Simple Digital Proportional Encoder



Simple Digital Proportional Encoder Revision 4

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Thank you for purchasing this product. This encoder was developed for use as a retrofit encoder for vintage transmitters, as an encoder for control-line transmitters, or a basis for robotics transmitters.

Revision 4 has improvements in both hardware and software to improve quality, particularly in timing stability. If you are familiar with Revision 3, you have nothing additional to learn as it is form, fit, and function identical to that version.

Features

• Eight channels, six proportional plus two switched

• Reversing on all channels

• End-point adjustment on all channels

• Direct hardware connection to Futaba style FP-TP style modules including TM-8, Hitec, Corona, etc.

• Center and stick range calibration in-situ. No diddling with stick pots!

• Center alignment to trim to 1.5ms

• Stick pots interface with servo pigtails, allowing easy control order adjustment

• Simple onboard programming, no need to connect to an external PC.

• Dimensions 3" x 1.5"

• Programmed microcontrollers are available separately for integration into custom circuits

• Two model selection

• Extended endpoint range for switched channels 7 and 8

• Channel 7 can have three positions by using a SP3T switch

• SMD technology

Features different from Version 3:

• Crystal controlled timebase

Description

The Simple Encoder hardware is based on an Atmel ATMEGA168 microcontroller, which uses six 2K analog-to-digital converters for proportional inputs and two I/O connections for channels seven and eight, which are non-proportional. The original intent was to emulate an NE5044 encoder, which is no longer in use.

A simplified schematic diagram is shown in Figure 1.

Transmitter voltage Vin (Typically 9.6V, but could be 7 to 12V) is regulated down to 5V for use by the encoder circuits. This also sets the range for the analog-to-digital converters from zero to +5V and this must not be exceeded. User inputs are a rotary BCD switch (0 to F) and three pushbuttons REV, LESS, and MORE. The only display indicator is a single LED.

The microcontroller encoder output is buffered from the external transmit module to protect it from possible high RF levels.

Hookup to external control potentiometers for channels 1 thru 6 (PC0 thru PC5, respectively) are shown on the upper right side of Figure 1. Channel 7 (Lower center) can use an SPDT or SP3T if a center position is desired. Channel 8 and model select switches are shown on the lower right side connecting to external SPST switches.

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Figure 1

Installation

Determine a location to mount the Simple Encoder. Make sure you have reasonable access to the controls so you can make adjustments at the field.

The control pots and switches are wired to the bare end of servo pigtails that will plug into the encoder board. Make sure you have allowed enough pigtail length to reach the required distance. The pot arms are wired to the signal line of the pigtails (white for Futaba, orange for JR). The other two lines go to the pot ends. See Figure 6. Switch wiring details are in figures 7a and 7b.

Refer to the board schematic Figure 2, Layout Figure 3 and simplified diagram Figure 5. The pot pigtails are plugged into J1 thru J6 (channels 1 thru 6, respectively) as required and the switched channels are plugged into J7 and J8.

Wire another servo pigtail to the switched transmitter power. Note that positive voltage is connected to the first (signal) line of the pigtail and the ground is connected to the last line of the pigtail. Signal=white for Futaba, orange for JR. Ground=black for Futaba, brown for JR. This pigtail plugs in to J9 of the board. Note that all connections to the servo pigtails are oriented in the same direction, to the top of the board per Figure 3.

If you are installing into a retro transmitter with a meter, you may wire the meter to the board and adjust it to read battery voltage. To do this, first determine the polarity of the meter connections and then connect a servo pigtail red to the positive terminal. Connect the negative terminal of the meter to the ground line of the pigtail (black or brown). Plug this line into J10 of the board.

Locate the optimum position for the transmit module. Prepare a servo pigtail to connect J11 of the encoder board to the transmit module. Refer to Figures 1 and 2 for connections. The pigtail provides power, ground and signal via corresponding pigtail connectors. If you are using a Futaba compatible transmit module, a connection diagram may be seen in Figure 1. Note that as viewed from the connector side the signal connection is closest to the edge of the module, followed by positive, an unused connection, ground and antenna. If the module is 2.4gHz, an external antenna will be provided with it and no connection to the antenna will need to be soldered. For a module on lower frequencies the antenna connection must be made with a short piece of wire to the base of the whip antenna.

For extremely tight installations using a Futaba compatible module, J12 may be used instead of the J11 pigtail. In this case the provided 5-pin connector is soldered directly onto the J12 pads and the module plugged onto the non-component side of the encoder board. Be sure to test-fit thoroughly before soldering, as this connector is difficult to remove.

If you are using a different type of transmit module, refer to note 2 and connect to J11 as required.

Always double-check connections before applying power!

Initial Turn-On

Check all connections visually and compare with the Figures. If possible use an ohmmeter to check continuity. All grounds are common.

If everything checks, a fresh battery may be installed. Unplug RF module for initial power-up.

