I would like to construct a project using a stepper motor and a force sensor. The goal will be to measure force vs distance. Does anyone have any suggestions on a interface board and construction tip using a USB port.
Any constructive input is welcome.
Chris
There are many ways to go about this. Consider a normal USB interface for each? Just an idea:
robotshop.com/en/phidgetstep … 067-0.html
robotshop.com/en/phidgetstep … motor.html
robotshop.com/en/phidgetbrid … rface.html
robotshop.com/en/catalogsear … t&dir=desc
I have a similar need. However, in my case, the linear actuator does not back-feed, so I can’t really use the motor current, or even a single load sensor on the actuator end. I will need to use the bathroom-scale type of set up, with the 4 sensors.
I was hoping to use the Firgelli Linear Actuator Control Board RB-Fir-12, but substitute the position sensor, with a voltage from a force sensor.
There only seems to be one item that accepts the very low voltage of the load cells, and converts it to analog. The DFRobot HX711 Load Cell Interface RB-Dfr-519 says it has analog output. But, I cannot find in the literature where it describes the analog output. I.e. what voltage range does it put out? And since there are a few language issues with the literature, I am wondering if the analog output is actually there.
I am also frustrated that it seems that it takes a lot of expensive components to gather the data that a $15 bathroom scale collects, and that is not even including the built-in display that the bathroom scale has.
$45.00, 200 kg Load CellRB-Phi-123 [Qty 4]
$16.90, Load Cell Interface RB-Dfr-519 [Qty 4]
With the above, I have 4 separate signals to somehow combine, so I need to somehow do that.
So, I am hoping that I am just going down the wrong road, and need to get a different type of load sensor.
I did see the $90 PhidgetBridge Wheatstone Bridge Sensor Interface RB-Phi-107, which can accept input from 4 of the load cells. But, it only outputs a USB signal. Is there a motor controller that will directly accept the digital signal from this device?
Thank you for any suggestions.
-Joe
Can you give us a bit of info about your application?
This is for an end-effector that must grip an object with a controlled amount of force. However, this is not you most typical gripper design, and it needs a fairly large surface area. Also, my linear actuators, and the linkages mean that I really need to sense the applied force at the gripping surface. The bathroom scale concept would seem to be the best approach to measuring the force.
-Joe
Can you provide a drawing or CAD of the setup? Trying to visualize but don’t want to make assumptions.
Attached is a screen grab showing one of the initial concept sketches. Note that the actuator cannot be back driven. The flat area at the top is approx 10" square, and is where the load is applied. The top is constrained in its motion by some linear slots shown in green.
Not what I was envisioning - thanks for clarifying! Can you add force sensors at the supports of the plate?
robotshop.com/en/force-sensors.html
I am not clear where the “supports of the plate” are that you are referring to. I also realize that the force sensor should be protected against significant shear loads.
The forces that the actuator must counter are only being applied in the down direction. And when the linkages in the image are mostly vertical, the amount of force that the actuator sees is quite minimal. The plate will see substantial side loads from the item we are gripping, and those forces will be carried by the slots in the picture.
My current design for force sensing, has a new plate added to the top of the existing plate, and is constrained to it by simply putting a shoulder bolt, which will carry the side loads. See the attached image for that detail.
-Joe
It’s a design issue indeed. We understand the constraints but don’t have any evident ideas. If we come up with something we’ll reply. Perhaps someone in the community might also have an idea.
First, thank you for sticking with me on this subject. Most vendors do not go to the level of effort and support that your company does on this forum.
Do you have any more information about the Load Cell Interface RB-Dfr-519, that is supposed to have analog output. Can you tell me the voltage output of that device?
Another approach might be to use one of the load cell interfaces that put out a digital signal, and then convert that digital signal to an analog voltge using a microprocessor. Though, that seems to be an odd thing to do.
I am speculating about one other approach. The PhidgetBridge Wheatstone Bridge Sensor Interface RB-Phi-107, can read my 4 load cells. Perhaps somewhere on the circuit board is a place where there is an analog voltage that represents the sum of all the load cells. If so, perhaps I can simply tap that spot for my voltage. Do you all have enough knowledge about that board to say if my idea is viable?
-joe
RB-Dfr-519 is meant to be connected to another device such as a microcontroller.
The resolution should be good as it uses a 24 bit ADC chip. The voltage output is 0-5V.
RB-Phi-107 connects directly to a computer; we’re not aware of anywhere on the board where you can get the signal via other means.
I just want to make sure I understand one aspect of the load cells you offer.
I believe the 50 Kg Micro Load Cell RB-Phi-120 has a Wheatstone bridge built into it, and therefore only one of these can be connected to the
Load Cell / Wheatstone Bridge Interface RB-Dfr-51
But, rather than use that sensor, if my understanding is correct, I can connect 4 of the SFE Load Sensor - 50kg : RB-Spa-488 to form the Wheatstone bridge.
So, using a RB-Dfr-51 interface with four of the RB-Spa-488 load cells, will permit me to make a scale sensor that functions like the typical electronic bathroom scale.
Note that I understand that this sensor is a bit under the target load I want to sense, so I will need to off-load it a bit, perhaps with a simple compression spring in the system. Also, that interface only has digital output, so I will need to interface it to an Arduino, or the like, rather than the analog input of my motor/servo driver.
Am I correct?
-Joe
No, it’s just a load cell.
You’ll need one RB-Dfr-519 for each load cell.
If you add a compression spring, you won’t know the actual load since the force exerted by the spring depends on its displacement (you can try to work out the equations if you want)
I have done more reading, and I am not sure if you are mistaken, or I am simply not familiar enough with the terminology and therefore am misunderstanding your answer.
There seem to be two different types of strain-gauge load sensors. The one like the RB-Phi-120 actually has 4 strain sensors inside, and are arranged internally like a Wheatstone bridge. They have the 4 wires necessary to make the connections.
But, the RB-Spa-488 is different and has only 3 wires. If you use 4 of the RB-Spa-488, you can form a Wheatstone bridge. Your circuit board Load Sensor Combinator RB-Spa-122, is an easy way to do this.
This page explaings the differences;
learn.sparkfun.com/tutorials/ge … load-cells
So, for my application, I have two possible approaches, both of which require that I have something like an Arudino that can accept the digital data;
1st approach;
Qty 4 of the $7, 50 Kg Micro Load Cell RB-Phi-120
Qty 1 of the $90, Phidgets PhidgetBridge Wheatstone Bridge Sensor Interface RB-Phi-10 (this accepts 4 separate Wheatstone bridge strain type sensors)
2nd approach;
Qty 4 of the $9.95, SFE Load Sensor-50kg RB-Spa-488
Qty 1 of the $1.95, SparkFun Load Sensor Combinator BOB-13281(this wires the sensors as a Wheatstone bridge)
Qty 1 of the $9.95, SparkFun Load Cell Amplifier - HX711
(or alternatively, Qty 1 of the $16.90, DFRobot HX711 Load Cell / Wheatstone Bridge Interface RB-Dfr-519)
Obviously, option 2 is the cheapest. And if I go with the DFRobot RB-Dfr-519, I have the option of just using the analog voltage to drive a separate servo board.
For other readers, I should also mention that there is a 3rd type of load sensor that you sell. This is the variable resistor type. They simply change their resistance to pressure, and can be read directly by an Arduino without using an amplifier. However, they are prone to drift if the load is kept on them, and are not nearly as accurate as the straing gauge type of sensors.
-Joe
You just need to check the output of the sensor - if the output is really small, it will need to be amplified.
