03261 [alt-beam] Re: Who was it that made that?Servo Bots / Inspiration

Alt-BEAM Archive

Message #03261

To: Sean Rigter rigter@cafe.net, Chiu-Yuan Fang chiumanfu@home.com,
From: Dennison dennlill@buffnet.net
Date: Fri, 14 May 1999 06:49:26 -0400
Subject: [alt-beam] Re: Who was it that made that?Servo Bots / Inspiration

Actually, surprisingly, SOmeone a while ago came up with this. I don't know
what happened to them, but they were very happy with it. It was a
'microcore' but it controlled the Servo Motors directly. Creating the
propper pulses, and they had it going so that they had a walker that never
saturated. It's was pretty neat, anyone remeber who that was? Or have a copy
of their e-mails?

Dennison

>Hello all,
>
>I had no idea of the volume of stuff I have written since I joined the
>list 6 months ago. I definitely take up the bandwidth but it seems to be
>appreciated. Chiu, thank you for organizing Rigter's Nupages. It would
>have bogged me down a lot to do this myself and just while i'm on a
>"creative roll".
>
>So when the subject on the list touched on my current research project I
>thought I better go on record.
>
>So as a little taste of things to come here are three schematics for a
>new class of Beam Tech : Beam Servo Bots. I have been working on the
>idea of using hobby servos, without ripping their guts out, for a long
>time but only recently had the time to try it out. These three servo
>controller circuits use hobby servos the way they were designed to be
>used but controlled in true Beam fashion with Microcore, Bicore and many
>other Beam like circuits. No proprietary circuits or microcontrollers
>just plain old Beam tech. This could blow the pants of low end RC market
>as well: the implications are truly amazing. Imagine 6 "voltage to
>servo" controllers using a single 74HC14. No more springs or limit
>switches! Proportional controlled steering!
>
>Hobby servos use pulses to adjust the angular position of the output
>shaft. According to the standard these servos are designed to rotate
>approximately 180 degrees. They require a pulse train of positive
>pulses with a width of 300us to 2.5ms for maximum clockwise and counter
>clockwise rotation. The motor hits the mechanical limits beyond these
>values and this can damage the gear box as well as damage the
>electronics. (so warning to all: there is no guarantee that these
>circuits will work for you and the experimenter uses these circuits at
>his own risk).
>
>Servo Heads - proportional control
>
>Hobby servos operate in two modes, digital mode and proportional
>(analogue) mode. The Beam servo controller servohead1 schematic shows
>full-blown linear voltage to pulse width converter using a 74HC14
>Schmitt trigger oscillator which take the output of a pair LDRs, reverse
>(green) LEDs, or PDs and proportionally positions the servo output shaft
>controlled by the voltage at the midpoint of the "optical half bridge"
>(OHB). This OHB approach is nice in that the output voltage is
>proportional to the difference in light level falling on the sensor pair
>and is virtually independent of the ambient light level. Servohead2 uses
>the same idea but with a using a 74HC04 or 74HC240 cmos oscillator.
>This oscillator is a close cousin to the Bicore but uses less
>components. Both of these circuit use the same principle to control of
>the pulse width with a control voltage by converting the control voltage
>to a current with the transistor current source! When a transistor has a
>resistor in the emitter, the current sourced from the collector is
>proportional to the base voltage. Only the time constant of the positive
>pulse is controlled with the control voltage. The time between pulses is
>roughly 2.5ms but can vary a lot. This is not important in this
>application as the servo is not sensitive to small variations in the
>pulse frequency. For these oscillators the current source linearizes the
>relationship between control voltage and pulse width and this is
>important to be able to predict the final position of the servo rotor
>for a given input voltage.
>The action of the Servo Heads is a little different from the
>conventional Beam heads in that they are more like the head of living
>creatures turning 180 degrees and not like the spinning top (bring in
>the exorcist) Beam heads rotating 360 degrees or more. This is nice
