Serial API does not provide a way of sending a relative angle setpoint. The solution you can use as a workaround: 1. Receive actual angles IMU_ANGLE using CMD_GET_ANGLES (do it only once) 2. Calculate relative angle and send in the CMD_CONTROL + MODE_ANGLE. Remember the last angle you send, and in next time you need to do a relative movement, just increment it. 3. Angle parameter is limited to +-720 degrees, but it's not a problem to go further: just use a native overflow property of a 16-bit variable. With this math the next angle after +720 will be -719.