.
4 terminal shunts are only required when very high precision is needed. More precision than your arduino could measure.