PICmicro + servomotor = big galvanometer

NoureddineBENABADJI

L.A.A.R. Laboratory (DTL team 2), Faculty of Physics, 
University of Sciences and Technology - Oran,

B.P. 1505, El M’nouar, ORAN, ALGERIA,  phone: (213) 041.42.15.81

The following circuit (Figure 1) shows a home made non conventional galvanometer I designed a while ago to sense slow variable physical parameters, like temperature, humidity, atmospheric pressure, etc..., as I was definitively fed up after looking for a big analog galvanometer in local stores (not available, or very expensive…). I have build this prototype to add robustness (classic galvanometer are not suitable for harsh environments) and modularity for maintenance purpose (if the internal coil of a classic galvanometer is damaged, you have to throw it).

Small servomotors are used for precision positioning, generally in robotics. They are driven with pulses of 1ms to 2 ms duration (critical), over a period of 20 ms (not critical). The maximum angle is 90° for the low cost models, most operated at 5V. So, a low pin count PICmicro chosen from the simple baseline family can do the job. With an internal ADC module, the microcontroller peeks periodical measures to be translated into precises duration pulses. In order to keep low-power operation in mind, the microcontroller is put in sleep mode immediately after the active pulse, and the periodic process is provided with the watchdog time-out. (no interrupt has been used, nor PWM mode as sleep mode inhibit it).

For test purpose, a PIC10F220 has been chosen as it is the smallest and simplest 6-pin microcontroller with built-in 8-bit ADC, and, for pedagogical reason, the firmware has been fully commented but not optimized,  and takes less than 256 words. Doing so allows the user to choose any other pin-limited PICmicro (in particular, the XLP generation of Microchip if someone wants extreme low power consumption).

Other circuits I designed with PIC10F.

Other PICmicro circuits I wrote for EDN design ideas.