Page 272 - Making PIC Microcontroller Instruments and Controllers

sElTtt{c uP THE HARDWAnE
CO |{ECT|O|{S 271

If we mount the sensor on rhe base that will be tilring, we will gel euor signals thar
we will interpret, and then output the corecrions ro rhe target table. We lnay have ro
jn
create a lookup table if there is any nonlircarity e:ther lhc respoDse of the sensor or
the mechanical linkages that connect the base to the top. In this case, rhe signal we detcct
is absolutely related to the position ofrhe base. We are rcading rhe actual eror at the
base. What we do with the sigml is up to us, and how we design rhe ljnkages 1() level
thetableis also up to us. This is not the best way to do it in mostcases, but this method
may be the only one available to us in some situarions.
Ifwe mount the sensor to the top table, the signal we get will be a neasure ofhow
far the table has tilted. We can make the table come back !o horizonral if we keeo
mahng an inlegicring cnrreclion !ill rh< rlrble becnmec hori,/onnl: ue 5rup \r hen the enor
signal goes to zero. It is an integntive process over time. (If we wanred ro know how
large the coffection \'r'as, we would have Io keep track of how far we had moved the
table to get it back to horizontal, bur we do not need rhis information in an inlegraring
systern.)
Simply stated: it is better to mount lhe deiector. to the actual table because rhat is the
sudace we are interested in keeping level. It is besr ro ger the error signal as directly as
possibler at the soulce.

Setting the Hardware
Up
Connections
The instnrment/controller
we will create needs to read two inputs ftom the sensor and
put out two oulputs to the servos. Thc inputs are in the form ot two fiequencies we
will read ftom the X-Out and Y-Out conneciors ofthe sensor, and our ouiputs are rhe
two L to 2 millisecond pulses wc will output to rhe radio conrol hobby servos_ See
Figure 19.2.
The PBP language has a conrmand (PULSOUT) that lets us outpur appropriate pulses
on PORTC.O and PORTC.l on the LAB,X l. but in this exercise we will create our own
pulses with the PAUSEUS command. However, we are srill ried !o PORTC.0 and
PORTC.
I because that is how the circuify lbr rhe LAB-X I is laid out. The servo ourpur
pins on the LAB Xl are hard wired ro PORTC.o and PORTC.I. The seNos are fed from
these lwo pins.Ifwe were laying out a new board, we could choose ro use any free por,
pins.
We will use a ten-pin connector P2 on thc LAB Xl for all our connections because
that is the connector that has PORTC.0 and PORTC.I on ir. This will atlow us ro use
PORTC.2and PORTC.3 as the inputs aom fte gravity sensorand so all the connecrions
can be on one fbur-pin cable. The se os can plug into the standard selvo connectors at
J? and J8- The circuitry for this is shown in Figure 19.2.
First, leas write a program to get a feel fbr wbar kind of pulses we are going ro be
getting from the Memsic sensor (we are interested in the pulse width, the frequency, and
the range). To do this, we need to read the pulses from just one of the outputs and dis-
play them on the LCD as we move one axis rhough I 80 degrees. Ler's arbirrarily decide
on the x axis output and write a program ro rcad it. See Program I 9. I _