Hi All,
Is there is simple way to hook up the Current Sensor (ACS714 Current Sensor Carrier -5 to +5A) via an input port on any of the Mestro Servo controllers to measure the input current suppled to the Maestro Unit? What do you recommend if the question is a reasonable.
Thanks
Mark
Hello, Mark.
The ACS714 Current Sensor Carrier -5 to +5A has an analog output that outputs voltages between 0 and 5 V and the Maestro servo controllers support analog inputs, so yes these two items should work together.
You would need to properly wire the current sensor so that all the current flowing to the servos gets routed through it and connect its output to a channel on the Maestro that has been configured as an input. Then the “Position” variable on that channel would correspond to the current.
Do you have any more specific questions about how to do this?
–David
Hi David,
The Theory is exactly what I assumed and thanks for confirming, now to actually successfully implement I am having some difficuly. Please let me outline what I have tried.
I am using the Maestro 12 port. Configuerd at least one port to be an input. I 1st verified the current sensor works by connecting a separate power supply to the 0.1" space Vcc/Gnd and the sensor records the voltage as desired. Now onto my second scenerio which is to measure the voltage being provided to the Maestro. If I connect the power source feeding the Maestro to both the Maestro and Current sensor with no diodes , resistors or capacitors the Maestro gets no power. That being the case I connected a diode between the Power source and the Current Sensor now the Maestro powers up but the Sensor doesnt register on the signal port.
So in simple terms I need help with a circuit to allow the sensor to work for measuring the power feeding the Maestro 12 port for example.
Any help or spoon feeding would be most welcomed 
Thanks
Mark
Hi David,
I did neglect to mention that I was using a single power source witht eh Jumper on the Maestro shorting the two pins to facilitate. I just removed the jumper and have two independent power sources (One for the Maestro and one for the servo/input ports). Now I am getting a different reading from the sensor when I connect/disconnect the sensor. Perhaps this is the key as to why I cannot check the main power source to the Maestro if the Jumper is shorted to support a single power source for Maestro and it’s ports.
What do you think?
Please note for the Maestro to power up with the current sensor connected to the source power for the Meastro I need to use diiodes to prohibit an opposing EMF. I hope my reasoning is sensible.
Thanks
Mark
The fact that the Maestro turned off when you wired your circuit a certain way and you had to add a diode makes me think that your wiring was wrong, and somehow caused the power supply to be shorted out.
Could you simplify your system down to the simplest possible thing (one power supply, one current sensor, one Maestro, no diodes) that exhibits unexpected behavior, and then fully describe all the wires you have connected?
–David
Sorry to use these old posts but the subject comes close to my question…
I would like to monitor the current that is going to a 6V servo. The purpose is to have an idea of how much force the servo is asking (no accurate/absolute value). The servo signal comes from an Arduino board instead of a servo controller. So my plan is to have the ACS714 current sensor between the Arduino and the servo power lines. The current sensor’s output would then be connected to a analogue input of the Arduino board. Would it work well to monitor the servo’s power (the servo drives a hand that has to hold an egg without breaking it…) ? I have no experience with this so I hope its not a stupid question.
thanks a lot,
Fons Jena
Hello, Fons.
It might work if the egg puts a substantial load on the servo before it breaks. If you use a servo that is too strong, it could crack the egg without generating much of a current spike. You might also consider using a force-sensing resistor to get feedback about when you are holding the egg tightly enough.
- Ben
Those force-sensing resistors are interesting, I didn’t know about them, thanks. I wanted to use current sensing because mounting sensors on the fingers themselves is not as easy as simply monitoring the current spikes. So I will have to use a motor that has a stall torque that is close to the desired torque?
I ditched the servo idea and I am planning on using a 25Dx54L metal gearmotor with a MC33926 Motor Driver that has current sensing. Would it be better to use the high power version with higher stall current ? I guess they are more current/load sensitive making it easier to track the current …
Thanks for the fast response,
Fons Jena
That motor driver is not really powerful enough for the high-power 25D motors, and I suspect you don’t need or want that much power, anyway. You should be able to detect when the non-HP 25D motor meets some resistance and the current rises by a few hundred milliamps. You might also consider one of our micro metal gearmotors.
- Ben
OK,
The MC33926 Motor Driver has a limited current output wehn operating between 5-8 volts. I would like to use the non-HP 25D motors at 6 volts. Stall current is 2,2 amps so I wonder if I can reach the 2 amps with this setup (motor driver / gearmotor combination)?
I could use a higher voltage but then it would lower the current which isn’t a good thing if I want a more sensitive system…
Fons Jena
Using a higher voltage will increase the current, not decrease it. As I said in my last post, you should be able to detect current spikes of only a few hundred milliamps, so you don’t need to come up with something that will draw many amps of current just so that you can detect it.
- Ben
OK,
But I still need about 2 amps to reach the torque limit of the motor because I have to have enough power to grasp heavy/rigid objects too… I assume that the MC33926 motor driver can deliver that amount of current at 6V?
Fons Jena
It will be close. You’re right at what we claim is the continuous current rating for the board, and ultimately the performance will depend on how long you need to deliver that current and how well you can keep the board cool while you do. The driver performance is derated slightly below 8V, so the continuous current it can deliver might be a little lower than the 2.5 A we claim.
I’m not convinced you’ve actually done the calculations to determine how much torque/current you need, but if you think it’s a lot, you might consider a higher-power motor driver or controller along with an external current sensor.
- Ben
I must admit I know too less about it (whcih makes your job harder… sorry) but I made some simplified calculations (no friction) and I need about 480 Nmm of torque (thats about 70 ‘ounce inch’). I added an extra margin for the friction (the tendon actuation is fas from frictionless…) and for the motor (I suppose using the motor close at its stall torque isn’t good). The 25D motors fits perfectly in my gripper that is why I would like to use them. Also, I like to use the MC33926 because of its built in current sensing and because I already used them in the past (they’re great). I can easily use a 9 volts supply for the 25D motor with the 75:1 gearing but 6 V would probably be enough (because you say: little lower than the 2.5 A).
Sorry for the time consuming questioning from my part and thanks for the help,
Fons Jena
I’ll go with the non-HP 75:1 25D motor with the MC33926 motor driver using a 9 volt supply. It could be an overpowered solution but I must make a decision. I hope my thesis promotor approves this solution !
Just make sure you’re not planning on using a 9V battery. Such batteries are not good at supplying high currents and will probably work very poorly in your application. I’d suggest using something like a 7- or 8- cell NiMH pack (8.4 or 9.6V).
- Ben
