[simpits-tech] USB Servo controllers, Synchros, & Steppers - Course 101
Marv De Beque
simpits-tech@simpits.org
Sun, 17 Mar 2002 10:09:44 -0500
Steve,
The best solution for gizmos like that would be to use either a synchro
or a stepper motor. Servos can't rotate continuously (normally).
Steppers are nice, but you still need to have a mechanism to recognize
home position and then count from there. They are also great for jobs
that require torque like HSI and ADI. Their down fall is resolution.
Each step has a finite amount of movement and the result can look jerky
if the resolution between steps is not very fine. You can accomplish
this with gears to reduce the movement between steps.
Synchros are really cool. I have been learning about them (haven't
forgot you, Gene) There is a reason why they are used in real
instruments and that is because they are VERY precise! However, they
don't have much torque, so they can't drive large amounts of mass like a
stepper can.
A synchro is essentially a compass needle with two electromagnets
located at the perimeter of the needle, one at the North pole and one at
the East. By varying the magnitude and the polarity of the current
induced in the two electromagnets, you can make the needle point
anywhere on the dial! The two electromagnets are fed voltages that are
multiplied by the sine and the cosine of the desired angle which you
want the needle to point. If you feed the two electromagnets AC signals
that are 90 degrees out of phase the needle will spin, completing one
revolution for each cycle of AC.
You can also reverse the process and induce two AC signals with a
synchro and use it as a means to indicate the rotational position of an
object.
Another issue for the synchro is the interface. There are chips made to
do this, but they cost about $100. A hobbiest can roll their own. You
simply need two digital to analog converters and a small
microcontroller. The micro would contain a sine lookup table and could
translate a digital value (say for instance, fuel quantity) to the two
digital equivalents which are then output to the two digital to analog
converters that feed the synchro. You need a good set of digital to
analog converters with enough resolution and speed or it will look like
a stepper motor when it moves. You can add a small capacitor to the
outputs to take some of the edge off of the movement.
Servos are great for gauges that don't require continious revolution in
any one direction. You can gear a servo to go beyond the 180 degree
limit with external gears or you can modify them to go up to about 270
degrees. I have heard of some people modifying them to do continuous
revolution, but I don't think they work very well. Servos require a
square wave input to produce movement. The degree of rotation is a
function of the square wave's duty cycle (the ratio of time the square
wave is at full voltage versus the time it is at zero volts). There are
limits to the frequency that the signal operates. A microcontroller is
a great way to generate the square wave. You can do it by bit banging
or some micros have a PWM port (pulse width modulated) to create the
square wave. Unfortunately, since you are using a micro to generate the
signal, there is a granularity to the revolution of the shaft. Chester
cites a 0.1 degree resolution. He probably does this with a fairly fast
mico which will make the movement acceptable to the eye.
I look forward to see more of Chester's work!
Marv
On Saturday, March 16, 2002, at 09:58 PM, Steve Wilson wrote:
> Hell yeah! A tenth of a degree of resolution sounds great. But what
> about
> that 90 - 180 limit? Don't we want servos that will revolve round and
> round
> to support things like an altimeter?
>
> Steve
>
> ----- Original Message -----
> From: "David Lawley" <dlawley@carolina.rr.com>
> To: <simpits-tech@simpits.org>
> Sent: Saturday, March 16, 2002 5:42 PM
> Subject: RE: [simpits-tech] USB Servo controllers
>
>
>> Ok, I have one of these on the way. They are $110 ($50 controller
>> only)
>> for a quad unit that comes with 4 servos. During email between myself
>> and the developer I had written that I wanted to us these to drive
>> gauges. He said it would send out a newer version of the software that
>> allows 0.1 degree resolution. This sounds much better than what the
>> SSC
>> unit allows at .36 @ 90 and .72 @ 180 degrees of motion plus its USB
>> driven.
>>
>> If this sound interesting to any of you I'll ask if I can post my
>> contact's email for more information.
>>
>> -----Original Message-----
>> From: simpits-tech-admin@simpits.org
>> [mailto:simpits-tech-admin@simpits.org] On Behalf Of David Lawley
>> Sent: Tuesday, March 12, 2002 8:08 PM
>> To: simpits-tech@simpits.org
>> Subject: [simpits-tech] USB Servo controllers
>>
>> Anyone know about this device?
>>
>> www.cpsc.ucalgary.ca/grouplab/developers/USBPhidgets/GlabServo.html
>>
>>
>>
>>
>>
>> _______________________________________________
>> Simpits-tech mailing list
>> Simpits-tech@simpits.org
>> http://www.simpits.org/mailman/listinfo/simpits-tech
>> To unsubscribe, please see the instructions at the bottom of the above
> page. Thanks!
>
> _______________________________________________
> Simpits-tech mailing list
> Simpits-tech@simpits.org
> http://www.simpits.org/mailman/listinfo/simpits-tech
> To unsubscribe, please see the instructions at the bottom of the above
> page. Thanks!
>