Smallest RC Throttle Circuit

[safe]

Smallest RC Throttle Circuit

There are a few RC throttle circuits already posted on the internet:

http://endless-sphere.com/forums/viewtopic.php?f=28&t=8160

http://endless-sphere.com/forums/viewtopic.php?f=9&t=11877&p=195857&hilit=+throttleizer#p195857

...and so I'm not the first to be playing around with the idea.

What I want to do is to find the "easiest" way to get something to work. My idea goes like this...

555 Timer Chips

http://www.horrorseek.com/home/halloween/wolfstone/TechBase/com555_555TimerCalc.html

The 555 Timer has a "Control" pin that most people seem not to be using. What the "Control" pin does is it adjusts the upper threshold so that an increase in it's value means a LONGER duty cycle and that's exactly what we want to have happen for our throttle. Most of the servo testers use a potentiometer along with a 555 timer, but the "Control" pin just sits there. So I was thinking you could just connect to the "Control" pin (or start from scratch with a 555 timer) and integrate the throttle signal (0 - 4 volts) so that the throttle goes directly inside the 555 timer to adjust the pulse.

Op Amp Scaling

http://www.stefanv.com/calculators/hp67_offset_gain.html

So now you have a situation where the 555 Timer has a stable position internally of about 2 volts or so when the servo is closed and the motor power is off. If you simply can raise the internal voltage of the 555 Timer it will increase the duty cycle and that moves the servo to the open position. So all that needs to be done is for an Op Amp to scale a range from the throttle of from 0V to 4V so that it instead produces a range of about 1.5V to 4V. That's what this circuit would do.

Okay... now I haven't tried this and am not sure what I might be missing... but it seems like this might simplify things a great deal.

You would just build this simple circuit and then attach it to a servo tester circuit through the "Control" pin.

People that know circuits please give me your opinion...

 

[safe]

This might be even easier... all you need is an accurate voltage reference (can be done with an off the shelf chip) and two resistors.

The way to go about this would be to accurately measure the low and high voltage readings for the "Control" pin on your chosen servo tester and then figure out what you need by reverse engineering. Alternatively you could build in some adjustability, but then that's getting complicated again. (maybe a couple of pots)

 

[safe]

This one uses a single 5 volt voltage supply for:

1. Throttle supply

2. Servo Tester supply (not shown in circuit, but assumed to be 555 timer based)

 

[safe]

Low Cost, Low Part Count

This only requires six resistors, a capacitor, a 555 timer and an accurate 5V voltage regulator and this would allow you to buy an ESC without a BEC. If the ESC had a BEC then you could drop the 5V voltage regulator and just use the ESC supply instead.

..I'm not saying the idea is perfected yet, but it looks pretty good.

You could then add another circuit (separate) that could be for current limiting that would be connected to the throttle before it goes through the resistor and pull the voltage down. By building it up as separate modules you can test each separately to more easily identify problems.

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Note: I'm not actually planning on using an RC motor myself as I'm working on my own Halbach Axial Disc motor concept, but the ESC controllers are cheap and they are sensorless, so it's as good a way as any to go about adapting a brushless controller to a motor. My preference was to avoid doing any of this... (but it's looking to not be as bad as I had first thought) Also, I'm really cheap and so if it can be built for next to nothing that's something I like.

 

[safe]

SPICE

This is the preferred SPICE file. There are a couple out there that really are hard to get not to crash in my SPICE program, but this one works pretty well:

* ICM7555 MACROMODEL
* ----------------------------
* Revision 1.0 4/2006
* ----------------------------
* The ICM7555 is a general purpose RC timer capable of generating accurate
* time delays or frequencies. This device feature and extremely low supply
* current combined with virtually non-existant current spike during output
* transitions.
* ----------------------------
* Connections
* 1 = GND
* 2 = TRIGGERB
* 3 = OUTPUT
* 4 = RESETB
* 5 = CONTROL VOLTAGE
* 6 = THRESHOLD
* 7 = DISCHARGE
* 8 = VCC
*Parameters which are modeled.
* 1) Full timer functionality
* 2) Supply bias current and load current
* 3) Output rise/fall times
* 4) Reset threshold
* 5) Output VOL/VOH
* 6) Discharge voltage with current
**************************
.SUBCKT ICM7555 1 2 3 4 5 6 7 8
XOPAMP 1 2 3 4 5 6 7 8 ICM7555_S

