R/C Servo Pulse Width Meter
Products included in this specification
Product features
General description
The R/C Servo Pulse Width Meter is a measuring instrument designed to measure the widths of the control pulses sent to standard radio control pulse width modulated servos from the control system receiver. The instrument is also capable of generating and sending a pwm pulse train to a servo, this mode being entered automatically when a valid input pulse stream is absent.
The instrument consists of a hand held case fitted with a 2 line by 16 character liquid crystal alphanumeric display. One end of the case features three momentary action push buttons labelled “Scan Up”, “Scan Down”, and “Fast/Set”. On the other end of the case is a hole for access to the lcd contrast adjustment trimmer and two, three wire flying leads, one for connection to the servo to be driven and the other for connection to a receiver or battery pack.
The instrument is active at all times that it is connected to a power source.
PWM Servo control
Regardless of the rf modulation method used, almost all standard radio control systems control servo position by means of a pulse width modulated control signal.
Note: Servo is a misnomer but will be used throughout this text because it is generally accepted industry terminology. The technically correct term is remote positioner.
The industry standard consists of positive square pulses repeated every 20ms. This is called the frame width. Each pulse is nominally 1500µs wide and is varied 500µs either side of this. Thus, standard pulses vary from 1000µs to 2000µs with a centre pulse width of 1500µs.
Not all radio control systems use this standard, however all systems have centre pulse widths somewhere between 1000µs and 2000µs. Frame widths also vary between systems, generally lying between 20 and 30ms. Most servos are immune to changes in the frame width across this range.
The perfect servo has an output which is linearly proportional, through a range of typically ±55°, to the input pulse width deviation from centre.
Hardware description
The three active components in the instrument are:
The three passive components in the instrument are two ceramic capacitors in the crystal oscillator circuit and one preset potentiometer for lcd contrast adjustment.
Three momentary action SPST switches are also used for user input.
The printed circuit board is single sided, gold plated, solder masked, and silk-screened on the component side. The pre-assembled lcd module is fitted above this board and connected via a 16-pin header and 4 standoff supports.
The entire assembly is fitted inside an abs plastic case with a cut-out in the top for the lcd. A plastic front panel with membrane keys and a window for the lcd provides for the user interface. At the end of the case is a hole for access to the lcd contrast adjustment and a connector for leads to make attachments to the equipment under test.
Principles of measurement
The principle of measurement is quite straightforward. The instrument waits for the first pulse to arrive. When this pulse is stable, it then waits for the first negative edge to start the internal real time counter. At a clock speed of 16MHz this counter counts at 4MHz thus providing 250ns resolution. Since the internal counter is only 8 bits wide, interrupts are enabled on the counter overflows and these interrupts are utilised to implement a 24 bit counter.
The instrument then monitors the input line for a positive edge and on capture, saves the current counter value as T1. On the next negative edge the counter value is saved as T2. The counter is then reset and timing of the next cycle begins.
The frame width is exactly T2 and the pulse width is T2 - T1. Measurement continues for 8 pulses then the calculated results are divided by 8 and prepared for display.
Note: This is not a moving average but an averaging of blocks of 8 measurements. Since some pulse edge jitter is present in all r/c systems, it has been found that this scheme satisfactorily reduces lsd bobble.
An exact copy of the input pulse stream is also generated and sent to the servo output signal line.
During the measurement process, the instrument also monitors the input line for valid control pulses. Input levels must remain stable for 100µs in order to be accepted and upon loss of acceptable signal the input line will be polled for a further 300µs.
In the event that no valid input signal can be found, the instrument will alter it's operating mode to that of generating pulses. Whilst in this mode the input signal line will be monitored continuously for a valid and acceptable stable signal using criteria similar to those described above. If a valid signal is found, then the operating mode will revert to the pulse measurement mode.
In the pulse generating mode, pulses are produced with a frame width of approximately 20ms. Pulse widths have a nominal centre equal to a value stored in eeprom memory, which can be set by the user, and a deviation of ±500µs.
User interface
The user can change the width of the generated pulses by pressing the “Scan Up” or “Scan Down” buttons which increase or decrease respectively the pulse width by 10µs. Holding either button down will cause the pulse width to change continuously in increments of 10µs every 200ms. If the “Fast/Set” button is pressed at the same time as one of the scan buttons then the output pulse width will change in increments of 10µs every 20ms.
If the “Fast/Set” button is pressed alone and held for 1½ seconds then the current displayed value of the output pulse width is latched as the new default centre pulse width and stored into eeprom memory.
Product availability
Printed circuit boards and membrane keypads are expected to be available prior to the end of 1999. These are expensive to tool for and since this is a small production item, tooling must be included on compatible prototype commercial jobs. When this is done, the product will be offered in kit form or assembled and tested.
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