12ADC-1 ADC Card - for higher resolution position feedback

 

In the motion platform servo drives described on the site standard potentiometer position feedback to the 64SPU-1 signal processor card is at 10 bit resolution - 1 in 1024.

 

12 bit (1 in 4096) MA3 absolute position encoders provide a significant improvement in the position feedback resolution for the DIY motion platform drives and provide smoother motion control. They are however a bit expensive and can only be purchased directly from the US based manufacturer US Digital.

 

The new 12ADC-1 card is a 12 bit resolution analogue to digital converter card that can be used with the 64SPU-1 signal processor card to increase the resolution of your motion platform position feedback - without the cost of MA3 encoders. It can be used with standard precision potentiometers to provide a level of position reporting very similar to that of the MA3 absolute encoders and, when used with the 64SPU-1 card, at a faster refresh rate.

 

 

The 12ADC-1 uses the Microchip Technology MCP3204 ADC chip to provide 12bit conversions for up to 4 input channels - although only three channels are used in the 12ADC-1 default programming for 3 DOF motion drives. The MCP3204 chip is interfaced using an on-board PICAXE 20X2 64MHz microcontroller which is programmed to export the 12bit position conversions via a single wire serial data connection to the 64SPU-1. The card can export smoothed 12bit conversions for 3 channels at approximately 70 Hz and can lift the 64SPU-1 servo loop refresh speed to the same rate. Each data set at 70Hz is the sum of 5 analogue to digital conversions for each channel to produce the smoothed output.

 

The 12ADC-1 is easy to wire - see the datasheet for details.

 

PERFORMANCE

 

The relative performance of the 12ADC-1 is illustrated in the image left (click to open in a new tab).

 

The purple traces show sample position feedbacks to the 64SPU-1 for standard 10bit potentiometer, 12bit MA3 encoder and 12bit 12ADC-1 pot feedback. The traces are highly zoomed to make the noise levels visible - each trace has a vertical division of 0.1 parts in a scale of 255.

 

You can see the improvement in both 12bit feedback methods over the top 10bit pot feedback. Also clear is the similarity between the two lower traces showing the 12bit MA3 encoder and 12bit ADC-1 pot feedback. Note that the larger steps in the bottom trace are due to the platform changing position slightly.

 

The 12ADC-1 was connected to a Vishay Spectrol 533 precision potentiometer using shielded grounded cable. Care was taken to electrically isolate the pot shaft and body from the drive electric motor to prevent noise transmission to the feedback line.

 

AVAILABILITY

 

The 12ADC-1 card is available to buy now - follow the link to the order page.

 

64SPU-1 firmware B programming will be required to enable the 12ADC-1 card to be used with the 64SPU-1. Owners of firmware A 64SPU-1 cards (ie cards ordered prior to 8/8/2010) will need to re-program their cards with firmware B - please email me for the firmware B flash program and instructions.

 

 

HARDWARE DEVELOPERS

 

The PICAXE 20X2 chip used on the 12ADC-1 card to interface with the MCP3204 ADC chip is socket mounted and can be removed from the card for reprogramming. Options for reprogramming could be to increase or reduce the number of ADC channels, change the sampling for faster refresh rates, change the serial output baud or format or to add two-way comms to implement "conversion on request" functionality etc.

 

For assistance to programmers the default .bas programming is available on request to customers to illustrate the core MCP3204 SPI interface coding and data output options.

 

This capability will mainly be of interest to developers familiar with PICAXE microcontroller programming. For readers who are not it is probably worth explaining that PICAXE microcontrollers are easy to program using a BASIC type language and free programming software provided by the supplier.

 

The mapping between the 20X2 pins and the 12ADC-1 output connections and 5 pin header are shown on the image right. The 20X2 hardware serial input and output pins and pwm output pin are mapped to the main digital output connector on the 12ADC-1. The hardware serial pins can be configured to run with either normal or inverted TTL level serial comms at high baud rates. The 5 pin header provides options for programmed jumper settings.

 

 

 

 

 

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