>because the head is less likely to strangle itself on it's umbilical
>cord (not a problem for autonomous or solar Beam heads also) To
>stabilize the rotation avoid the shakes and generally reduce power I add
>a cap to the midpoint of the OHB usually 1000pf for LEDs 0.1 for PDs and
>up to 500uf for LDRs. Also watch the current consumption of the PDs and
>LDRs, if very sensitive they can draw a lot of current and should be
>protected with a 1K resistor in series with the OHB +V connection. Green
>LEDs never have this problem and since the input current for the
>Proportional Beam Servo Controller is extremely low it is perfectly
>compatible with the small (ua) reverse LED currents.
>Another application for this Proportional Beam Servo Controller circuit
>would be as the proverbial hothouse controller using two thermistors in
>the half bridge to control a vent based on the difference in
>temperature. Or use a single thermistor and a resistor or a LM34 or
>similar to control vents and valves for the VAC of your hothouse or home
>or ??
>
>
>Servo Core - Digital Servo Controller
>
>Beam circuits require a complimentary power output stage like a BiCore
>with a bridge or 2 microcore stages with 245 buffers for each motor.
>Beam Servo Bots take advantage of the hobby servo build in electronics
>power output circuit. In the Servo Core example a conventional ucore has
>been adapted by greatly reducing the time constant of each stage to
>generate 4 servo pulses which are send to 4 servos with the pulse width
>of the pulses controlled via 4 control voltages. These control voltages
>are generated by a second conventional 4Nv microcore as used in 2 motor
>walkers. Notice that only one microcore stage is used for each motor
>and that the digital output of each Nv supplies the fully left and fully
>right signals.
>As a result when the Nv output is high (75% of the time) the servo is
>turned full clockwise and when the pulse goes low (25% of the time) the
>servo slews to the counter clock position and return to it clockwise
>resting position after the low pulse terminates.
>The effect of this is a wave like (millipede) motion of the servos as
>the process goes around the microcore loop. Of course this can be
>extended to a larger number of Nvs and servos. I haven't used this on a
>platform but it seems like a useful type motion. The 4 servo core is
>just a starting point which can be connected to master slave Bicore and
>even a microcontroller outputs to provide a convenient digital to servo
>interface
>Well I will stop here although there is much more but I am sure there
>will be many other contributions to this fascinating application of Beam
>tech to Servo Control.
>
>Thanks to all the great ideas on this list : they truly are an
>inspiration for me (and I am sure for Mark Tilden as well).
>
>A special thanks to Steven Bolt for the idea of the current source to
>control pulse width , the optical bridge and his many other unique
>contributions. And of course a many thanks to Mark Tilden for starting
>so much of this, staying at the center and somehow keeping the magic
>going.
>
>enjoy
>
>Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
>
>
>Chiu-Yuan Fang wrote:
>>
>> http://www.geocities.com/SouthBeach/6897/rigter/wr_1.html#jan18
>> Wilf posted this message about driving steppers.
>
>Dennison
>
>> I have/had a bicore controlled RC car. It's kinda neat. I used a bicore >
to control the forward/backward motiong, and another suspended bicore to
>> controll the rack and pinion. So it steers twards the light.
>
>James Taylor wrote:
>
>> attach the steering servo to a light seeking bicore,
>> you could easily make a phototropic car.
>> I was thinking about this the other day. if you
>> attach the steering servo to a light seeking bicore,
>> you could easily make a phototropic car.
>
>Josh
>
>> I just got some great inspiration to turn my Radio Controled Porsche
>> into a PhotoTroph.
>> No I haven't learned all that but I will eventually turn it into
>> a Phototroph i am studying its circuts and figuring out how to add
>> photodiodes and such. Also Nv's for aestetics I was going to put the
>> Photo Diode on the Tail and Head Light Holes :) Mail me with any
>> suggestions, Idea,or tips. Thanx !

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