.SUBCKT ICM7555_S 18 11 12 13 14 15 16 10
*****************************************
*COMPARATORS
ES1 A11 18 10 18 1
IABIAS A11 A12 10UA
MA1 A13 A16 A12 A11 MOSP
MA2 A14 A15 A12 A11 MOSP
VAOS A17 A16 0.5M
RAD1 A13 18 10K
RAD2 A14 18 10K
DAC3 A15 10 DA
DAC4 18 A15 DA
GCA1 18 A20 A13 A14 1000M
RCA1 A20 18 100K
VCA1 A21 18 1V
DCA1 A20 A21 DY
DCA2 18 A20 DY
**************
IBBIAS A11 B12 10UA
MB1 B13 B16 B12 A11 MOSP
MB2 B14 B15 B12 A11 MOSP
VBOS B17 B16 0.5M
RBD1 B13 18 10K
RBD2 B14 18 10K
DBC1 B16 10 DA
DBC2 18 B16 DA
GCB1 18 B20 B13 B14 100M
RCB1 B20 18 100K
VCB1 B21 18 1V
DCB1 B20 B21 DY
DCB2 18 B20 DY
**************
RB1 10 B22 60K
RB2 B22 B23 60K
RB3 B23 18 60K
VR1 B23 A15 0
VR2 B22 B17 0
VIN1 A17 11 0
VIN2 B15 15 0
VIN3 B22 14 0
*******************************************
*SR LATCH
EL1 A25 18 10 18 1
RLA A25 A26 10K
CLA A26 18 1P
MLA1 A26 A27 18 18 MOSN
MLA2 A26 A28 18 18 MOSN
RLB A25 B26 10K
CLB B26 18 1P
MLB1 B26 B27 18 18 MOSN
MLB2 B26 B28 18 18 MOSN
MLB3 B26 B29 18 18 MOSN
VFB1 B27 A26 0
VFB2 A27 B26 0
EFB1 A28 18 POLY(2) A20 18 10 18 0 0 0 0 1
EFB2 B28 18 POLY(2) B20 18 10 18 0 0 0 0 1
****************************
*PD
EPD1 B33 18 B26 18 1
RPD1 B33 B34 10K
CPD1 B34 18 50P
*OUTPUT
MO1 B30 B34 A25 A25 MOSPA
MO2 B30 B34 18 18 MOSNA
MO3 B31 B30 A25 A25 MOSPA
MO4 B31 B30 18 18 MOSNA
CO1 B31 18 0.1P
CO2 B30 18 0.1P
VO1 B31 12 0
*DISCHARGE
MD1 16 B30 18 18 MOSNA
*RESET
RRST B29 A25 20K
MRST B29 13 18 18 MOSN
RRSTB 13 18 50G
*SUPPLY CURRENT
ISUP 10 18 12.3U
FSUP 18 A36 VO1 1
DSUP1 18 A36 DZ
DSUP2 A36 A37 DZ
RSUP A37 18 1
GSUP 10 18 A37 18 1
*******************************************
.MODEL DA D(IS=100E-14 RS=0.5K)
.MODEL MOSP PMOS(VTO=-0.7 KP=12.57E-4)
.MODEL MOSN NMOS(VTO=0.7 KP=12.57E-3)
.MODEL MOSPA PMOS(VTO=-2.0 KP=78.5E-4)
.MODEL MOSNA NMOS(VTO=2.0 KP=78.5E-4)
.MODEL DX D(IS=100E-14)
.MODEL DZ D(N=10M)
.MODEL DY D(IS=100E-14 N=0.1M)
*******************************************
.ENDS
.ENDS

 

[safe]

50 Hz Desired

Unfortunately it looks like analog servos do not like high frequencies. Servos are usually designed for a frequency of about 50 Hz and that means an average period of about 20 ms. The pulses are supposed to vary from 1.25 ms to 1.75 ms, so the actual duty cycle is down in the 10% range.

So I need to add one more thing... a diode.

This diode combined with adjustments in the resistor and capacitor values deliver the sparse pulse structure (low duty cycle) that a servo is going to want.

 

[SimpleSimon]

Safe, in case you haven't noticed till now, I'll state it quite bluntly. From the lack of responses here I'd guess no one is terribly interested. Which isn't surprising, as I don't think anyone is very keen on a wireless servo-actuated throttle on their Mb, or even in throttle by wire on one.

 

[safe]

RC Motors are very popular in some circles.

AussieJester used to post here and he has built a bike that uses one.

Anyway... I was hoping to get feedback on this, but it looks like I'm on my own with it.

 

[safe]

Adjustable

This could be it.

If you add a 10K pot into the 555 Timer circuit it will shift the pulse width from narrow to wide across the whole spectrum.

I'll have to post back onto this thread when I build it. (probably in the spring)

5 Resistors
1 Pot
1 Diode
1 555 Timer
1 Capacitor
1 Voltage Regulator

...total part count 10, cost would be maybe $5-$10.

 

[safe]

Efficiency

Speaking of efficiency... I was a little worried about using a linear voltage regulator instead of a BEC because the rumor is that the linear voltage regulator is not very efficient.

So I had to check it out.

http://digikey.com/scripts/DkSearch/dksus.dll?Detail&name=KA278R51CTU-ND ($0.88)

I found a nice little voltage regulator that manages to only drop 0.5 volts for a one amp current. My circuit only draws about 0.3 amps (see the graph) so that comes to:

0.5 volts * 0.3 amps = 0.15 watts

...and if you compare that to a 1000 watt motor draining at full power you get:

0.15 watts / 1000 watts * 100 = 0.015%

So maybe it's true that the MOSFET based voltage regulators can save a little power (somehow) but it's not much. I'll design the wiring assembly so that when I'm not riding the voltage regulator is disconnected. I normally disconnect the power wires after every ride (and plug them into the charger) so if the battery is not connected it can't drain while the bike is parked. I remember a guy that spent $1000 for LiFePO4 batteries and he designed his circuit with a linear voltage regulator that drained the batteries when the bike was parked and that destroyed the batteries when he left the bike in storage for some time. So that's what needs to be avoided.