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DC and Servo Motors

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stobbz

New Member
Hello, I'm curious to know if the following circuit would operate in the same way using a servo motor instead of a DC motor.

**broken link removed**

If so, would it have to be a particular variant of servo motor?

Any help appreciated :)
 

ronsimpson

Well-Known Member
Most Helpful Member
It looks to me that the OP-amps look at the position of the motor and compares that to the position of the 10k Pot. The difference is amplified and used to drive the motor. I thing this will almost work. The motor will osculate, turn right/turn left very fast. Place a capacitor across the 50k pot to slow down the amplifier. The amplifier's response needs to be slightly slower than the motor's response time.
 

stobbz

New Member
Ahh that's a relief, thanks a lot. As for the general operation of the circuit, the output from the photo resistors goes to the buffer and then to the comparator. The output from the comparator biases the PNP/NPN transistor pair to turn the motor either clockwise or anti-clockwise as required. The circuit was designed for a DC motor, but I wasn't sure of the implications of using a servo, thanks again!
 

stobbz

New Member
It can be any variant of servo motor, whichever would work best with this particular circuit I'd imagine. My project description just insists on using 'a servo motor', doesn't specify a particular type.
 

ronsimpson

Well-Known Member
Most Helpful Member
If I understand right; Servo motor is any motor with feedback. Your CDS and amplifier makes this project 'servo'.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Servo Mount can also mean the particular mount. It's an eccentric ring and a clamp. It makes it really easy to mount so it engages a gear.
 

jpanhalt

Well-Known Member
Most Helpful Member
I don't see how the op-amps get feedback on the motor's position? It looks to me like the original circuit simply turns on or off the DC motor depending on the light intensity.

I suspect the servo motor would want something besides a straight DC supply.

John
 

stobbz

New Member
I don't see how the op-amps get feedback on the motor's position? It looks to me like the original circuit simply turns on or off the DC motor depending on the light intensity.

I suspect the servo motor would want something besides a straight DC supply.

John

That's correct, it also changes direction of rotation depending on which sensor is receiving the most light. So this design may not work with a servo motor?
 

ronsimpson

Well-Known Member
Most Helpful Member
We don't know how the sensors work. He may have a way of detecting angle of rotation. I was thing of a offset piece on the shaft that at full CCW would block the light from one sensor and let light to the other. In the middle 50% of the light would be blocked for each sensor.
 

xainnasir

New Member
the simplest servo motor circuit is ...... PIC18F452 ====> CD4050BE =====> Servo Motor ...... n u have to do a little programming in PIC ..... if u r intrested i can provide the whole schematic and code as well ..... as i just did this project today ......
 

jpanhalt

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Most Helpful Member
A voltage-controlled servo motor might work, but I don't suspect that is what you have, nor is it what you have drawn. Most servos are controlled with a pulse-width modulated signal on a third lead (straight DC motors don't have a signal lead). Simply applying voltage to the power leads of a servo motor does nothing.

John
 

stobbz

New Member
Ok, that's a shame. I thought there may be some variant of servo motor which would work with this particular design. It's looking more and more likely that, if I have to use a servo motor, I'll probably be better off incorporating some kind of microcontroller, rather than searching for a completely analogue solution. Thanks.
 

jpanhalt

Well-Known Member
Most Helpful Member
I think an analog solution to making a servo motor move in proportion to the intensity of light would be reasonably simple. You need to convert the voltage into an appropriate pulse width at set repeat rate. I am sure it can be done with two LM555's*, maybe even one.

Is that what you really want to do, and do you really need servo control? Why?

John

*One would be set up as a voltage controlled one-shot to give the right pulse width (e.g., the voltage on pin 5 would control pulse width) and the other would control the repeat rate by giving a pulse at pin 4 (reset) of the first 555.
 

stobbz

New Member
It's actually a solar tracker project and the initial specification requires a servo motor be used. Still in the initial stages and I think it was assumed that we would be using a microcontroller. Like I said though, unless it's going to be too awkward, I'd prefer to achieve the functionality without using a microcontroller.
 

jpanhalt

Well-Known Member
Most Helpful Member
Are you at all familiar with the 555 chip? Do you have access to an oscilloscope?

John
 

stobbz

New Member
I am familiar with the 555 chip, unlike servo motors, I've used them in previous labs haha. And yes, I do have access to an oscilloscope.
 

Reloadron

Well-Known Member
Most Helpful Member
I'll tell you what I think (operative word is think) the posted circuit is. This circuit looks like and reminds me of a circuit I have worked with, though a decade has passed. I do not think the motor shaft is linked mechanically to the pot. Years ago I worked with AVC (Arc Voltage Control) units. We looked at the actual arc voltage gap and the voltage across that gap. As the weld head traveled around a part with an uneven surface the idea was to keep the distance between the tungsten electrode and the workpiece a constant. The distance between the tungsten electrode and workpiece was a function of the voltage drop across the gap.

Forget the photo cells for a moment. They are a varying voltage. Depending on the light dark relationship, acting as a voltage divider, they output a voltage that is a variable. The reference pot in the circuit, however, is a fixed voltage. The pot is not ganged or mechanically connected to the motor shaft. The first opamp is a buffer. The second opamp is a differential amplifier that will output the difference between the variable from the photo cells and the fixed reference voltage. That difference will drive the motor seeking a null or zero difference. If I set Vref = 6 volts and the output of the LDR divider is 9 volts that delta of 3 volts will drive the motor till the delta is zero volts and the motor stops.

The gain pot in the circuit sets the gain. If the gain is too low the motor response will be sluggish or non existent, if the gain is too high the motor will jitter. The idea is to set the gain for a nice smooth motor response.

I ran a simulation on this that looks like hell for wiring as following a long work week and starting a nice 3 day weekend I am sitting here enjoying a few beers and tired. However, I believe the circuit is using the motor (like driving a rack and pinion gear) as a servo motor. I can clean up the simulation tomorrow and post it if anyone is interested. But that is what I believe you have in that posted circuit.

Ron
 
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