ramp up circuit

mikecer

I am trying to control a motor brake I need to be able to control voltage (0 to 24volts) and ramp-up time
any suggestions?

mikecer

anybody?

Bean

Mike,
This is not a chat room. Do not expect an instant reply.
Bean.

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KenM

Can you draw a picture of what kind of voltage ramp you want, or be a little more specific on exactly what the end result is.

There are a variety of ways to ramp up "energy" to turn a device on slowly instead of a step input.

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Ken

mikecer

I have a 24volt trigger to my circuit. I need to ramp up my output to my motor from 0v to 24v over an adjustable time period
between 0sec (instantly) up to a gradual increase over 5 seconds. Both the max voltage and the time needs to be adjustable
with potentiometers.

mikecer

The ramp will be linear.

PJAllen

I guess it's a question of making a variable frequency ramp generator whose output amplitude is variable.

You could use a STAMP to step through a DAC (0,1,2,... 255, 0,1,...).· You could use a pot to control a VCO (a square-wave or pulse generator) and use that as a Clock-In/Trigger for the STAMP to use in providing the DAC input data.· The output here would be a linear stair-case.

You could use a ramp generator IC (like from a TV, which may not get you a freq slow enough), but that wouldn't require a STAMP.· The output here wouldn't have a stair-case.

An amplifier is no big deal, depending on the current necessary, and controlling that with a pot is typical.

So, what do you really want here?



Post Edited (PJ Allen) : 1/7/2006 3:45:37 AM GMT

KenM

A constant current into a capacitor will generate a linear voltage ramp. Vary the current and vary the slope of the ramp.

If you need specifics on constructing such a circuit, please write back.

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Ken

Paul Baker

For the constant current -> capacitor setup to work, the current out would need to be constant. Since a motor has a variable resistance (a motor at standstill has a lower resistance than a moving motor), you would need to buffer the voltage. Ordinarily Id say use an omp amp voltage follower to buffer the voltage, but an opamp cannot supply enough current to drive a DC motor.

I think we may be trying to answer his question in the way he stated it and are missing the the obvious solution in the process: PWM (actually PJ Allen was leaning that way with the VCO idea). Instead of trying to drive the motor with an analog DC voltage of a specified value, the simplest (and most efficient method) would be to supply the motor with PWM of increasing duty cycle, this has the same effect as supplying it with an analog voltage but keeps everything in the digital realm (it also decreases complexity, part count, cost and energy loss of the driving circuit). Furthermore PWM is better at producing a linear correlation between motor speed and driving waveform (ie a PWM'ed motor overcomes standstill inertia more easily than a DC'ed motor at low driving rates (duty cycle/voltage), this is actually a theory of mine I have yet to prove, but I believe it to be the case).

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Post Edited (Paul Baker) : 1/7/2006 6:52:54 PM GMT

Paul Baker

mikecer, ask for clarification about using PWM to control the ramp up of your motor if you need it.

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·1+1=10