If the meter circuit is used, proceed as follows. Locate R6 and are prepare to adjust it. Switch the transmitter on while observing the meter. Adjust the pot R6 to provide maximum deflection of the meter. If perchance the meter fully deflects to it’s stop when turned on, turn the transmitter off, reposition R6, and try again.

Alignment and Calibration

Control Centering. Physically center the controls and any trims for channels 1 thru 6. Power on and rotate the BCD selector switch SW4 to position ‘A’. Momentarily press the ‘REV’ button. Centering is now set.

Control throw. Rotate the BCD selector to position ‘C’. Momentarily press the ‘LESS’ button. Move all control pots on channels 1 thru 6 to their extremes, holding those positions at least one second. If a control has a trim, move the trim fully in the same direction as the stick so the full extreme value will be recorded. When complete, momentarily press the ‘MORE’ button. When you press the ‘MORE’ button, the LED will give a short blink indicating the throw values have been recorded into EEPROM.

At this point, power down the unit and plug in module to provide an RF link. Power up the transmitter along with a matching receiver and servo(s) and function should be observed. Do not touch any trims.

Pulsewidth Alignment. For this step, an instrument for reading pulsewidth will be necessary. We highly recommend the Vexa ServoXciter, but any similar instrument will work (see note 1). Rotate the BCD selector to ‘E’. Plug the pulsewidth instrument into any receiver channel that has a pot attached to a corresponding encoder channel with a centered control. Using the ‘LESS’ and ‘MORE’ buttons, adjust for 1.5ms. Note that each click of ‘LESS’ and ‘MORE’ results in a step, e.g. holding these buttons does not result in continued change.

Return the BCD selector to ‘0’. Power down the system.

NOTE: To date all encoders have been shipped with the pulsewidth alignment factory set as described and should require no user adjustment. This information is included here for completeness.

System Programming for the User

Install the servos and receiver into your model.

Reverse. Determine if any channels need to be reversed. To reverse channel direction, rotate the BCD selector to a number (1 thru 8) corresponding to the channel number and press the ‘REV’ button. The LED will light when the BCD selector is set to any reversed channel. Note that this includes the two switched channels 7 and 8.

Endpoint Adjustment. Depending on usage, servo throw may need adjustment. To adjust an endpoint, rotate the BCD selector to a number (1 thru 8) corresponding to the channel number. Hold the control for that channel in the direction you wish to adjust and while doing so, use the ‘LESS’ and ‘MORE’ buttons to reduce or increase the endpoint on that side. Coarse adjustments of ‘LESS’ or ‘MORE’ may be made by holding down the button and finer adjustments may be made by blipping it.

Model Selection. The model select switch stores and recalls all the reverse and endpoint adjustment values programmed by the user. Note that for safety, model selection occurs only upon power up. Place the switch in the desired position while the transmitter is off, and then turn the transmitter on. Once on, the encoder may be programmed or the transmitter used for the selected model.

It is recommended that you always return the BCD selector to ‘0’ before flight operation.

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Figure 3

Note 1

If you do not have access to such an instrument, a reference servo will work quite well. Mount the servo on a board with a long pointer attached to the output wheel such that the center position can be read with one degree accuracy. Plug the servo into a modern RC system and create a new model in that system’s transmitter. Without adjusting any trims, mark the center position displayed by the servo pointer. This will be 1.5ms.

Note 2

Futaba and similar modules use an encoder input as shown in Figure 4. Some modules require the opposite polarity. In this case, the Simple Encoder microcontroller may be programmed to provide inverted encode output. To invert the output, rotate the BCD selector to ‘F’ and press the ‘REV’ button. The LED will light to indicate reverse encode mode. Return the BCD selector to ‘0’ before use.

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Figure 4

Note 3

2.4GHz modules seldom give problems, but VHF (35, 50, 72MHz) RF sections may result in random servo movements, etc. If this occurs try the following:

1) Ground the encoder board directly to the chassis.

2) Feed power to the RF module not from the encoder, but directly from the power switch. Ground the module directly to the chassis. Keep these power leads short.

3) Keep the RF output lead to the antenna short.

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Figure 5

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Figure 6 [pic] Figure 7a

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Figure 7b

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

Note a) 10K resistors are shown, but many values may be used (2K to 10K), as long as they are the same.

Note b) If using SPDT, the point shown will have to be disconnected during control centering procedure.

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Note: If you’d like to run separate aileron servos, this is a possible option. Channel six will receive the same input as channel one when wired as shown. Of course, these could be any two channels of the first six. This method allows you different reverse and endpoint settings for the two servos by the channel programming. Any two or more proportional inputs can be paralleled this way for similar functions.

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Sometimes trims can be a problem, especially if they are separate pots. This is a circuit used on a World Expert that had 5K control pots, but 100K trim pots. It should be reproduced four times for the four primary channels using two LM358s or one LM324. The 500K gives a trim range of about 35%. Adjust the trim if necessary to assure the op-amp output does not exceed the high limit of 3.5V.

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