DAC circuit to control a motorized device that has positional end stops in each direction

[John Dorsey]

Hello,

I would appreciate some advice for designing a DAC circuit to control a motorized device that has positional end stops in each direction.

The motorized device provides the following signals to us:
Ground
COM (+5V)
Signal Com +V (+7.5V)
Signal Com -V (+2.5V)

To move the motor in each direction to the end stop, we need to create a voltage that varies between +2.5V and +7.5V. A center voltage of +5V stops the motor.

I will be using a PIC18F26K22 micro-controller powered with 5V and will communicate with the DAC using I2C or SPI.

The DAC resolution can be 12 or 14 bit. I expect the current requirement is very low for the analog signal to the motorized device. <50 mA.

Thanks for any design suggestions.
John

 

[Les Jones]

I think you should start by finding out what the required drive current is before starting the design. Just a guess is not good enough. Have you considered using the PWM output passed through a low pass filter followed by an OP amp to shift the 0 to 5 volt signal that you would get out of the low pass filter to a 2.5 to 7.5 volt signal. This would be simpler than using a DAC.

Les.

 

[John Dorsey]

Thanks Les.

I agree that knowing the exact current requirement would be ideal and I will try and find out what it is. Meanwhile we can assume a worse case of 100 mA.

There are three motorized devices (MD) that need to be controlled by the PIC and that's why it seemed appropriate to use 3 low-cost DACs such as the Microchip MCP4725 on the I2C bus. The 18f26K22 has only two PMW outputs and they are 10-bit which is probably not a fine enough resolution. Your PMW suggestion is a good one but might not be the ideal solution here.

An important factor is that the motors are held in a stopped position with the +5V reference voltage provided by the MD. Perhaps that could be used as Vref on the DAC.

Do you think it would be feasible to use the MCP4725 DAC along with a suitable op amp to attain the +7.5V maximum and have a simple method of ensuring that the +5V for the motor stopped position is respected?

My analog electronics knowledge is quite limited so I really appreciate your insight.

John

 

[Les Jones]

I do not know of any op amp that would meet your requirements. My original suggestion was using a standard op amp followed by an output stage using transistors or mosfets to give the required current output. But because of your very high accuracy requirements it is beyond my expertise to design an interface to meet your needs.

Les.

 

[John Dorsey]

Thanks anyway Les. I appreciate your effort.

John