RF signal problem in Robot

[hamid159]

Hy Guyz,,
i'm making a robot..27Mhz transmitter is being used...i'm using relays as motor driver...when i don't connect the motor with relays,circuit works well at about 7 ft range ....but when i connect the motor with relays and keep the transmitter near to the receiver it works well..but when i take the transmitter a bit far,then relays turn off and on fastly....it energizes and then suddenly de-energize..and the process keeps on going...
Any help would be appreciated....
Thank You in Advance

 

[biff44]

when a relay closes, especially if it is attached to a motor, there is a tiny spark generated. That spark contains RF energy, and is quite likely jamming your receiver on the robot.

You could do a number of things:
* put a 1000 pf ceramic capacitor, 50v rating, across the relay contacts. If it is DC system, a diode across the same terminals (pointed the right way) will snub out the inductive kickback.
* you could shorten the wire length between the relay and the load (i.e. shorter wires = shorter transmitting antennas)
* You could put your receiver in an emi shielded box, with only one coaxial connection to the RF antenna. Put the antenna on different spots on the robot to find where it picks up relay noise as little as possible.
* You could use power transistors instead of the relays to do the control. But make sure the transistors are pretty high breakdown voltage, and use the diode/capacitor snubber, or the transistor may fail.
* change to a higher frequency system (433, 915, 2450 MHz) where the relay noise has died out to a very small level

 

[hamid159]

Thanks biff44...
transient current of motor is about 10A while steady state current is 4A.....i've also put a diode across relay coil terminals but nothing happened....how can i use power transistors....will i have to use h-bridge circuit??? what about solid state relay?can this relay solve my problem?

 

[chuckey]

The problem could be with the sparking at the motors commutator. Connect 2 X 1000PF capacitors from the two brush holders to the motor case. If this does not cure the problem , feed the motor leads through either ferrite beads or RF chokes, positioned as close to the motor as possible.
Also make sure that the motor current does not flow through any part of the circuit in common with the receiver power feeds.
Frank

 

[vfone]

You have a lot of DC current with that motor. At 10A most probably you have a voltage drop from the battery.
Check this voltage drop if not affect your receiver somehow (most probably shifting the oscillator frequency).

 

[hamid159]

yes there is voltage drop.but there is separate voltage supply for receiver circuit

 

[biff44]

Thanks biff44...
transient current of motor is about 10A while steady state current is 4A.....i've also put a diode across relay coil terminals but nothing happened....how can i use power transistors....will i have to use h-bridge circuit??? what about solid state relay?can this relay solve my problem?

NO, not across the relay coil...you want across the relay contactors that switch the motor on/off. That is where the sparking is, and what maybe generating RF energy.