FAQ
TL;DR: Line tracers using L297/L298 steppers can draw 1.0–1.4 A under load; “If efficiency matters then you don’t want to use stepper motors.” [Elektroda, Olin Lathrop, post #21658986]
Why it matters: Builders want longer runtime without sacrificing tracking accuracy; this FAQ shows where the current goes and what to change.
Quick Facts
- Battery used: 12 V, 1.8 Ah sealed pack reported by the builder. [Elektroda, Vinit Apte, post #21658981]
- Measured current, wheels off-ground: approx. 120–150 mA total. [Elektroda, Vinit Apte, post #21658979]
- Measured current, on-ground load: approx. 1.0–1.4 A total. [Elektroda, Vinit Apte, post #21658981]
- Expected runtime at 1 A load: roughly 1–2 hours for a healthy 1.8 Ah pack. [Elektroda, Joe Wolin, post #21658982]
- Quick diagnosis: test battery with a 12 Ω ≥12 W load and log voltage every 5 minutes. [Elektroda, Joe Wolin, post #21658984]
How long should a 12 V, 1.8 Ah battery run my line tracer?
At 1.0–1.4 A on-ground draw, expect about 1–2 hours if the battery is healthy. That aligns with a back-of-the-envelope 1.8 Ah ÷ ~1 A ≈ ~1.8 hours. If you see far less, the pack may be weak or the current is higher than measured. [Elektroda, Joe Wolin, post #21658982]
Why does current jump from ~150 mA to over 1 A when it’s on the floor?
Load and stepper inefficiency cause the jump. Steppers waste energy as heat and momentum is lost between fixed steps. As speed rises, square-wave coil drive further reduces efficiency. Wheels on-ground add mechanical load, so current rises sharply. [Elektroda, Olin Lathrop, post #21658986]
Are stepper motors efficient for a line tracer?
Not particularly. They offer simple positioning but poor electrical-to-mechanical efficiency, especially with fixed-step, square-wave drive. As one expert noted, “If efficiency matters then you don’t want to use stepper motors.” Consider alternatives if runtime is critical. [Elektroda, Olin Lathrop, post #21658986]
Will microstepping or sine-wave drive help reduce battery drain?
Yes, somewhat. Driving each winding with an approximated sine (microstepping) improves power transfer and smoothness, cutting some losses compared with square steps. Use a suitable microstepping driver or a MCU with motor-control PWM to synthesize the waveform. [Elektroda, Olin Lathrop, post #21658986]
How do I test if the battery is the culprit?
Load-test it. 1) Disconnect from the robot. 2) Attach a 12 Ω, ≥12 W resistor as a fixed load. 3) Measure and log battery voltage every 5 minutes for an hour, then compare to expected discharge behavior. A fast sag indicates a weak pack. [Elektroda, Joe Wolin, post #21658984]
What baseline measurements should I take before optimizing power?
Measure total current at the battery in multiple modes: idle, sensors active, wheels off-ground, and driving on-ground. Then isolate sections (logic, motor drivers) and measure each. These baselines reveal which block dominates consumption. [Elektroda, Joe Wolin, post #21658978]
Do PWM tricks on the L298 reduce stepper current meaningfully?
Not much in this setup. The builder tried PWM and saw little improvement. With fixed-step square drive, steppers remain inefficient, so PWM alone won’t deliver large runtime gains. Microstepping or a different motor type helps more. [Elektroda, Vinit Apte, post #21658971]
What do the L297 and L298 actually do here?
The L297 generates step and phase control for a stepper, while the L298 H-bridge drives the motor coils with required current and polarity. Together they form a classic stepper-control chain used in the described robot. [Elektroda, Vinit Apte, post #21658971]
My robot dies after ~15 minutes—what does that imply?
That observed runtime indicates a mismatch with the expected 1–2 hours. It suggests either reduced battery capacity or higher real load than earlier measurements captured. Start with a controlled load test and re-verify on-ground current. [Elektroda, Vinit Apte, post #21658983]
Should I switch from steppers to DC servos for better efficiency?
If runtime matters, yes. A servo with position feedback typically converts electrical power to motion more efficiently than a stepper in continuous drive applications like line tracing. Expect better watt-hours per meter traveled. [Elektroda, Olin Lathrop, post #21658986]
What’s a simple way to compare no-load vs load current correctly?
Use a DMM at the battery leads and record current with wheels off-ground and then on-ground. Note the large delta; one report showed ~120–150 mA off-ground versus ~1.0–1.4 A on-ground, a useful diagnostic contrast. [Elektroda, Vinit Apte, post #21658981]
What’s a quick 3-step plan to extend runtime without new batteries?
1) Improve drive efficiency: enable microstepping/sine-like coil drive. 2) Reduce mechanical load: check friction and weight. 3) Verify battery health via resistive load test before further tuning. “Efficiency matters” most in motor choice. [Elektroda, Olin Lathrop, post #21658986]
