The Emergency Board gives the operator of the ConceptCar a chance to remotely do an emergency stop. To seamlessly fit into the car, it adheres to our form factor. It has the following features:
- The emergency signal is given by an extra radio transmitter, sending on a frequency other then the main control unit.
- The emergency signal reaches up to 1.5 km (transmitter is a SHR-7)
- The emergency stop is performed completely independent of the rest of the electrical system's state
- It works even when the power supply of the electrical system is depleted
Pushing any of the 3 Buttons on the emergency remote control sets the car in a stop mode. This means that the brakes are closed, throttle is set to zero and the steering is set to straight forward. Only if all 3 buttons on the emergency remote control are pushed simultaneously or if the driving battery is disconnected, the car will leave the stop mode and reset to running mode.
The current mode is shown on the red EmergencyLED which is switched on when in stop mode, and off when in running mode.
How the Emergency Board works
The Emergency Board is in the signal path of the PWM signals for actuators, between the ActorBoard and the motor and steering. If an emergency signal arrives, the signals that come from the ActorBoard are being replaced with stop signals generated from the EmergencyBoard itself. The replacement is done by simply switching a multiplexer.
The signal transmitting part works with a SHR7 decoder board and a fully assembled transmitter unit. The decoder board receives the signal from the transmitter and generates a code that contains information on the sending device and on which buttons on the 3-button-control were pushed.
The SHR-7 Decoder receives signals via the 433,920 MHz band and decodes them to a digital signal (shown in the next section).
- Sending via 433,920 MHz
- Bandwidth: 20 Hz to 2kHz
- Supply Voltage: 7 to 14 V DC
- Current (active mode): 6,4mA
For more Information refer to this documentation (PDF).
The signals arriving from the shr7 decoder are binary. The information is encoded by the length of the high and low periods, a long period is 1ms, a short one 0.5ms (with a deviation of +/- 5% to be in safe margins). As depicted in the figure to the right, four distinct character codes are transmitted. From multiple characters a whole transmission frame is constructed.
- Synchronization (S)
- A transmission frame is always lead by a synchronization preamble. Before this, there is a high-signal (logical "1") period of at least two long signal-time-periods. Unlike the other signals it is 9 pulses long.
- - (Minus)
- Character code short-long-short-long
- 0 (Zero)
- Character code long-short-short-long
- + (Plus)
- Character code long-short-long-short
From these signals transmission frames are composed. A frame starts with a synchronization character, and then consists of 18 following +, - or 0 characters as shown in the figure below. The Remote-ID characters x1 to x8 hold the keycode that is configured in the SHR-7 remote device. The "KEYS" characters y1 to y3 encode if keys are pressed, a + character means key is pressed. For the x and y characters +, - and 0 are valid, all other characters must be a precise match.
The measurement of the pulse length is done by a hardware timer/counter. The decoding of the signal is then handled by a state machine, depicted below.
This state machine is built with Simulink Stateflow, and from it C code is generated with the Realtime Workshop Embedded Coder. This code is deployed on an ATMEGA88.