IRobot 690 - raw wheel encoder counts change with no movement command issued

Question

I am using a Raspberry Pi 3, B+ pertinent libraries are WiringPi, with WiringSerial. Code is written in c.

EDIT: the Serial connection is handled through GPIO 14(Tx) & 15(Rx), 5v (Tx) is stepped down through 165 ohm resistor and, a 3.3v Zener Diode.

The following code is what I use to retrieve and and convert Raw Encoder Count to 16bit value.

int16_t sensors_left_encoder_init() {
int16_t left_encoder_start = 0;
char buffer_l[2];
int i = 0;

serialFlush(fd);
serialPutchar(fd, 142); // Request sensor packet
serialPutchar(fd, 43); // Left encoder request
do {
    buffer_l[i] = serialGetchar(fd);
    printf(" -> Left Serial Value: %d\n", buffer_l[i]);
    i++;
} while (serialDataAvail(fd));
left_encoder_start = (int16_t)(((buffer_l[0]) & 0xFF) << 8 | (buffer_l[1]) & 0xFF);
printf(" -> Left Encoder Start: %" PRId16 "\n", left_encoder_start);

return left_encoder_start;
}

int16_t sensors_right_encoder_init() {
int16_t right_encoder_start = 0;
unsigned char buffer_r[2];
int i = 0;

serialFlush(fd);
serialPutchar(fd, 142); // Request sensor packet
serialPutchar(fd, 44); // Left encoder request
do {
    buffer_r[i] = serialGetchar(fd);
    printf(" -> Right Serial Value: %d\n", buffer_r[i]);
    i++;
} while (serialDataAvail(fd));
right_encoder_start = (int16_t)(((buffer_r[0]) & 0xFF) << 8 | (buffer_r[1]) & 0xFF);
printf(" -> Right Encoder Start: %" PRId16 "\n", right_encoder_start);


return right_encoder_start;
}

I pulled the battery, ran the code and the first run produced:

-> Left Serial Value: 0 -> Left Serial Value: 1 -> Left Encoder Start: 1 -> Right Serial Value: 0 -> Right Serial Value: 2 -> Right Encoder Start: 2

However the second run through produced this:

-> Left Serial Value: 0 -> Left Serial Value: 3 -> Left Encoder Start: 3 -> Right Serial Value: 0 -> Right Serial Value: 4 -> Right Encoder Start: 4

I have a feeling its how serialGetchar() is receiving the serial output from the interface, but I am not 100%.

From reading the wheel encoder topics on here, and the OI Specs, I understand that the raw values should not change unless a drive command was issued to the interface.

Answer 1

I'm not familiar with the Rpi3 UART capabilities as I'm using a different controller with a hardware UART, but I can report that others have had difficulties using software-based UARTS with later generation of Roombas, i.e. Series 500 and later

https://www.ez-robot.com/Community/Forum/Thread?threadId=10953

I suggest adding a USB-based UART dongle and see if that solves the issue

I think you are indicating that you are using a Roomba 690, which I thought was Wi-Fi only. If that's the case, how are you connecting to it?