Understanding the differences between Gyroscopes and Accelerometers

Question

Hello all,

I apologize for what is likely an ignorant question, but I am fairly new to electronics and have little confidence. I am working on a personal project where I am using a servo motor to move a rod back forth 90 degrees (not exactly, but this analogy should suffice), and I am interested in measuring the instantaneous (angular?) acceleration (I want to use it to calculate torque). Surprisingly, I have come across trouble while researching online for a sensor to do this. My understanding now is that an accelerometer is only capable of measuring linear acceleration (which means the object can't be rotating, right?), while a gyroscope only measures angular velocity when an object is spinning about an axis. Technically, the rod in my project isn't spinning-it's just rotating back and forth between two angles (0 and 90) on the XY plane. Is there a simply way/sensor to do this? More broadly, am I completely misunderstanding the uses of gyroscopes and accelerometers? I hope i explained the issue properly...

michaelkellett · Answer

Part of your problem is that for some reason most manufacturers and users of MEMS accelerometers and angular-sensing-things usually describe rate sensors as gyroscopes.

Take an example from Farnel, 2433080 - may not do for your purposes but cheap and OK to explain:

from the second paragraph of the description in the data sheet:

It includes a sensing element and an IC interface

able to provide the measured angular rate to the

external world through digital interface (I2C/SPI).

So it actually measures angular rate. Your rod is twizzling back and forth so the rate will be changing (only you know by how much and how fast) but this device has selectable ranges and bandwidth (from 1 to 56Hz) so it may be OK. To measure the acceleration you will need to differentiate the rate signal - so you'll need a micro to read the sensor and do the maths (you might be able to find an analogue sensor and avoid using a micro). This system will not measure the angular acceleration instantly - there is inherent delay in the filtering and signal processing (not to to do with how fast the processor is but more fundamental).

If you must actually detect acceleration I can tell you how to make an angular accelerometer but you don't want to go there unless you have to.

MK

michaelkellett · Answer

It may be that the cheapest solution is to use a low cost optical shaft encoder,

Farnell 2401104 (about £40) does 3600 cycles per rev which gives you 4 x 3600 = 14400 edges per revolution or 40 edges per degree. At 100 degrees per second this is 4000 edges per second and an Arduino should be able to keep up and record the data.

This will be much more accurate than a rate gyro.

MK

michaelkellett · Answer

If you want to know torque then a really good rule is to measure it as directly as possible - if the rod speed is largely independent of the human load but it's the human load you want to measure then trying to estimate it by acceleration will be really inaccurate. The simplest way to measure torque is to measure the motor current but this may not be very accurate due to losses in the gearbox etc. If the rod is accelerating sufficiently fast that the acceleration torque is comparable with the human torque you can calculate the amount of torque needed to accelerate the rod and subtract it. You could make or buy a torque sensor. You could use a low cost load cell to measure the force the human applies so long as you can arrange for that force to applied via the load cell. This will be the cheapest and most accurate way. What's your budget for the sensors ? MK

michaelkellett · Answer

You can buy little load cells quite cheaply (they are designed for weighing scales and might https://www.sparkfun.com/products/10245 - this isn't really sensitive enough for your purposes. In order to use two of these you would need to be able to machine metal parts (or get that done for you) . There are loads of cheap torque sensors made for automotive power steering but of course they are not documented for one off use and are hard to get except as spares. If you can glue strain gauges to a metal shaft you can make your own torque sensor but you don't want to try this unless you have or can borrow some experience in bonding gauges. If we stick with your motor current then all you need to do is to average the pulse width modulated signal - if you can sample fast enough you can do this digitally. Calibration is easy - wind a string (thin) round the rotor of your servo + gearbox (possibly adding something to increase the diameter.) Hang a weight off the string and hold the weight still against gravity with the servo - you know the force and the diameter of the shaft so the torque is easy to calculate. It all gets much harder with dynamic systems because the inertial forces must be estimated or measure - in your system just make the rod light and ignore it. Using the motor current is so cheap I suggest you try it first and see how you get on. What sort of current sensor did you get ? MK

michaelkellett · Answer

@ John, torque=moment of inertia * angular acceleration, true, but only applies to the torque needed to move the bar, tells you nothing about the torque applied by the human. If the bar has significant mass you will need to measure the torque applied by the motor, subtract the bar acceleration torque and possibly subtract the torque due to gravity acting on the mass of the bar. I've attached a pair of drawings produced on a special high tech mechanical simulation package. The top sketch shows the calibration setup. The lower sketch shows what I think your set up looks like (how about some photos from you - it would help). Remove the rod to calibrate, the servo should be holding the weight still so the force of the servo is balanced against the force of gravity acting on the weight. If the pulses from the current sensor are too fast then integrate them with a filter (anything from an RC to something active and exciting but without numbers I can't advise). MK

Understanding the differences between Gyroscopes and Accelerometers

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