RC Hardware

What is an RC transmitter?

SMACCMPilot may be flown using a Radio Control (RC) transmitter. When we say “RC” we are talking about the general range of hobbyist radio control systems built for flying RC airplanes, helicopters, and the like.

An RC transmitter is a simple and reliable way to manually fly a small quadcopter. In the stabilize flight mode, the SMACCMPilot flight controller essentially serves to make a quadcopter system manually controllable - manual inputs command throttle, pitch angle, roll angle, and yaw rate. We will map these inputs to the four joystick axes of the RC transmitter. We’ll also map the control sequences to arm and disarm the flight controller to use a switch on the RC transmitter as a kill switch, which can turn off the quadcopter motors at any time. This is a critical safety feature.

RC Transmitter Joystick Functions

RC Transmitter Joystick Functions

A Safety Controller

SMACCMPilot uses the RC system primarily for safety. Because SMACCMPilot is still experimental and cannot safely fly without human guidance, the RC system provides a pilot a way to disable autonomous flight or the helicopter motors themselves at any time.

We envision that one day, SMACCMPilot will be smart enough to fly & land safety without requiring an RC system for safety. However, for now, we recommend that SMACCMPilot users get comfortable flying the vehicle under stabilize mode, and always have a safety pilot holding the RC transmitter when flying in autonomous mode.

Improper operation of any quadcopter can cause injury. Only operate a safe distance away from people, and always have an operator spotting the vehicle and ready to hit the kill switch.

Compatible RC Transmitter and Receivers

RC transmitter systems come in many shapes in sizes. Each transmitter system will also be compatible with some set of RC receivers. We need a transmitter and receiver system with the following features:

  • Aircraft or helicopter style transmitter with two joysticks
  • Transmitter offers at least 6 channels of control
  • Receiver offers PPM output

Mixing information

RC transmitters typically require some amount of setup to configure the mapping of control inputs to output channels. SMACCMPilot requires a radio with at least 6 output channels.

RC transmitters use various schemes to “mix” input sticks and switches to the channel outputs. Fundamentally, each channel is sent from the receiver to the flight controller as a pulse-width modulated (PWM) wave. (Pulse-position modulation, PPM, is a scheme used to multiplex multiple PWM signals serially.) Whether modulated as PWM or PPM, each channel has a width in the time domain, measured in microseconds. RC transmitters generally output a channel with a width ranging from 1000us to 2000us. If your RC transmitter uses a different range, it may not work with SMACCMPilot.

SMACCMPilot expects radio channels according to the following scheme:

  • Channel 1 controls roll.
  • Channel 2 controls pitch.
  • Channel 3 controls throttle.
  • Channel 4 controls yaw.

  • Channel 5 selects flight control mode. At this time, SMACCMPilot supports three flight control modes: stabilize, and altitude hold, and autonomous. The user will typically map channel 5 to a 3-position switch on the right side of the controller. Channel 5 pulse widths correspond to the following modes:
    • 1000-1250us: autonomous mode
    • 1250-1750us: altitude hold mode
    • 1750-2000us: stabilize mode
  • Channel 6 is the arming switch. It is designed for safety: if, at any point, the arming switch is released, all of the motors will disarm (no more power will be applied) instantly. The arming switch must be set before the motors are armed via either the RC controller or via a telemetry command. The user will typically map channel 6 to a 2-position switch on the top left of the controller.

Below is a recommened configuration for FS-TH9X radio controller, shipped as a default with Iris+: FS-TH9X radio controller

Iris+ switches

3-position MODE SWITCH is located on the right side of the controller, and the pilot operates it with their right index finger (while keeping the right thumb on the roll/pitch stick). The modes are:

  • TOP: Manual mode (all control inputs are from the pilot)
  • MIDDLE: Alt-hold mode (pilot controls roll/pitch/yaw, the altitude/throttle is maintained automatically)
  • BOTTOWM: GCS mode (same as Alt-hold, except the GCS operator can send roll/pitch/yaw commands over datalink)

Two position KILL SWITCH (labelled as THROTTLE CUT) is your best friend. Located at the left side of the controller. It is operated by your left index finger (so you can keep your left thumb on the throttle stick) and you should always have a finger on the KILL switch. When engaged (push away with your index finger), the vehicle immediately kills all the throttle and stops the motors. When disengaged (push towards you) it allows the vehicle to be armed.

RC switches

RC switches

Iris+ radio configuration

Your radio should be configured exactly the same:

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Arming via RC controller

The user can arm the motors from the RC controller with the following sequence:

  1. Channel 6 is set (pulse width greater than 1500us)
  2. Channel 5 set to stabilize mode (pulse width greater than 1750us)
  3. Throttle stick is at the low limit (pulse width lower than 1050us)
  4. Yaw stick is at the high limit - fully to the right (pulse width greater than 1950us)

After this sequence is complete, the motors are armed. When the throttle stick is raised, motors will start spinning.

The vehicle is disarmed at any time by resetting the channel 6 switch.

More information

See our examples page for recommendations and setup instructions for some radio systems we have used.

The PX4 Project wiki has more information about various RC radio systems. Note that the PX4 project software can support more types of radio system input than the SMACCMPilot software. SMACCMPilot only supports PPM input at this time.