Speed and direction converter circuits

V1.01 2-Aug-04

1. Introduction

Some of the circuits on these pages require speed and direction circuits in different formats. For example, a speed controller may require an analogue speed signal, and a digital direction signal. However, the handset may use a single pot that just generates a signal in the RC receiver between 1ms and 2ms.

The signals may be in a variety of different forms depending on what equpiment you may have bought, and other circuits you may have made from these pages or from other sources:

This page describes several converter circuits for converting between the most common formats. Note that the conversions here do not relate directly to steering of the robot. The direction referred to relates to the rotational direction of a motor, not to the direction steered by the motor (although of course it does affect the steering of the robot).

If you are interested in a converter to change two servo channels for robot speed and robot direction (steering) into two PWM signals for driving the left and right motors, see the steering mixer page.

Signal

Form

Speed [1] Analogue value (0 - 5v) for whole robot
[2] Analogue value (0 - 5v) for one motor
[3] Servo signal for whole robot
[4] Servo signal for one motor
[5] PWM signal for one motor
Speed and motor direction [6] Single servo signal
Motor Direction [7] Digital (backwards / forwards)
[8] Single servo signal

2. Convert [4] to [2]

(Converting a servo signal to an analogue signal)

This is accomplished using the Receiver Decoder circuit described here.

3. Convert [2] to [5]

(Converting an analogue signal to PWM for driving a motor)

This is accomplished using the PWM Generator circuits described here.

4. Convert [4] to [5]

(Converting a servo signal to PWM for driving a motor directly)

To perform this conversion without the middle form of an analogue signal, use the digital circuit described in section 2.2.3. here.

5. Convert [6] to [2] & [7]

(Converting a servo signal to analogue speed and digital direction)

This performs the conversion described in the introduction: a speed controller requires an analogue speed signal, and a digital direction signal, but the handset uses a single pot that just generates a signal in the RC receiver between 1ms and 2ms. The centre position of the pot should represent no movement. The leftmost position should represent maximum speed in one direction and the rightmost position should represent maximum speed in the other direction:

A table and graph can be drawn of the required conversion:

Input pulse from RC receiver Analogue speed signal Digital direction signal
(0 to 5v) PWM ratio
1 ms 5 v 100% 0
1.2 ms 3 v 60% 0
1.4 ms 1 v 20% 0
1.5ms 0 v 0% X
1.6 ms 1 v 20% 1
1.8 ms 3 v 60% 1
2.0 ms 5 v 100% 1

A block diagram of a system for generating this conversion is shown below:

The signal is first decoded to a 4-bit word using the digital receiver decoder circuit presented in the Rx Decoder page. The most signficant bit of this is the direction signal, which is a digital signal that changes state half way through the range of values from the Rx decoder. This is used to optionally invert the remaining three bits, inverting them when it is 0 and leaving them unchanged when it is 1. Inverting these is effectively negating the 3-bit value of them. It is this that causes the ‘V’ shape in the required graph. The input and output values of these 4 bits are as follows:

Pulse width (ms) Rx decoder output Output from XOR gates Value of output
1 0 000 111 7
1.0625 0 001 111 7
1.125 0 010 110 6
1.1875 0 011 101 5
1.25 0 100 100 4
1.3125 0 101 011 3
1.375 0 110 010 2
1.4375 0 111 001 1
1.5 1 000 000 0
1.5625 1 001 001 1
1.625 1 010 010 2
1.6875 1 011 011 3
1.75 1 100 100 4
1.8125 1 101 101 5
1.875 1 110 110 6
1.9375 1 111 111 7
2 1 111 111 7

The DAC can be just a simple weighted resistor ladder into an opamp summing junction. This can generate the required output voltage between 0 and 5v. Note that since this is a digital circuit, the V characteristic will be stepped rather than smooth as in the graph above, but this won’t matter much for speed control – it will be barely noticeable. If necessary, the 0-5v signal can be inputted to a PWM generator circuit to generate a PWM waveform for the speed signal.

The complete circuit diagram (except for the digital RxDecoder and PWM generator which are shown elsewhere) is shown below:


Click on the circuit diagram to open it in a new window.

A SPICE simulation of the circuit is shown below which confirms its required operation.

The XOR gates conditionally invert the Q0..Q2 signals when Q3 is high, that is when the receiver pulse width is between 1.5ms and 2ms. Otherwise, for receiver pulse widths between 1.0ms and 1.5ms, Q0..Q2 are left un-inverted. The outputs of the three XOR gates are fed into a DAC, which is implemented as a simple adder circuit. This sums a +5v signal and the negative of the three inputs according to the following gains:

Input Gain
Q2 4/7
Q1 2/7
Q0 1/7

The output voltage is then given by the equation:

Vout = 5 – (4/7·VQ2 + 2/7·VQ1 + 1/7·VQ0)

Note that the opamp must be a rail-to-rail type for inputs and output to be able to accept inputs in the range 0 to 5v, and to be able to output 0v - 5v. Given the inputs are logic 0-5v, the output voltage, depending on the input code of Q2..Q0, is:

Q2.Q1.Q0 Q Value Output voltage
000 0 0.00v
001 1 0.72v
010 2 1.43v
011 3 2.15v
100 4 2.85v
101 5 3.57
110 6 4.28v
111 7 5.00v

As explained previously, the output is stepped because of the digital nature of the input pulse width measurement.

6. Devices used in this circuit

The following devices were used in this circuit. Click on the manufacturer’s name to go to their web site, or the device name to go to the device datasheet.

Manufacturer Device
Linear Technology LT1218 rail-to-rail opamp

Philips Semiconductors

74HC86 Quad XOR gate

7. Links

Most of these links are to commercial products. I neither endorse nor decry these products, since I've never used any of them. If you feel you are not up to making the converter circuits yourself, then you can buy one of these.

The Team Delta R/C to 4QD Interface sits between the RC receiver and a 4QD speed controller, performing the necesary conversion:
http://www.teamdelta.com/products/prod2a.htm

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